Smith-Kettlewell TECHNICAL FILE



2318 Fillmore Street, San Francisco, CA 94115

VOL. 3, NO. 4, Fall 1982











The circuits appearing in this article are auditory; a closed circuit causes the tester to produce an audible tone. (Where possible, testers which can provide sufficient tactile output power will be mentioned for use by our deaf-blind readers.) Circuits of two classes will be discussed--the first is simple "go/no-go" buzzers, and the second can be grouped as "current-controlled oscillators." The latter systems actually give the user information as to the resistance of the test circuits.

It can be argued that the continuity tester is the most important test instrument in the blind technician's equipment. Whereas a sighted person finds occasional use for his ohmmeter in checking for burned-out coil windings and defective switches, his blind counterpart finds great utility for audible "buzzing" devices in tracing colored wiring, tracing foil on printed circuit boards, checking polarity of diodes and electrolytic capacitors, testing his soldering, and ad infinitum.

Simple "Go/No-Go" Buzzers

The Sonalert Series (by Mallory)

Sonalerts are encapsulated beepers (the innards of which are a secret akin to the Coca Cola formula) which are commonly heard in "beep balls," air terminal security stations, and warning sounders in burglar alarms. They produce a tone of 2900 Hz. The most basic units, the SC628 (6 to 28 volts) and the miniature version, SNP428 (4 to 28 volts), operate just fine from a low-current source, such as a 9-volt battery going through a large test resistance. With 2 to 10 uA (the test resistance being 500K to 100K), you will get a usable audible tone. Higher voltage units (110VDC) are also available, which might be appropriate for making high-voltage leakage tests.

A series of AC units can also be gotten which contain a diode bridge rectifier. The 110-volt AC unit, the SC110, can be substituted directly for the neon lamp in high- voltage test bench setups (found in appliance repair shops); simply connect the unit through a 2-wire cable to the base of a broken neon lamp of appropriate size.

Basic Sonalert Circuit

Taking the low- voltage DC unit as an example (the SC628), the positive lead of a 9-volt battery goes to the plus terminal on the back of the Sonalert. The minus terminal on the Sonalert goes to the positive test lead, while the negative test lead goes to the negative side of the 9- volt battery. Since the larger versions of the Sonalert have screw terminals, this con- nection can be done using no soldering. If sense of the aesthetic dictates, you can mount your unit in a box (requiring a 1-1/8 inch hole for the large version and a 1-inch hole for the small one).

Electronic Buzzers

Recently, a new class of buzzers has become available which are "contactless." They use a transistor oscillator to drive a coil, against one pole of which a permanent magnet is made to bounce. These are very small rectangular units which have wide application in miniaturized IC projects. Many have pins for mounting them directly on PC boards. There are units with screw holes and flexible wire leads, such as the Mouser 25MS120 (6 to 16 volts). These can be screwed down to a breadboard or mounted on the apron of a cabinet. (Mouser Electronics, 11433 Woodside Avenue, Lakeside, CA 92040; phone (7l4) 449-2222.)

Basic Circuit for Electronic Buzzer Testers

The positive battery terminal goes to the plus terminal of the buzzer. The negative buzzer terminal goes to the positive test lead, while the negative test lead goes to the negative side of the battery. Of course, the voltage of this battery is chosen so as to be appropriate for your particular buzzer.

These buzzers draw fairly high currents, commonly 25mA. Therefore, with the above circuit, these testers are only appropriate for low-resistance continuity tests, such as checking of switches and of direct wiring. Checking transformer windings will no doubt be fatal to the buzzer and can give you the surprise of your life on a step-up winding, since the pulsations of the buzzer will be stepped up to a very high voltage.

An embellishment on these buzzers is the presence of a control pin by which the unit is triggered. The advantage in using these units is that the test circuit is not called upon to pass the buzzer's supply current. My favorite brand of these is the Star Micronics CMB-12 (7 to 17 volts) and the CMB-06 (3 to 7 volts). (Star Micronics, Inc., 200 Park Avenue, Suite 2308, New York, NY l00l7; tel. (2l2) 986-6770.)

Circuit for the Star Micronics Buzzer with Control Pin

These units have four pins which are in the location of the corner pins on a 14-pin DIP socket. The "speaker holes" are closest to pins 7 and 8, with the pins on the other end being numbered 1 and 14.

Pin 1 is grounded and goes to the negative terminal of the battery (9V for the CMB-12). Pin 14 goes to the positive battery terminal (no switch is necessary). The positive test lead goes to pin 14 and to the positive of the battery. The negative test lead goes to pin 8, the control pin.

These buzzers will sound when pin 8 is brought high (up to pin 14). They will trigger reliably with a test resistance of less than about 68K.

All of these electronic buzzers produce considerable mechanical vibration. Deaf- blind users would have no trouble feeling them go off, and they are small enough to hold in the palm of one hand while testing is being done.

Relaxation Oscillators

Back in the days when we had steel men and wooden test instruments, our auditory continuity testers were made using a neon lamp in a "relaxation oscillator." These units are the first in a series of "current-controlled oscillators," the use of which gives the user feedback as to the amount of resistance in his test circuit (the lower the resistance, the higher the pitch). Here follows a brief discussion of theory regarding relaxation oscillators.

The operation of the neon-type oscillator is simplest to describe. The lamp is placed in a series circuit which contains a charging resistor. This lamp is shunted by a "timing capacitor." The capacitor is charged through the resistor until the ionization of neon in the lamp occurs (60V plus). At this point, the ionized neon presents a low impedance across the capacitor, causing it to discharge to a point at which the neon de-ionizes (relaxes), whereupon the charging cycle re-occurs.

Circuit for Neon Relaxation Oscillator

The negative terminal of the battery (90, 135, or 180 volts) goes to the tip of an open-circuit earphone jack, with the sleeve of this jack going to ground and to the negative test lead. The positive of the battery goes through per- haps 1 megOhm to one side of the neon lamp, with the other side of this lamp going to the positive test lead. The lamp is shunted by perhaps .001uF.

The earphones used must be high-impedance. Dig out your ol' Trims, Brandes, Baldwins, or Army surplus relics.

These circuits operate on very high voltages in order to ionize the neon in the lamp. This makes them ideal for testing the leakage of big ol' capacitors and may lead to discovering flaws in the installation of big ol' equipment.

One good reason for explaining the theory of the above neon oscillator is that we now have an excuse for rehashing the NE555 oscillator, whose concept is similar.

The NE555 is a fancified relaxation oscillator in its free-running connection. The charge on a "timing capacitor" is monitored by "sensing pins" (pins 2 and 6, the "Trigger" and "Threshold" pins respectively). As a matter of academic interest, pins 2 and 6 are one input each of two comparators whose job it is to sense 1/3 and 2/3 VCC respectively. Ignoring test leads for the moment, consider the following arrangement. VCC goes through 100K (a charging resistor), then through .002uF (a timing capacitor) to ground. The junction of this resistor and capacitor goes to pins 2 and 6 (which are tied together), and this junction also goes through 10K to pin 7, a "Discharge" terminal. As soon as the capacitor's voltage gets up to 2/3 VCC, the "sensing pins" dictate that pin 7 shorts to ground, so as to discharge the capacitor through the 10K resistor. Then, as soon as the capacitor voltage drops to 1/3 VCC, pin 7 opens (relaxes), allowing the charge cycle to re-occur.

Thus, pin 6 and pin 2 serve the same purpose as the ionization and de-ionization phase of the neon lamp, respectively. It may interest you to note that if you put your high- impedance earphones in series with the 100K charging resistor, this circuit will sound and act just like the neon relaxation circuit which it emulates.

Why not combine pins 2 and 6 into one terminal? The reason is that a one-shot can be gotten by separating pin 2 from the RC junction. A cycle can be "triggered" by bringing this pin down below 1/3 VCC. In some chips, these two "sensing pins" are indeed combined and brought out as one pin-- for example, pin 7 on the XR2242 timer chips in the battery charger (SKTF, Winter 1981). Since the oscillator in the 2242 is never to be operated as a one-shot, the manufacturer decreed that pin 7 be such a combination.

Circuit for NE555 Tester

(Note that this circuit needs an on-off switch, since the timer chip draws current at all times.) Pin 1, the negative supply pin, is grounded. Pin 8, the positive supply pin, goes to the VCC line, which in turn goes through an on-off switch to the positive side of the battery (3 to 9 volts). Pin 4, the "Enable" pin, goes to pin 8 and to VCC. (Bringing pin 4 to ground stops the oscillator from operating.)

The positive test lead goes to VCC, while the negative test lead goes through 100K, then through .002uF to ground. The junction of this resistor and capacitor goes to pins 2 and 6 as well as going through 10K to pin 7, the "Discharge" terminal. The output, pin 3, goes through 47 ohms, then through the speaker to VCC. (Pin 5 is not used here; it is the 2/3 VCC point, and is brought out externally so that this voltage level can be shifted for FM'ing.)

On the 555, the output (pin 3) comes from a flip-flop which is operated by the "sensing pins." This output, which produces nice square pulses and is capable of handling currents up to 200mA, can be used to drive the loudspeaker directly.

The NE555 can supply enough power to a speaker (or tactile transducer) to be felt by the user, especially if the oscillator frequency is drastically reduced. A deaf-blind user might well consider building this circuit with a timing capacitor of perhaps .15uF. The pulses from the output can be increased in width by increasing the value of the discharge resistor to perhaps 27K.

Uni-Junction Transistors

Not to be neglected in a discussion of relaxation oscillators are those which use the uni-junction transistor. Once again, the charge and discharge of a timing capacitor (supplied through a charging resistor) is orchestrated by the "firing" of the transistor as discussed below:

A bar of silicon with ohmic contacts at either end, the base 1 and base 2 contacts, forms a voltage divider (through a load, which may be a speaker) across the battery supply. A third lead from the device, the emitter, goes through a resistor to VCC and also through a capacitor to ground. When the charge on the capacitor reaches about 1/2 the battery voltage, a diode junction between the emitter and the base bar becomes forward- biased; the resultant flow of charge carriers causes a drastic reduction in resistance between base 1 and base 2. The emitter cannot help but be involved in this confusion--it is brought down to a lower voltage which thereby discharges the capacitor connected to it. At some point, the charge on the capacitor is bled off such that the emitter base diode junction is no longer polluting the base bar with charge carriers; it "relaxes" and the charge cycle is allowed to resume.

Circuit for Uni-Junction Transistor Oscillator

Base l goes through the speaker to ground, while base 2 goes through an on- off switch to the positive side of a 9-volt battery. The negative side of the battery is grounded. The emitter of the transistor goes through .05uF to ground, and also through 4.7K to the negative test lead. The positive test lead goes to the base 2 side of the on-off switch.

Suitable uni-junction transistors (UJT's) are: 2N2646, 2N2647, 2N485l, 2N487l, HEP3l0, and Radio Shack 2762029. Holding the UJT with its leads pointing upward and with the flat side toward you, the three leads are, from left to right, base 2, emitter, base l.

You will notice that the test leads are not common to ground in the latter two of these circuits. This means that, if you insist that your tester's cabinet be at ground, the test lead jack or jacks must be insulated from it. Another way around this problem is to use a "current mirror" which will allow the negative test lead to be at common ground. Proper current mirrors (not available at Pierre Cardin) use a matched pair of transistors; however, since our unit gives only relative indications as to the test circuit current, we will forego this luxury.

The current mirror consists of two PNP transistors, the first of which is used for its base-emitter diode and sets the bias for the second such that current is duplicated in mirror image. Both emitters are tied together and go to VCC. The collector of the second transistor goes to the top of the timing capacitor, simply leaving out the charging resistor. The base of the second transistor goes to both the collector and the base of the first, which also goes through the charging resistor (which has been moved over to this portion of the circuit), then to the positive test lead. The negative test lead is grounded.

Transistor Blocking Oscillator The Gimmick and the "Audicator"

A logical extension of the Transistorized Auditory Gimmick (developed by Bob Gunderson in l955 and discussed in SKTF, Spring 1981) is to use the oscillator section as a continuity tester. An extremely popular instrument, the "Audicator," using this circuit, has been available for decades from Science for the Blind (SFB Products, P.O. Box 385, Wayne, PA 19087). Because of its wide range of uses, no innovative vocational specialist can afford to be caught without an Audicator, and it is the Editor's choice of continuity testers.

A blocking oscillator is a circuit in which a transistor self-biases itself (using transformer-coupled feedback) until "blocking" terminates this process. " Blocking" is the point where the transistor saturates and is no longer able to vary the current in the trans- former winding. Once the dynamic activity ceases and hard-biasing of the transistor begins to fall away, the transistor self- biases itself in the other direction; the transistor quickly works to decrease the current in the transformer winding until it is cut off.

In the continuity tester circuit to be described, the resistance of the test circuit provides the initiating bias to start the oscillations; otherwise the circuit lies dormant and draws no current. The bias current determined by the test circuit profoundly affects the frequency of oscillation. This is true, since the charge state of a coupling capacitor on the base is profoundly affected by the impedance of the external biasing circuit.

While the above theoretical treatise is elegant (don't you think?)--"a transistor self-biases itself in accordance with guide- lines from the Department of Redundancy Department"--the "blocking oscillator" is a great deal easier to build than it is to understand. If it makes you feel better, you can take comfort from the fact that the Editor built and used them for 20 years before he knew how they worked.

The circuit is built around a push-pull output transformer having a center-tapped primary winding of 500 or 1000 ohms and a secondary winding of 8 ohms. The transistor can be any general-purpose PNP silicon unit, such as a 2N2907. (The transformer is avail- able from Radio Shack as a 273-l380, and the transistor is a Radio Shack 276-2023.)

Audicator Circuit

The center tap of the primary goes to the emitter of the transistor; the collector of the transistor is grounded. One end of the primary winding goes through 0.1uF to the base of the transistor, while the other end of the primary goes to plus 9 volts. The negative end of the 9-volt battery is grounded. The negative test lead is grounded, while the positive test lead goes through 4.7K to the base of the transistor. The secondary of the transformer drives the speaker.

Embellishments on the Audicator

Often, in checking for intermittent circuits or for verifying the contact of components being soldered (see next article), listening to the high-pitched squeal of a low-resistance circuit for a prolonged period can be annoying and can threaten the family structure. There are two simple circuit arrangements which you can add to the basic unit to ease the pain.

Pitch Control

A rheostat can be put in series with the positive test lead to permit insertion of additional biasing resistance, thus bringing the pitch down to a comfortably low frequency. In series with the 4.7K base resistor, connect a rheostat of perhaps 500K. Mount this rheostat on the tester and fit it with a control knob so that it can be operated conveniently.

Discontinuity Tester

In operation, the tester is made to sound until a test circuit appears across the test leads. In this way, the tester will be silent until a break in the test circuit occurs. Use a shorting jack (closed-circuit phone jack with its switch contact wired to the sleeve) for connection of the continuity tester's leads; when the test leads are removed, the tester will sound. Then, include another jack into which these test leads can be inserted (simple open- circuit phone jack) with its sleeve going to the transistor base and its tip going to plus 9 volts. Shorting the discontinuity tester's leads biases the transistor to cut-off.

Past References to Continuity Testers

Continuity tester circuits have been touched on in previous Technical Files. The two main articles are "Auditory Gimmick Circuits - Old and New," Spring l98l, and "The K3VTA Auditory Gizmo," Summer l982.

Polarity Reversing Switch

In testing diode junctions and transistors (see next article), it may come in handy to have a switch by which the polarity of the test leads can be reversed easily. Be sure to mark this switch or remember which is the right way round, since knowing the proper polarity of your test leads will be very important in testing such items. (I always mark the positive lead with a knot, piece of string, or tape.) The circuit configuration for the polarity-reversing switch will come in handy for other purposes; the Editor has long since committed it to memory.

A double-pole, double-throw switch is used. Position l of Section A goes to Position 2 of Section B; Position l of Section B goes to Position 2 of Section A. You can remember this configuration by noting that the above jumper wires form an "X" on the back of the switch. Position l of Section A shall be deemed negative, and shall go to the negative test point in the tester; Position l of Section B goes to the positive test point in the tester. The negative test lead goes to the swinger of Section A, while the positive test lead goes to the swinger of Section B. When the switch is in the "1" position, the test lead polarity is normal; when the switch is in the "2" position, the polarity is reversed.



Uses for the test instruments of the previous article are enumerated here. While particular attention is paid to the testing of electronic components and circuits, mention is made of other uses -- light probes, liquid level indicators, and rain alarms.

{Editor's Note: My thanks to Dr. T.A. Benham of Science for the Blind for the excellent demonstration tape which is available with the Audicator. Many of the ideas and techniques in this article came from there.} It can be argued that the simpler the instrument, the more complicated will be its use. This certainly applies to the appropriate use of continuity testers. Inappropriate use can mislead the technician, and can even damage equipment. While an article like this can tickle your imagination, it could never list all the pitfalls. Proper use of such a simple device is nothing short of an interpretive art, and the mistakes one makes will be the best teacher.

Checking Cables

Whenever you suspect a bad plug-in cable or whenever you solder connectors on a cable, the continuity tester will give the best assurance that positive contact has been made and that no short circuits have developed.

Check all connections individually from one end to the other of the cable. Next, at one end of the cable, check for shorts between adjacent connections. Finally, short every- thing together at one end of the cable with clip leads; hook the tester onto connections at the other end and wiggle the cable as it enters its connectors. If the tester signal cuts in and out, there is an intermittently open connection, which is very often due to a "cold solder joint."

It may not be possible to check for shorts in certain instances. Consider the following examples: Audio cables of the "attenuator type" usually have a 10-ohm resistor across both ends. Wires which go to a speaker and/or to an output transformer can be tested for open circuits, but they cannot practically be tested for shorts. Many antennas, especially those having transformer matching at the feed point, present to the tester a very low- resistance path--hence, the cable will appear shorted unless it can be disconnected from the antenna.

In this and other direct wiring checks, simple buzzer-type testers are as effective as their fancier counterparts.

Tracing Wiring

Wires in a bundle or wires through a conduit can often be identified (traced) with a continuity tester. For example, if you get hold of an identified wire with one continuity tester lead, this wire can be found at the other end of the bundle by searching around with the other tester lead. The success of this procedure depends heavily on the electrical characteristics of associated circuitry --resistances encountered had better not look like shorts to your tester. Very long wires which preclude the possibility of reaching both ends with test leads can be traced by tying the far end of an unknown wire to a known conductor--ground or a known wire. One tester lead can then go to the known conductor, while the other is used to search for the unknown. In this way, testing can be done at one end of the cable.

Two dangers should be kept in mind when doing these tests. The circuit should not be live; voltages of the wrong polarity or volt- ages over the supply of the tester can damage it. On the other hand, currents and voltages supplied to the test circuit by the tester can damage sensitive semiconductors.

Tracing Foil on Printed Circuit Boards

One clip of the tester can be used to follow along a foil trace of a PC board. Hook one tester clip to a lead on the component side of the board, or simply hold it against the solder joint on the foil side. Then, the direction (or directions) of the trace emanating from this joint can be traced by first circumscribing the point of origin with the other test lead, after which this lead can be used to follow the trace. You can best keep track of the trace by purposely scanning the tester lead from side to side across the trace, thus treating it like a very irregular sidewalk being explored with your cane.

As with all in-circuit testing situations, you will find times when associated circuit resistances are low enough to fool you--they will look like short circuits to the tester. Also with in-circuit testing, you must be conscious of the tester's capability to damage sensitive circuit components.

Hmmm-mm-mm. ... Which Resistor is Which?

Current-controlled oscillators can be used to compare resistors. For example, if your parts collection for a project consists of four resistors, 6.8K, 22K, 47K, and 100K, the pitches generated by the tester will be enough different as to make their identification obvious by a process of elimination. If, as in full-fledged electronics labs, you have well-labelled cabinets full of all 5% or 10% values, you can use your tester as an ohmmeter--simply compare the unknown with "guesses" from the parts drawers.

Checking the Type and Polarity of Diodes

Since diodes conduct only in one direction, a continuity tester whose positive and negative leads have been marked is a natural for determining diode polarity. Junction diodes can be tested with any of the circuits appearing in the previous article (with the possible exception of high-current electronic buzzers). (There are very rare exceptions where the diode's current-handling capability may be exceeded by a given tester.)

Diodes conduct when their anode is positive with respect to their cathode. If connecting the unit to the tester elicits no sound, the positive lead is on the cathode. If a sound is heard, the positive lead is on the anode (except in the case of low-voltage Zeners, which will be discussed later).

The Editor once had a job of installing 600 diodes into a set of 200 PC boards (3 diodes per board). To speed things up, a short piece of model railroad track was connected to an Audicator so that dozens of diodes could be lined up along the track; they were oriented so that they were all back-biased. After the track was loaded, the units were slid off into a narrow box to preserve their orientation. In loading the track, each diode was checked by laying it across the rails; if the tester went off, the diode was forward-biased, at which point it was turned around and laid on the track so as not to make noise.

Incidentally, three of the diodes had their visual markings reversed and would have been installed improperly by a sighted technician. A half dozen leaky units, as determined with the tester, were removed before installation, thereby saving us some troubleshooting.

Although modern units are seldom leaky, you may find an occasional diode which will cause the tester to sing at a moderately high pitch in the reverse direction. This means that you have an imperfect one which should reside in the circular file.

Zener diodes whose voltage is lower than that needed to bias the tester into operation will cause your instrument to sound in either direction. However, current-controlled testers will often give you a lower pitch when the positive lead is on the cathode of the Zener, since the diode is holding the tester leads at its intended breakdown voltage.

Sensitive testers, such as the Audicator, will actually give you a different pitch for different diode junction voltages. For example, it is easy to tell the difference between a short circuit, a germanium diode, and a silicon diode--each causes the tester to produce a different pitch. It is often a good idea to keep a sample of each on hand so that your unknown unit can be matched to a sample.

Identifying Leads on JFET's and UJT's

Both uni-junction transistors (UJT's) and junction field-effect transistors (JFET's) will look similar on a continuity tester. The channel and/or the base bar will cause the tester to sound the same pitch in either direction. When you have found the control element (the emitter of the UJT or the gate of the JFET), it will exhibit the properties of a junction diode from this lead to either of the other two leads. When this control element is P-type, the diode will be forward- biased (and will cause the tester to sound) when it is connected to the tester's positive lead. When the control element is N-type, the tester will sound when this element is on the tester's negative lead.

Testing Bipolar Transistors

Not only can you tell the gender of your transistor (NPN or PNP), but you can tell whether or not it still works, and you can make a crude guess as to how much current gain it has. Finally, you can often determine whether or not it is silicon or germanium. For the latter two tests, it helps to have a couple of known samples with which you can compare the unknown unit.

A bipolar transistor looks like two diodes connected back-to-back (facing opposite directions), with the base being the junction between the two. Therefore, you will know you have the base when you detect a diode between it and the emitter, as well as between it and the collector. Furthermore, if these diodes are forward-biased when the positive tester lead is on the base, the base is of P-type material and the transistor is NPN. If these diodes are forward-biased when the negative test lead is on the base, it is of N-type material and the transistor is PNP.

Finding the base-emitter and base-collector junctions intact does not guarantee that the transistor is good. Very often, these will test OK, while a collector-emitter test will show a short.

Once you find out which kind of transistor you have (NPN or PNP), you can hook it up so as to watch it "transist" using an Audicator or other sensitive current-controlled oscillator. If NPN, put the transistor's emitter on the negative test lead and its collector on the positive test lead. You can then bias the transistor into conduction by pressing your fingers against both the base and collector leads; the tester will squeal with delight as the pressure of your fingers increases the base bias. You will notice that you can get much greater pitch variation using the "gain" of this external transistor than you could by squeezing the continuity tester's leads by themselves. In fact, the degree to which your irresistible touch influences the pitch is a crude indication of the transistor's gain.

As mentioned before, the pitch of forward- biased germanium junctions will be slightly higher than that gotten from silicon junctions. As yet another indication, note the leakage current with your tester connected from collector to emitter. (This current, termed ICBO, is collector current with base open.) The tester will make little or no noise if the unit is silicon, while a germanium transistor may very well produce a low buzz.

Checking Polarity of Electrolytic Capacitors

The polarity of capacitors can often be felt tactually. The positive end may have a rubber insulator and/or a crimped ridge in the body to hold this insulator, or the plus lead of new units may be longer than the minus lead. Sometimes, however, you will undoubtedly come across units whose markings are not tactually obvious. A good current- controlled oscillator, such as the Audicator, can be used to identify the polarity of many such units. Capacitors of greater than perhaps 10uF will cause a descending pitch to be emitted from the tester, and the rate of descent will vary in direct proportion to the capacitor's value. (Even a 1uF unit will make the tester produce a discernible "chirp.")

When connected correctly (the positive of the capacitor to the positive test lead), the pitch will descend to a point at which the tester actually cuts off, although leakage of the capacitor will cause the tester to emit periodic low-frequency bursts. When connected incorrectly (in reverse with the positive of the capacitor to the negative test lead), the capacitor will accept only a partial charge; the pitch of the tester will descend only part way, after which it will even rise slightly as nasty reforming of the dielectric occurs.

After having tested an electrolytic capacitor in the wrong direction, you will do it a big favor by reversing the test leads and charging it correctly. When making indiscriminate in-circuit tests (tracing wiring and checking for shorted socket pins), you will often come across capacitances as you probe around. If you get wind of the fact that a particular unit has taken a charge in the wrong direction, reverse the tester leads and put a charge of correct polarity on it.

Tantalum capacitors are often of such quality as to not break down or exhibit leakage when charged in the wrong direction; this makes polarity identification difficult. Ironically, these units are encapsulated in such a way as to make tactile polarity identification impossible as well. On new units the positive lead is sometimes longer than the negative lead. Where this cue is not present, it is time to seek sighted assistance.

Using Your Tester as Feedback in Soldering

There are two ways in which a continuity tester can significantly contribute to a soldering situation. The first is by giving the technician positive assurance that his connection has not been jostled out of position--a wire slipping off a socket pin, etc. The second idea is to use a continuity tester to "mark" the soldering target so that contacting it with the iron causes a "beep."

We know from preliminary discussions of soldering that all items must be in firm physical contact before they can be expected to heat up together. Connecting the tester to leads or terminals of two such items, a break in the path will evidence itself by interruption of the tester's signal, and the two work pieces are no longer touching. At this point, we can find out what went wrong before an unsuccessful attempt is made to solder them. Usually, the problem is that a lead was jostled away when you approached it with the solder and the iron.

As an example of an actual connection, let us consider the situation where you have a component lead which is bent over and lying on top of an IC socket pin. A piece of 26- gauge solid wire can be inserted into the socket hole on the component side of the board and then attached to one clip of your tester. The other tester clip can be attached to the component lead as it emerges from the body, or the tester can sometimes be clipped onto the component's other end (as long as a DC current path is established; i.e., the component cannot be a capacitor.) When the lead is in contact with the socket pin to which it is to be soldered, the tester will give you a positive indication.

If your soldering iron has a 3-wire plug and is consequently of the grounded type, the desired joint to be targeted can be connected through the continuity tester to ground. For example, one tester lead can be attached to an IC socket hole through a piece of 26-gauge wire, while the other tester lead is grounded. As soon as the iron touches the desired target, the tester will indicate a direct short circuit and will give you positive assurance that you have arrived.

In building amplifiers with the LM386, the Editor has often built himself into a corner by jumpering pins 2 and 4 together before the input, pin 3, has been soldered. In this case (remembering from preliminary soldering discussions that materials which are not being heated will not take solder), I used the tester to indicate when the iron is inadvertently contacting the junction of pins 2 and 4. By grounding one tester lead and inserting the other into socket holes 2 or 4, the continuity tester will assure me that I am avoiding these two pins, thereby preventing a bridged connection between them and pin 3.

You may wish to modify your tester (as discussed in the previous article). The tester may be fitted with a pitch control (a rheostat in series with one of the test leads). The other modification is to make it into a "discontinuity tester." Either of these modifications will be a comfort to your ears and an asset to your patience while contact is being maintained.

Troubleshooting Your Projects

Whether during the building of projects or if you later suspect a wiring error, the continuity tester can often do for you what visual tracing of wires does for a sighted technician. However, as circuits become complex, interaction between various parallel resistive paths will lead to false and meaningless indications of resistance values. As the circuit complexity increases, you must use your common sense to narrow down the question being asked with the tester, perhaps limiting yourself to testing for direct shorts.

As a procedural example, let us consider the wiring of an IC into perforated board using point-to-point wiring. Pick a socket pin whose connection you wish to verify. Connect one tester lead to the component lead which is supposed to go to this pin. (Only if absolutely necessary should you make your test from the far end of this component, since other resistances in the circuit will influence the results of your test.) Attach- ing a piece of 26-gauge wire to the other test lead, use the audible signals to verify a direct connection to the desired pin. After the desired connection has been verified, try adjacent socket holes to see if a bridge has been created between pins.

Of course, the continuity tester should not be your only test instrument; nothing can substitute for current and voltage tests on live circuits, and nothing can substitute for following signals with a signal tracer. However, the above tests can catapult freshly wired circuits into the realm of being "testable" before power is applied.

Light Probes

Light sensors can be directly connected to current-controlled oscillators to make them into light probes. Different tester circuits will create instruments of different sensitivities, with the 555 timer offering the best flexibility for changing circuit parameters. (In a future issue, we will present the circuit for the Smith-Kettlewell light probe with sensitivity control which uses the 555 as its oscillator.)

A very sensitive but slow-response sensor is the age-old cadmium-sulphide photo resistor (Radio Shack 276-116). This can be connected in either direction across the tester (it is not polarized), and will respond to light over a wide angle.

Photo transistors (which have an extremely fast response) are much more directional. Among their chief disadvantages is that they are much more sensitive to red and infrared than to any other light frequency (they may ignore neon indicator lamps).

Photo transistors are particular as to how they are connected. The emitter goes to the negative test lead, while the collector goes to the positive lead. The base is usually left open, although the sensitivity of the probe can be reduced by tying the base to the emitter through a resistor of perhaps l megOhm.

Liquid Level Indicators The "SaWhen"

Unless a liquid is absolutely devoid of ions, its conductivity can be detected with a continuity tester (either current-controlled oscillators or the Star Micronics buzzers). All that is needed is to fashion a pair of test leads that can be attached to the edge of the vessel.

A very simple way of making the sensor is to attach a couple of stiff wires (perhaps 14-gauge) to a small piece of perforated board. Arrange these wires so that their ends protrude beyond the end of the board, then bend them over so that these ends can be hooked over the edge of the cup. A cable can then be run from the board to the tester.

The Smith-Kettlewell "SaWhen" is a self- contained unit, including a 9-volt battery, which uses a Star Micronics buzzer (CMB-12). The buzzer is mounted over to one side on a 7/8 inch square piece of perforated board; this leaves a left-over l/4 inch margin along one side of the buzzer to which fang-like sensor wires can be attached. A 9-volt battery clip is cemented to the bottom of the board underneath the buzzer. The fangs are bent out and downward so that they can loop over the edge of the cup. Even though the components are solidly cemented together, the buzzer vibrates to such an extent as to make this unit usable by the deaf-blind.

Circuit for Smith-Kettlewell SaWhen

Pin l4 goes to the large battery snap (so as to fit on the positive battery terminal), while pin l goes to the small snap. The fangs go to pins 8 and l4.

The level indicator probes for the Science for the Blind Audicator are coaxial; an outer tube is one contact and a center wire is the other. A lengthwise saw cut slits the bottom of the tube part way up; this keeps liquid from becoming trapped in the probe, and also permits the liquid to serve as a "variable resistor" as it rises higher up the probe. Two lengths are available, the longer of which is handy for chemistry work and for mixing various levels of liquid in tall drinking glasses.

Rain Alarms

The more sensitive testers, current- controlled oscillators and the Star Micronics buzzer, can be used to alert you as to whether or not rain has fallen on a sensor. Moisture on the sensor creates a leakage path between two closely spaced conductors. Left outside and connected by a cable through the window to the tester, the "rain alarm" can alert you as to the presence of a drizzle. You may wish to include a normally-open pushbutton switch in series with the tester so that the system can be "interrogated" rather than going off by itself. The Star Micronics buzzer, if left unattached to a solid instrument housing, will vibrate so as to make it usable by the deaf-blind.

The rain sensor itself can be made by running several inches of bare wire as closely spaced parallel conductors on a piece of perforated board. The Science for the Blind attachment made for the Audicator is a fancy PC-board version having many interleaving foil traces; it is extremely sensitive. For those who want only to know about the torrential stuff, I have seen rain sensors made of heavy spring contacts held open by a sugar cube with these contacts used to control high- current buzzers. This latter approach is intended to let you know when to get out the sandbags and protect your wine cellar.


By Albert Alden


A hands-free directional compass with audible tone output is described. In contrast to locking-type braille compasses, this instrument provides the user with dynamic auditory feedback for determining direction and relative degree of veering from a desired path. It is based on the principle of the "Hall Effect," and a brief discussion of this phenomenon is also given.

Photograph of The Smith-Kettlewell Auditory Compass


A 9-inch sensor stick "Hall- Effect Transducer," is oriented so as to achieve a null of the auditory signal; this occurs when the stick is perpendicular to the earth's magnetic field; i.e., east-west. Rotating the sensor in one direction causes a "beep-beep" signal, while rotating it in the opposite direction produces a "ding-ding" sound. One end of the stick can then be arbitrarily marked with tape so as to assign this end with a particular direction (our unit "beeps" when the tape end approaches north).

This system has one disadvantage inasmuch as error is introduced if the stick is not level. This presents no problem near the equator. However, as one approaches either of the earth's magnetic poles (progressing toward increased latitude, north or south), the "dip" or angle of inclination of the earth's magnetic field becomes significant. An approximation of the resulting error can be expressed as follows: The error in the null direction that one can expect is approximately equal to the tangent of the angle of the "dip" times the angle to which the sensor is held off-level. For example, the angle of "dip" of the earth's magnetic field is approximately 62 degrees in San Francisco. The tangent of this angle is 1.88. For every 1 degree that the sensor is tilted off-level, the error in the null direction will be 1.88 degrees.

Where necessary, as in boating, the sensor should be mounted on gimbals and weighted so as to be kept level. For our unit, we fashioned a ball-and-socket arrangement, with a pendulum below the ball being kept in a solution of glycerine and water (to serve as damping), the sensor being mounted above the ball and above the container of glycerine. On the other hand, where random tilting of the sensor is not expected, such as when it is clipped to the user's belt, the error is not a problem; the sensor is arranged for a null in the audible signal, and veering will produce the same indications as if the sensor were held level.

The Hall Effect

In 1879, E.H. Hall discovered that applying a magnetic field across a current-carrying conductor causes a skewing of the charge distribution within the conductor. This results in an induced voltage appearing across the conductor which is mutually perpendicular to both the magnetic field and the direction of current flow.

To illustrate the above, picture a long, flat conductor carrying a current in its long direction, and being placed in a magnetic field so that the conductor's flat sides face the poles of the magnet. The Hall Voltage will appear between the edges of the conductor; electrons will be forced by the magnetic field to pile up on one edge, and positive charges will congregate along the other edge. Given a current "I" and a magnetic flux density "B," the Hall Voltage will be proportional to I times B.

Until recently, Hall-Effect devices (some- times called "Hall-Effect generators) were a laboratory curiosity, since metal conductors expressed extremely small Hall Voltages. Using semiconductor technology, Hall-Effect devices are now available which are thousands of times more sensitive (10 to 60 millivolts per kiloGauss).

{Editor's Note: Nowadays, sensitive and accurate clamp-on DC ammeters are becoming commonplace. They contain a Hall-Effect sensor in the probe which can measure the minute magnetic field around the conductor of interest. This makes breaking the circuit to measure its current unnecessary.}

Hall-Effect Sensor

The sensing device used for the compass is the H.W. Bell BH-850. (F.W. Bell, Inc., 6120 Hanging Moss Road, Orlando, Florida 32807; tel. (305) 678-6900.) It is about 9 inches long, l/4 inch thick, and about 1/2 inch wide. A thin Hall-Effect crystal is mounted on edge in the center of this bar, while iron pole pieces (said in the literature to concentrate the magnetic field) extend from the flat faces of the crystal to the ends of the bar.

Four wires are available, two for passing a current through the crystal in its long direction (red and black), and two for sampling the Hall Voltage (blue and yellow). {Editor's Note: Other than perhaps by trial and error, there will not be a way of determining these wires with test instruments. The crystal has a fairly low resistance in every which way-- about 4 ohms in its long direction--which makes using an ohmmeter for determining these leads impractical.}

The insulation on the wire leads is Teflon. This material is difficult or impossible to strip using conventional tools. Teflon wire strippers are available which have Nichrome heating elements in them to melt through the insulation. It is for this reason that cutting the already stripped and tinned wires is not advisable--they are best left as they are.

Circuit Operation

The current supplied to the Hall-Effect crystal is in the form of pulses, 10% on and 90% off. When in the presence of a magnetic field, a wave-form having the duty cycle of these proportions is available on the Hall Voltage wires, and the polarity of this signal is determined by the direction of the magnetic field. In other words, a zero-center voltmeter connected to the blue and yellow wires would show positive 10% pulses for a magnetic field of one direction and would show negative 10% pulses for a magnetic field in the opposite direction.

A differential amplifier (using pins l, 2, and 3 of the LM324 Quad Op-Amp) is used to detect this Hall Voltage--a balance control inone leg of the sensor takes care of any incidental offset voltages in the circuit and provides the user with a means by which the instrument can be "nulled" for an exact east- west orientation. The output of the differential amplifier is then fed into a stage having adjustable gain so as to provide the instrument with a sensitivity control (this second stage being pins 5, 6, and 7 on the 324).

At this point, the signal still consists of pulses, either up or down as determined by the magnetic field, with these pulses being referenced to a bias voltage VREF. This reference voltage is made up of an op-amp (pins l2, l3, and l4 of the 324) connected as a voltage follower looking at a voltage divider across the battery supply. Pulses from the above two stages are either above this reference 10% of the time or below it 10% of the time.

A scheme was then devised by which the lower portion of this wave form is always fixed at VREF. In other words, the desired signal either has positive-going pulses of 10% duty cycle or positive-going pulses of 90% duty cycle, the bottom of this wave always resting at VREF. To accomplish this, a diode clamp is used to hold the lower portion of the signal at VREF (this is an "active clamp" in which an op-amp, pins 8, 9, and l0 of the 324, is used to take up the slack of a diode's junction voltage). The signal into the clamp is capacitively coupled; when the signal tries to go negative, the clamp prevents its end of the coupling capacitor from doing so, thereby forcing the previous stage to charge this capacitor to the degree of negative swing. The result is that the output of the coupling capacitor (the input of the clamp) has on it the desired positive-going signal. The "holding time" of the capacitor was chosen to be just a bit short for the 90% time segment, so as to allow the long pulses to sag some- what; this gives the long beeps a bell-like quality.

The clamp-shifted positive-going signal is used to gate a MOS-FET, used as a variable resistor which intermittently couples a tone signal (from the second half of a CMOS-556) into an audio amplifier (an LM386). The first half of the 556 generates the l0% duty cycle pulses at about l.4Hz with these pulses then being applied to the sensor via a PNP switching transistor. The 4.5-volt battery supply, made up of 3 AA cells, is necessary because of the high current (160mA) fed to the sensor.

{The Editor would like to draw your attention to the novel circuit connection of the second half of the 556. In this arrangement, the charge resistor is being operated by the output; the charge on the RC circuit is being made to follow the output. The advantage of this connection is that it produces 50% duty cycle squarewaves.}

Circuit for the Smith-Kettlewell Auditory Compass

The blue and yellow output wires of the sensor (an F.W. Bell BH-850) each go through a voltage divider to ground, with the taps on these dividers going to the inputs of a differential amplifier. One output lead goes through 3.3 ohms, then through 16 ohms to ground. The other output leads goes through a 10-turn pot (used for "balancing"; not connected as a rheostat), then through 10 ohms to ground.

The junction between the 3.3 and 16-ohm resistors goes through 1K to pin 2, the inverting input of an LM324 quad op-amp. Between pins 2 and 1 is a 33K feedback resistor. The output of the second voltage divider, the wiper of the 10-turn pot, goes through 1K to pin 3, the non-inverting input, with pin 3 also going through 33K to a reference voltage, VREF. Pin l, the output, goes through 6.8uF (positive toward pin l), then through 390K to VREF.

The junction of the 6.8uF and 390K goes to pin 5, the non-inverting input, of the next stage. Pin 6, the inverting input, goes through 3K to VREF, with pin 6 also going through a 100K rheostat (sensitivity control) to pin 7, the output. Pin 7 also goes through luF (negative at pin 7), then through 5l0K to VREF. (This luF capacitor is the coupling unit into the clamp circuit discussed in the text.)

The junction of the luF and 5l0K goes to pin 9, the inverting input, of the op-amp associated with the clamp. As its feedback circuit, this op-amp has a diode connected between its output and inverting input; pin 8 goes through the diode to pin 9 (anode at pin 8). Pin l0, the non-inverting input, is tied directly to VREF. The output signal is not taken from the output of this op-amp, but comes from the junction of the luF and 5l0K which also goes to pin 9.

The fourth op-amp in the 324 package is used to make VREF. Pin l2, the non-inverting input, goes through 15K to VCC (4.5 volts), with pin l2 also going through the parallel combination of 7.5K and luF to ground (negative of the capacitor at ground). (This fixes the non-inverting input at 1/3 VCC.) The output is tied to the inverting input to make this op-amp a "voltage follower"--pin l4 is tied to pin l3--and pin l4 is then used as VREF.

Pin 4 of the 324 goes to VCC, while pin 11 is grounded. Pin 7 of an Intersil ICM7556 is also grounded, while pins l4, 4, and 10 (the VCC and Enable pins) go to VCC. The negative side of the battery (4.5 volts made up of 3 AA penlight cells in series) is grounded. The positive side of this battery goes through an on-off switch to the VCC line, this VCC line being bypassed to ground by 6.8uF (negative at ground). (This bypass should be located near the LM386 amplifier.)

On the first half of the 7556, pin 1 (Discharge) goes through 560K to VCC, and through 75K to pins 2 and 6 (Threshold and Trigger) which are tied together. Pins 2 and 6 also go through luF to ground (negative at ground). Pin 5, the output, goes through l.lK to the base of a PNP power transistor (MJE- 2955), with the emitter of this transistor going to VCC. Its collector goes through 22 ohms to the red lead of the sensor, while the black sensor wire is grounded.

The second half of the 7556, the tone generator, is free-running constantly. Pins 8 and l2 (Trigger and Threshold) are tied together and go through .0luF to ground. Pins 8 and l2 also go through a 47K charging resistor to the output, pin 9. Pin 9 also goes through l2K to the top of a 5K volume control, with the bottom of this control being grounded. To take some of the edge off the audible tone, this control is shunted by a .047uF capacitor.

The wiper of the volume control goes through .luF to the source of a MOS/FET (2N6660), with the drain going through 470 ohms to ground. This drain also goes to pin 3 of an LM386 amplifier. Pins 2 and 4 are grounded, while pin 6 goes to VCC. Pin 7 is bypassed to ground by luF (negative at ground). Pin 5 goes through 33uF (positive at pin 5), then through the speaker to ground. To suppress oscillations, pin 5 also goes through 20 ohms in series with .02uF to ground.

The gate of the MOS/FET goes to the output of the active clamp; i.e., to pin 9 of the 324 and to the junction of the luF capacitor and the 5l0K resistor.

Calibration Adjustments

The balance control in series with the output lead of the sensor (the l0-ohm, l0-turn pot) is adjusted by laying the sensor stick on a level surface away from magnetic materials and man-made fields and noting the relative direction of the two null positions as the stick is turned. The balance adjustment is made so that these two nulls occur exactly l80 degrees apart.

The sensitivity control (the l00K rheostat) can be adjusted to suit the user's taste, given the instrument's application. For example, this sensitivity shall have to be turned down from maximum if the compass is to be used while walking, since the natural rotation of the body will prevent the user from maintaining an absolute null. In effect, the sensitivity adjustment "broadens the null," determining the off-null angle at which the instrument saturates, after which no further rotation will increase the amplitude of the signal. At its least sensitive, the device will saturate for off-null angles of about 20 degrees.

Parts List

Resistors fixed 1/2 W 5%

Resistors fixed 1/4 W 5%

Resistors adjustable


Diodes and Transistors

Integrated Circuits



The following tips on cassette repair were sent along to us by Jim Gibbons, WA2FVQ, of Colonia, New Jersey.

A few days before receiving your magazine containing the article on cassette repair, I was talking to a friend of mine on the radio, and he explained a method of reattaching the tape directly to the take-up reel without splicing. I didn't see this covered in the article, so I thought I'd pass it along for what it's worth.

If you look at an empty cassette reel, you will find two small slits around the circumference about l/4" apart. This l/4" section is removed--it slides out perpendicular to the circumference. The end of the leader is then stretched or placed across the resulting gap, and the small section of the reel is replaced by pressing it against the leader at the gap until it snaps back into position. There may be a free end of leader sticking out, but this can be trimmed off with scissors or with a razor blade. I tried this technique with some scrap I had laying around, and was able to attach the reel in a relatively short time.

I might also add that some of the cheaper, paper-boxed cassettes have plastic inserts between the two reels to keep them from rolling freely. I find it handy, when rethreading the tape, to use these inserts to lock the reels until the tape is passed through all of those crazy pulleys and pressure pad assembly. You can then simply replace the top half of the cassette, screw it all back together, and then remove the insert.

Thank you, Jim Gibbons.

Paul Stebbins of Millbrae, California, assures me that he has consistently good luck with the Edi-Tabs (either from Nortronics or 3M). The handle of the tab can be lined up with the track in the editing block, thus assuring that the splice will be straight with the track. Mr. Stebbins also suggests that the tape ends can be joined by leaving the razor blade in the cutting slot and bringing the tape ends up against the sides of the blade--remove the blade and butt the tape ends together.

Thank you, Paul Stebbins.

Daveed Mandell of Los Angeles suggests that the editing point can be marked by pinching the tape sharply. Once transferred to the block, this crease can be felt.

Thank you, Daveed Mandell.


The following books are listed in the l981/82 Supplement to the catalog from Recording for the Blind, Inc., 215 East 58th Street, New York, NY 10022, under the heading "Engineering."

Alerich, Walter N.:

American Radio Relay League:

Caristi, Anthony J.:

"Electronic Telephone Projects." Howard W. Sams, Cl979.

Cooke, Nelson M. & Herbert F.R. Adams:

"Arithmetic Review for Electronics." McGraw-Hill, Cl968.

Del Toro, Vincent:

"Electromechanical Devices for Energy Conversion and Control Systems." Prentice- Hall, Electrical Engineering S.E., Prentice- Hall, Cl968.

Diefenderfer, A. James:

"Principles of Electronic Instrumentation," 2nd Ed., W.B. Saunders, Cl979.

Forier, Louis C. et al (Editors):

"Motor Auto Engines and Electrical Systems." 7th Motor, Cl977.

Friedman, Arthur D.:

"Logical Design of Digital Systems" (Digital Systems Design Series). Computer Science Press, Cl975.

Gajda, Walter J. & William E. Viles:

"Engineering: Modeling and Computation." Houghton Mifflin, Cl978.

Gerrish, Howard H. & W. E. Dugger, Jr.:

"Transistor Electronics: Basic Instruction in Electricity and Electronics." Goodheart- Willcox, Cl979.

Gilli, Angelo C.:

"Electrical Principles for Electronics," 3rd Ed. McGraw-Hill, Cl978.

Gorsline, G. W.:

"Computer Organization: Hardware/Software." Prentice-Hall, Cl980.

Greco, J.:

"Combinational and Sequential Circuits: Analysis and Design." J. Greco, C.

Gustafson, Robert J.:

"Fundamentals of Electricity for Agriculture." AVI Pub., Cl980.

Hayes, John P.:

"Computer Architecture and Organization" (McGraw-Hill Computer Science Series). McGraw-Hill, Cl978.

Herrick, Clyde N.:

"Instruments and Measurements for Electronics." McGraw-Hill, Cl972.

Kelly, Anthony J.:

"Electricity" (Parts I, II, III). Center for Degree Studies, Cl970.

Kyle, James:

"Electronics Unravelled. A New Commonsense Approach." TAB Books, Cl974.

Langley, B.C.:

"Electric Controls for Refrigeration and Air Conditioning." Prentice-Hall, Cl974.

Lemons, Wayne:

"Transistor Radio Servicing Course," 2nd Ed. Howard W. Sams, Cl977.

Lurch, E. Norman:

"Electric Circuit Fundamentals." Prentice- Hall, Cl979.

McPartland, J.F. & J.F. McPartland III (Eds.):

"McGraw-Hill National Electrical Code Handbook." McGraw-Hill, Cl979.

Malvino, Albert Paul:

Malvino, Albert & Donald P. Leach:

"Digital Principles and Applications," 2nd Ed. McGraw-Hill, Cl975.

Master Publications:

"Repair Master for Electric Ranges and Controls." Barnee Schollnick, Ed., Master Publications, Cl978.

Mileaf, Harry (Ed.):

"Electricity One-Seven" (Hayden Electricity One-Seven Series). Hayden Books, Cl966.

Mullin, Ray C. & Robert L. Smith:

"Electrical Wiring, Commercial: Code Theory, Plans, Specifications, Installation Methods. Based on l978 National Electrical Code." Delmar, Cl978.

Noll, Edward M.:

"73 Dipole and Long Wire Antennas." Editors and Engineers, Cl969.

Orr, William I.:

"Radio Handbook." Editors and Engineers, Cl978.

Orr, William I. & Stuart D. Cowan:

Pike, Charles A.:

"Transistor Fundamentals. Book I, Volume II: Basic Transistor Circuits." Howard W. Sams, Cl968.

Rieger, K.:

Roth, Charles H., Jr.:

"Fundamentals of Logic Design," 2nd Ed. West, Cl979.

Schwartz, Martin:

"Amateur Radio Novice Class Theory Course." AMECO Pub., Cl977.

Smith, Robert L.:

"Electrical Wiring, Industrial: Codes, Theory, Plans, Specifications, Installation Methods." Delmar, Cl978.

Taber, Margaret R. & Eugene N. Silgalis:

"Electric Circuit Analysis." Houghton Mifflin, Cl980.

Technical Education Research Center:

"Course V: Light Sources and Wave Optics." C.O.R.D., Cl980.

Temes, Lloyd:

"Schaum's Outline of Theory and Problems of Electronic Communication" (Schaum's Outline Series). McGraw-Hill, Cl979.

Trejo, Paul E.:

Wiatrowski, Claud E. & Charles H. House:

"Logic Circuits and Microcomputer Systems" (McGraw-Hill Series on Electrical Engineering). McGraw-Hill, Cl980.

Williams, Gerald E.:

"Digital Technology Laboratory Manual." Science Research Associates, Cl977.

Zbar, Paul B.:

The following are taken from the l980/8l Supplement to the same catalog, also under the heading "Engineering."

American Radio Relay League:

Audio Engineering Society:

"Loudspeakers: An Anthology of Articles on Loudspeakers from the Pages of the Journal of the Audio Engineering Society," Vol. I to Vol. XXV (l953-l977). A.E.S., Cl978.

Babb, Daniel S.:

"Resistive Circuits." International Textbook, Cl968.

Buck Engineering Company, Inc.:

"Introduction to Electricity and Electronics," Instructor's Ed. Buck Engineering, Cl974.

Burke, W.E., et al (Editors):

"This is Electronics. Book I: Basic Principles." Howard W. Sams, Cl970.

Carr, Joseph S.:

"Elements of Electronic Communication." Reston, Cl978.

Comer, David J.:

"Modern Electronic Circuit Design" (Addison- Westley Series on Electrical Engineering). Addison-Westley, Cl976.

Curtis, Anthony R. & Judith G. Curtis (Eds.):

D'azzo, John J.:

"Linear Control System Analysis and Design: Conventional and Modern" (McGraw-Hill Electrical and Electronic Engineering Series). McGraw-Hill, Cl975.

Deem, Bill R.:

"Digital Computer Circuits and Concepts," 2nd Ed. Reston, Cl977.

Edminister, Joseph A.:

"Schaum's Outline of Theory and Problems of Electric Circuits" (Schaum's Outline Series). McGraw-Hill, Cl965.

Garland, J.D.:

Gerrish, Howard H.:

"Learning Experiences in Electronics: Teaches Modern Concepts," 2nd Ed. Buck Engineering Company (Lab-Volt Educational Systems), Cl967.

Grob, Bernard:

Ham Radio Publishing Group:

"The Golden Years of Radio: Amateur Radio Comes of Age." Ham Radio Publishing Group, Cl978.

Harfenist, Sylvan:

"Refrigeration License Manual: Complete Test Preparation for the Written and Practical Examination," 2nd Ed. ARCL, Cl975.

Hayt, William H., Jr. & Jack E. Kemmerly:

"Engineering Circuit Analysis," 3rd Ed. McGraw-Hill, Cl978.

Johnson, Kenneth W. & Willard C. Walker:

"The Science of High Fidelity." Kendall- Hunt, Cl977.

Kubala, Thomas S.:

"Practical Problems in Mathematics for Electricians." Delmar, Cl973.

Larson, Boyd:

"Transistor Fundamentals and Servicing." Prentice-Hall, Cl974.

Layne, Ken (Editor):

Loper, Orla E. & Arthur S. Ahr:

"Introduction to Electricity and Electronics." Delmar, Cl973.

McLaughlin, Terence:

"Make Your Own Electricity." David & Charles, Cl977.

Malvino, Albert Paul & Gregory F. Johnson:

"Experiments for Electronic Principles: A Laboratory Manual for Use with 'Electronic Principles'." McGraw-Hill, Cl973.

Marcus, William & Alex Levy:

"Elements in Radio Servicing," 3rd Ed. Webster Div., McGraw-Hill, Cl967.

Mix, Floyd M.:

"House Wiring Simplified: Tells and Shows You How." Goodheart-Willcox, Cl977.

Mullin, Ray C.:

"Electrical Wiring, Residential Code: Theory, Plans, Specifications, Installation Methods," 5th Ed. l975 Code. Delmar, Cl975.

Nathanson, Fred E.:

"Radar Design Principles: Signal Processing and the Environment." McGraw-Hill, Cl969.

National Fire Protection Association:

"National Electrical Code," l978 Ed. with "Tentative Interim Amendment to the l978 National Electrical Code." Cl977 & l978.

Philco-Ford Education Operations:

"Electronic Circuits and Systems; Vol. V: Advanced Electronic Circuit Technology." Philco-Ford Corp., Cl960.

Philco-Ford Technical Education Program:

"Electronic and Electrical Fundamentals; Vol. VIII: Student's Laboratory Manual for Vacuum Tube and Semiconductor Fundamentals," Rev. Ed. Philco-Ford Corp., Cl96l.

Philco Tech Rep Div., Technical Dept.:

Ruiz, J.:

"Color TV: Theory and Servicing." Data Design Laboratories, Cl972.

Rutkowski, George B.:

"Solid-State Electronics." Howard W. Sams, Cl972.

Schumacher, Alice Clink:

"Hiram Percy Maxim: Father of Amateur Radio, Car Builder, and Inventor." Ham Radio Publishing Group, Cl970.

Schwartz, Leland P.:

"Survey of Electronics," 2nd Ed. (Merrill International Series on Electrical and Electronics Technology) Charles E. Merrill, Cl977.

Schwartz, Martin:

Schwartz, Martin & John Kenneally:

"Advanced Class Radio Amateur License Guide." AMECO, Cl979.

Scott, Ronald E.:

"Linear Circuits, Part II: Frequency Domain Analysis." With the editorial assistance of Martin W. Essigmann. (Addison-Wesley Series in the Engineering Sciences) Addison-Wesley, Cl960.

Sears, Roebuck & Co.:

"Simplified Electric Wiring Handbook." Sears, Roebuck, Cl960.

73 Magazine Staff, Ed.:

Shiers, George:

"Electronic Drafting." Prentice-Hall, Cl962.

Slurzberg, Morris & William Osterheld:

"Essentials of Electricity & Electronics," 3rd Ed. McGraw-Hill, Cl965.

Tocci, Ronald J.:

"Fundamentals of Pulse and Digital Circuits," 2nd Ed. Charles E. Merrill, Cl977.

Villanucci, Robert S., et al:

"Electronic Techniques: Shop Practices and Construction." Prentice-Hall, Cl974.

Woram, John M.:

"The Recording Studio Handbook," with an introduction by Norman H. Crowhurst. Sagamore, Cl977.

Woram, John M.:

"The Recording Studio Handbook," with an introduction by Norman H. Crowhurst. Sagamore, Cl977. E. Merrill, Cl977.

Villanucci, Robert S., et al:

"Electronic Techniques: Shop Practices and Construction." Prentice-Ha.


Cranmer-Modified Perkins Brailler as Computer Terminal

The following announcement comes from the Kentucky Bureau for the Blind, State Office Building Annex, Frankfort, KY 40601:

We are pleased to announce the completion of a documentation package describing in complete detail the procedure for modifying the Perkins Braille Writer so that it will function as an automatic page embosser and computer terminal. {Please note that the units themselves are not for sale from the Kentucky Bureau.}

As a braille printer, the Kentucky Modified Perkins may be used as an output device for a timesharing computer, Versa- Braille machine, or most other sources of electronic data using the ASCII code, and the RS232C interface. The printer runs at approximately 10 characters per second and embosses as the carriage moves in both directions.

As a terminal, the Kentucky Modified Perkins operates in half-duplex, full-duplex, at a variety of baud rates, etc.

The documentation consists of approximately 36 pages of narrative, three appendices, parts list, etc., several pages of electronic and mechanical drawings, and photographs.

If you are interested in obtaining this documentation, please send a check in the amount of $10 to the Technical Services Unit of the Bureau for the Blind, P.O. Box 758, Frankfort, KY 40602. The check should be made payable to "Kentucky State Treasurer"; on the "for" designation line, please put "Bureau for the Blind."

If you have a question on this material, please telephone us at 502-564-4754.

Smith-Kettlewell Training Program

We now offer a training program in order for blind students to learn techniques of assembly, such as parts layout, soldering, and the mounting of hardware. This program does not include a regiment of technical study; however, it is perhaps the ideal supplement to one's formal education in electronics and the pursuit of one's hobby interests.

This training is free on a first-come, first-served basis. The student determines his own goals, work schedule, and length of stay. No sponsorship from a rehabilitation agency is necessary; however, living arrangements and accommodations other than use of our laboratory are not provided by us. Because of its informal structure, certification upon completion of the "course" can go only so far as to evaluate the student's performance upon request.

For more information, write or call the Training Program Manager, Jay Williams, Smith- Kettlewell Institute of Visual Sciences, 2232 Webster Street, San Francisco, CA 94ll5; tel: 561-1677.


What's to come? There are a few projects I promised you which have not found their way into these pages -- our light probe, our oscilloscope, etc. There is a function generator chip (Intersil ICL8038) which we use in our low-frequency tape indexer, various Morse Code devices, and which can be made into a full-fledged sine/square/triangle wave generator. Alas, your often-requested, long-awaited discussion of op-amps will be forthcoming.

We are currently experimenting with the idea of providing printed circuit kit sets for some of our more popular projects. Included with the circuit boards will be Thermoformed models of parts layout for stuffing the boards. Our prototype kid, the Auditory Meter Reader, was successfully assembled by three blind subjects other than myself, suggesting that our system shows promise. You will be the first to know how this project is getting on, giving you ample opportunity to be the first on your block to assemble a kit accessible to the blind.

As a matter of reflection, I decided to do a statistical breakdown of the material contained in the preceding eight issues and compare this with my impression of your plaudits and suggestions. These statistics appear as follows:

(Note that these percentages do not add up to 100%; this is true because the categories are not mutually exclusive. For example, the LM386 project in "Point-to-Point" of Fall 1980 is included in both "Projects" and "Techniques." Also, the discussions of chips in the digital articles were put in the "Component Information" category as well as being counted under "Technical Discussions.")

To me, the above figures suggest a well-balanced magazine which, as stated in the Preface, seeks to close the gaps which stand in the way of blind people pursuing their interest. Your overall positive response to the Technical File confirms its usefulness. Plans are afoot to formally evaluate the Technical File by way of a questionnaire; this will not be conducted by me, but by our Human Factors Evaluation Staff.

To me, the most impressive category is the last one -- 26% of our material came from you. Not only do I appreciate your good work, but this demonstrates how effective the "forum" of the Technical File has been. Smith-Kettlewell Institute is a well-known establishment throughout the world, and yet with all our work in rehabilitation and engineering, we benefit greatly and cannot do without your interaction. This speaks well for the power of the consumer, and we are honored to have your active participation.

As for your Editor, these past two years have been the most fulfilling and rewarding of my professional career. As the Yuletide and New Year festivities progress, I wish for you the same degree of joy and fulfillment that it has been my pleasure to gain from your inspiration.

P.S. Subscription time is imminent. For the vast majority of you, the 1983 "Volume 4" subscription is now due. Until March 1, the subscription price will stay the same, after which the hard copy issues will be $15 per year and the tape version will be $8 per year. In order to save your money, please hurry. (If your subscription is not due to expire with this issue, a code to that effect appears on your mailing label.)