TABLE OF CONTENTS

CONTINUITY TESTER CIRCUITS--OLD & NEW

CONTINUITY TESTER USES

THE SMITH-KETTLEWELL AUDITORY COMPASS

HINTS AND KINKS

LEXICOGRAPHY

BULLETIN BOARD

EDITOR'S CRYSTAL BALL

CONTINUITY TESTER CIRCUITS--OLD & NEW

Abstract

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.

CONTINUITY TESTER USES

Abstract

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.

THE SMITH-KETTLEWELL AUDITORY COMPASS

By Albert Alden

Abstract

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

Description

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%

• 1 - 22 ohm

Resistors fixed 1/4 W 5%

• 1 - 3.3 ohm
• 1 - 10 ohm
• 1 - 16 ohm
• 1 - 20 ohm
• 1 - 470 ohm
• 2 - 1K
• 1 - 1.1K
• 1 - 3K
• 1 - 7.5K
• 1 - 12K
• 1 - 15K
• 2 - 33K
• 1 - 47K
• 1 - 75K
• 1 - 390K
• 1 - 510K
• 1 - 560K

Resistors adjustable

• 1 - 10 ohm 10 turn
• 1 - 5K
• 1 - 100K

Capacitors

• 1 - .01uF
• 1 - .02uF
• 1 - .047uF
• 1 - .1uF
• 4 - 1uF
• 2 - 6.8uF
• 1 - 33uF

Diodes and Transistors

• 1 - 1N4148
• 1 - 2N6660 transistor, Siliconix VN67AF,
  Radio Shack 276-2071
• 1 - Motorola MJE 2955, Texas Instruments
  TIP32, Radio Shack 276-2025

Integrated Circuits

• 1 - ICM7556
• 1 - LM324
• 1 - LM386

Miscellaneous

• 1 - Hall Effect Generator--
  F.W. Bell BH-850
• 1 - Loudspeaker
• 3 - AA alkaline batteries

HINTS AND KINKS

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.

LEXICOGRAPHY

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.:

• "Electric Motor Control." Delmar, Cl975.
• "Electricity III: Motors, Generators,
  Controls." Delmar, Cl974.

American Radio Relay League:

• "The ARRL Antenna Anthology" (by Marian S.
  Anderson). American Radio Relay League,
  Cl978.
• "The ARRL Antenna Book." ARRL, Cl974.
• "The Radio Amateur's Handbook." ARRL, Cl980.
• "The Radio Amateur's License Manual." Ed.
  by Wiland, Dale, Clift, ARRL, Cl98l.

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:

• "Electronic Principles," 2nd Ed., McGraw-
  Hill, Cl979.
• "Resistive and Reactive Circuits." McGraw-
  Hill, Cl974.

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:

• "All About Cubical Quad Antennas" (l978),
  2nd Ed. Radio Publications, Cl959.
• "Beam Antenna Handbook." 5th Radio
  Publication, Cl976.
• "Simple, Low-Cost Wire Antennas for Radio
  Amateurs." Radio Publications, Cl972.

Pike, Charles A.:

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

Rieger, K.:

• "Electrical Schematic Diagrams, Part I," 3rd
  Ed. International Textbook, Cl973.
• "Electrical Schematic Diagrams, Part II,"
  2nd Ed. International Textbook, Cl973.

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.:

• "AC Circuits: An Individualized Approach
  to Electronics." Cambridge Books, Cl972.
• "DC Circuits: An Individualized Approach
  to Electronics." Cambridge Books, Cl972.

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.:

• "Basic Electronics: A Text-Lab Manual,"
  4th Ed. Gregg Div., McGraw-Hill, Cl976.
• "Electronic Instruments and Measurements:
  Laboratory Manual for Electronic
  Technicians." McGraw-Hill, Cl965.

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

American Radio Relay League:

• "ARRL Ham Radio Operating Guide." ARRL,

  Cl976.

• "Specialized Communications Techniques for
  the Radio Amateur." ARRL, Cl975.

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.):

• "Tune In the World With Ham Radio,"
  2nd Ed. ARRL, Cl976.
• "Tune In the World With Ham Radio, Student Workbook, ARRL, Cl978.

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.:

• "National Electrical Code Reference Book,
  Based on the l978 Code," 2nd Ed. Prentice-
  Hall, Cl979.
• "l978 National Electrical Code Questions
  and Answers." Prentice-Hall, Cl979.

Gerrish, Howard H.:

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

Grob, Bernard:

• "Basic Electronics," 3rd Ed. McGraw-Hill,
  Cl97l.
• "Basic Television: Principles and
  Servicing," 4th Ed. McGraw-Hill, Cl975.

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):

• "Automotive Electrical Systems: Classroom
  Manual." Canfield Press, Cl978.
• "Automotive Electrical Systems: Shop
  Manual." Canfield Press, Cl978.

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.:

• "Electronic and Electrical Fundamentals;
  Vol. I: Basic Concepts in D/C Circuits."
  Philco Corporation, Cl960.
• "Electronic and Electrical Fundamentals;
  Vol. III: Vacuum Tube and Semiconductor
  Fundamentals." Philco Corporation, Cl960.

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:

• "'AMECO' Extra Class Radio Amateur License
  Guide." AMECO, Cl979.
• "General Class Radio Amateur License
  Guide." AMECO, Cl979.

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.:

• "Amateur Radio Extra Class License Study
  Guide." TAB Books, Cl970.
• "73 General Class License Study Guide."
  73, Inc., Cl978.

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.

BULLETIN BOARD

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.

EDITOR'S CRYSTAL BALL

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:

• Projects, general; 31%
• Projects, lab bench equipment; 26%
• Projects, orientation and recreation; 5%
• Component and device information; 18%
• Technical discussions; 7%
• Articles contributed by readers; 26%

(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.)