The Smith-Kettlewell Technical File

A Quarterly Publication of
The Smith-Kettlewell Eye Research Institute’s
Rehabilitation Engineering Research Center

William Gerrey, Editor

Issue: [current-page:title]

Original support provided by:
The Smith-Kettlewell Eye Research Institute
and the National Institute on Disability and Rehabilitation Research

Note: This archive is provided as a historical resource. Details regarding products, suppliers, and other contact information are original and may be outdated.

Questions about this archive can be sent to


Operational Amplifiers

The Smith-Kettlewell Active Light Probe

The Smith-Kettlewell Receptionist Mat--An Introvert Detector

The Handling and Storage of
Magnetic Recording Tape

Hints and Kinks


Editor's Corner


By Albert Alden


This is the first in a series of articles
on the subject of operational amplifiers (Op
Amps). This article will describe the ideal
errorless amplifier and give some applications. Future articles will treat the limitations of practical amplifiers, DC errors,
dynamic characteristics, and additional


The Op Amp is a high-gain differential
amplifier circuit (these days almost exclusively an integrated circuit) with two input
terminals, two power terminals, and an output
plus a couple of other connections to be
described later. The symbol for an Op Amp is
a triangle, usually drawn with the base vertically on the left. The input terminals are
on the base with the one labeled "-" above
the one labeled "+". The output is at the
apex on the right. A positive input signal
on the + terminal with reference to the -
terminal will produce a positive output
signal and vice versa. We shall henceforth
call the + terminal the non-inverting input
and the - terminal the inverting input. Two
power supplies are usually used, plus and
minus l5 volts being a typical value. It
should be noted that no power ground connection is made to the Op Amp. Ground (i.e.,
the common connection of the plus and minus
supplies) is used as the reference for the
input and output signals.

The properties of our ideal Op Amp are:

  • Gain = Infinite
  • Input Impedance = Infinite
  • Bandwidth = Infinite
  • Output Impedance = Zero
  • Input Bias Current = Zero
  • Input Offset Voltage = Zero
  • Common Mode Rejection = Infinite

The consequences of each of the above are
as follows.

  • Gain = Infinite. This means that any
    signal imposed between the input terminals
    gives a very large output signal, or conversely it takes an infinitely small signal
    to produce any output signal.
  • Input Impedance = Infinite. This means
    that changing the signal at the input terminals does not result in a change of the
    current flowing into the input terminals.
  • Bandwidth = Infinite. Infinite frequency
    response or the output will change instantly
    for an instant change in the input.
  • Output Impedance = Zero. This means that
    we can change the load on the output without
    the output itself changing.
  • Input Bias Current = Zero. This means no
    current flows in or out of the input terminals.
  • Input Offset Voltage = Zero. This means
    that if the input terminals are shorted together and connected to ground, the output
    signal will be zero volts with respect to
  • Common Mode Rejection = Infinite. This
    means that the shorted input terminal described above may be connected to any voltage,
    and the output signal will remain at zero

Although you can't buy an Op Amp with the
parameters described above, analysis based on
them is usually adequate for an understanding
of the working of an Op Amp circuit.

In the majority of applications of Op Amps,
negative feedback is employed. The feedback
is realized by the connection of a resultor
from the output back to the inverting input.
This is called the feedback or output resistor. With the non-inverting input grounded,
any deviation of the output from ground will
feed back through the resistor a signal into

the inverting input with the proper polarity
to cause the output the correct to zero. Or
conversely the feedback forces the signal at
the inverting input to be at ground. This is
called a virtual ground: it is at ground
potential but not connected to ground.

With the virtual ground concept and the
zero input bias current feature of the ideal
Op Amp, we can describe our first application.

The Inverting Amplifier

The circuit of the inverting amplifier is
as follows. There is a feedback or output
resistor (Ro) from the output to the
inverting input as described above. The non-
inverting input is grounded. An input
resistor (Ri) is connected to the inverting

input; the other end is the input terminal of
our circuit.

If we apply a positive signal (Ei) to the
input resistor, a current I = Ei/Ri will
flow. Remember the other end of Ri is at
virtual ground and therefore the voltage
across it is the input signal. Since no
input current flows into the input terminals,
it must flow through the output resistor.
The negative feedback and infinite gain of
the Op Amp forces the output voltage to be
exactly the correct value to "accept" this
current; that is, Eo = -IRo. Notice the minus sign. From the above two equations we get Eo = -Ei (Ro/Ri). The
relationship between the output and input is
a function only of the ratio of the values of
output and input resistors (with a negative

A good way to think of the inverting
amplifier is to use the analogy of the lever.
The fulcrum is the virtual ground. The
lengths of each end are proportional to the
value of the resistors. Lifting the input
end (a positive voltage) causes the lever to
rotate about the fulcrum (virtual ground) and
the output end to go down (a negative
voltage). The longer the output end, the
greater the output movement (larger gain).
The infinite gain in the amplifier
corresponds to a rigid fulcrum in our

An extension of the inverting amplifier
circuit is the summing amplifier. To the
inverting amplifier circuit we can add any
number of additional input resistors
connected to the inverting input of the Op
Amp. The free ends of these resistors
constitute new inputs to the amplifier. Each
resistor contributes a current In = Ein/Rin
that passes through the output resistor and
thus adds to the output signal. The general
equation is

Eo = -{Ei1 (Ro/Ri1) + Ei2 (Ro/Ri2) + Ei3 (Ro/Ri3) + ...}

The contribution of each input is
determined by the ratio of the output
resistor to its input resistor. Note,
however, that the gain of all inputs is
controlled by Ro. This ability to
adjust individual gains and overall gain is
very useful.

The Non-Inverting Amplifier

We shall now describe the non-inverting
amplifier. This circuit retains the output
resistor Ro, connected from the output back to the inverting input. The input resistor Ri is
connected to ground. The non-inverting input
is not ground but becomes the input to the
circuit. The output resistor connection and

the infinite gain of the amplifier causes the
voltage at the inverting input to be
identical to that at the non-inverting input.
There is no virtual ground because the non-
inverting input is not grounded, but there is
a virtual signal input voltage at the
inverting input. This virtual signal
Ei (equal to the actual input signal) causes
a current I = Ei/Ri to flow through Ri to
ground. This current comes from the output
of the Op Amp. The output must be such that Eo = I (Ro + Ri). Substituting Ei/Ri for the current I, we
get Eo = Ei ((Ro + Ri)/Ri) or Eo = Ei ((Ro/Ri) + 1).

Notice two differences between this and the
inverting amplifier equation:

  1. there is no negative sign.
  2. there is a +1 appended to the resistor ratio.

If Ro is shorted and Ri eliminated, the
gain is unity, and we have a "follower."

Another way of looking at the non-inverting
amplifier is as a backwards attenuator. The
output and input resistors are connected as
an attenuator where the output of the
amplifier must adjust itself to give a
voltage at the resistor junction (inverting
input) equal to the input signal (non-
inverting input). Writing the equation for
an attenuator made from Ro and Ri and
swapping Eo and Ei in the equation gives
the equation for our non-inverting amplifier.

In our lever analogy, one end of the lever
is fastened to the earth, the input is the
fulcrum, and the output is the free end of
the lever. As before, lengths of the lever
correspond to resistor values.

Input impedance of Inverting and Non-inverting Amplifier

The input impedance of the
inverting circuit is equal to Ri, while that
of the non-inverting is equal to the input impedance of the Op Amp itself. This is infinite for our ideal device.

The difference amplifier

As before, the output goes through Ro to the inverting input, while Ri goes from this inverting input to one input signal. On the other hand, the non-inverting input goes through a resistor to ground (equal to Ro), while this non-inverting input also goes through a resistor equal to Ri to another input signal.

In other words, we are simply adding a resistor equal to Ro from the non-inverting input to ground plus a resistor equal to Ri connected to the non-inverting input. There are now two input points and the output is
equal to the difference of these two signals
times Ro/Ri or Eo = (Ei1 - Ei2 (Ro/Ri). This formula may be derived by shorting one
input, calculating the output due to a signal
on the other input, repeating for the
opposite case, and summing the two
expressions. This is possible due to the law
of superposition.

The derivation is left to the reader.
Hint: One situation is treated like the
inverting amplifier and the other like the
non-inverting amplifier, but with the signal
being attenuated before getting to the non-
inverting input to be amplified by ((Ro/Ri) + 1).

741 op-amp

In order to try some of these circuits,
here are the pin connections for the popular
74l op-amp.

  • 1 - offset null
  • 2 - inverting input
  • 3 - non-inverting input
  • 4 - -V (-5 to -18 volts)
  • 5 - offset null
  • 6 - output
  • 7 - +V (+5 to +18 volts)
  • 8 - N.C.

The offset null connections are not used in
this discussion. As mentioned previously,
there is no power ground connection. The
common connection between the two power
supplies is the input and output signal

Resistor values in the 3.3K to 47K range
should be used as starters for experimentation.

Next issue: More applications and the real



The described unit is a pocket-
sized light probe with a self-contained light
source. It can be used as a "passive" detector of external light sources, or it can
"actively" detect reflections of its own
internal source. The latter mode permits it
to be used as a print detector and for reading gauge pointers behind a glass face. The
unit has a sensitivity control, allowing it
to be adjusted for a wide variety of ambient
light conditions. Although it is commercially
available, the circuit has such versatility
as to afford "special purpose" probes to be
constructed, thereby enhancing its value for
typists and experimenters. Modifications are
included so that the circuit will provide
sufficient power for tactile output, thus
making it usable by the deaf-blind.

A Sketchy Background--
The Editor's Perspective

Somewhere (where I can no longer find it)
is the mention of a Polish inventor who, in
1879, devised a light detector for use by the
blind as a mobility aid (as I remember, his
name was something like Noisiewski). Your
Editor first got his little hands on a "light

probe" at the ripe age of 4 years. In 1951
my father, who was also blind, contracted
with a local "inventor" to build a detector
which he could use in turning off the various
lights and displays in his piano store--one
which he could also try as an aid to mobility.
This instrument was a neon relaxation oscilla-
tor controlled by a selenium photoresistor.
This piece of handiwork cost him $35, the
crime of which is alone enough motivation to
run a technical magazine to reduce the number
of such instances.

By the late l950's, the work of Swail and
Gunderson, as well as developments at the
Royal National Institute for the Blind in
England, brought forth a number of hand-held
probes "containing even the whole battery!"
Throughout the 23-year course of the Braille
Technical Press, several units containing
their own light sources were described by
Gunderson, Swail, and Earl Quay; these could
be used for the detection of print on a page
and for even looking through a glass crystal
to find a pointer beneath.

Until the early l970's, "active" light
probes (those containing a light source as
well as a "passive" light sensor) were made by
carefully positioning a lamp next to the sen-
sor, either or both of which also required a

condensing lens. Soon, however, all-new
solid-state sensors were devised for industrial use--made for guiding rolls of
material, counting rotations of shafts which
were scribed with a line, and reading holes
in punched cards. These sensors made construction of the probes' "front ends" much

With the advent of these new sensors, two
more "inventors" came on the scene: Southwest
Research Institute in Texas brought out the
"paper money identifier;" it was a fine
active light probe, but it was marketed for
the wrong use, in addition to which its fancy
wooden cabinet kicked the price well up over
$100. Bill Gerrey, on the other hand, built
Gunderson's ol' Transistorized Auditory
Gimmick around the new sensors, and then set
about finding industrial uses for this very
inexpensive and almost pocket-sized instrument. (I had it doing everything from
reading non-electrical gauges to positioning
holes in fabric in the jaws of an eyelet-
inserting machine.)

It then became the task of "inventors" (in
number now rivalling those who created the
lights we were detecting, including the inventor of the candle) to make the instruments
small and inexpensive. The smaller we made

them, the more expensive were the components
and the more intricate was the machining of
their cases. I may hold the record in this
antiquity; my final design was only 4-1/2
inches long, l inch in diameter, had an $8
switch, a $l0 speaker, a $6 battery, and just
a little machining . . . bringing its price
well up to $l00 along with the rest.

By l979, the 100th anniversary of the
original Polish invention, production
engineering was completed on a unit which
broke both the size and cost barriers. Bill
Loughborough, at Smith-Kettlewell, eliminated
the tubes and housings we were grappling
with--he glued the parts together and dipped
the assembly in tool-handle plastic, leaving
the battery to stick out and serve as a
handle. His is the only unit I have ever
seen being routinely carried in people's
pockets, and its cost is little more than
what my father paid for its primitive
counterpart 30 years earlier.

Description and Operation
of the Production Model

The assembly of parts is done on a small PC
board which is the same size as the cross-
section of the 9-volt battery which powers
it. The light sensor is glued to the back of

the speaker, which is then cemented to the
top of the 555 timer and other components at
one end of the board. Next, the sensitivity
control is positioned back-to-back with the
speaker and glued to the tops of the components on the other end of the board.
Finally, a pair of battery snaps is glued up
under the bottom of the board.

Vulnerable parts of the assembly are then
covered with tape in preparation for dipping
the unit in self-curing plastic (Plasti-
Dip*1). The assembly is snapped to the top
of a dead battery, turned upside down and
immersed so as to cover perhaps one-third of
the battery. This procedure is repeated
three times.

The resultant instrument is about the size
of a cigarette lighter, with the assembly of
the probe protruding about one inch beyond
the top of the battery. A hole for the
speaker is cut through the plastic on one
side and a control knob for the sensitivity
control is fitted to the potentiometer shaft
on the opposite side. Plastic is cut away on
the very top of the probe to expose the end
of the sensor, which sticks out slightly
beyond the speaker.

The sensor used is a so-called "line finder"
(Spectronics SPX1404); it is slightly concave
so as to accommodate a rotating shaft onto
which a line has been scribed--its industrial
use being that of an optical tachometer. Its
phototransistor and its LED source are pointing slightly toward each other so as to intersect at a focal point a short distance ahead
of the sensor. (The manufacturer claims this
distance to be about l/2 inch, although it is
nominally shorter than this and varies considerably between units.)

The least expensive pot we could find was
used; it is a typical panel-mount unit with a
split shaft designed to fit a control knob.
On the light probe, this shaft has been cut

so as to protrude only 3/l6 inch beyond its
bushing. A special knob is machined for the
resultant oddball diameter and length of the
shaft. A fillister head set screw was chosen
so that the screw could be used as a "pointer."


Whatever the task, I advance the
sensitivity control to the point where the
frequency of oscillation levels off (the point
of saturation), after which I back the control
down to a point where I get a moderate frequency. This latter setting is chosen so
that changes in light level will bring about
a pitch change in either direction. The more

light which the sensor sees, the higher the
frequency of oscillation, and vice versa.

On the other hand, in the passive mode a
higher sensitivity will have the effect of
broadening the angle of acceptance to which
the probe will respond. Therefore, if you're
looking for light in the middle of nowhere,
turn the probe all the way up and look around
till you find it; backing off on the sensitivity will then allow you to pinpoint the
source exactly.

The fact that the LED and the phototransistor are pointing so as to intersect ahead of
the sensor enables this device to look
through a thin piece of glass; when the
sensor is up against the glass, specular
reflections from the glass surfaces are out
of position so as to miss the phototransistor.
In this way, the probe can be used to detect
a meter pointer behind the glass cover, or
you can detect the level of a liquid in a
thin glass vessel. For best sensitivity to
changes in reflectance of the items behind
the glass, be sure that the sensor is up
against and perpendicular to the glass
surface and that the control knob has been
adjusted to give you an intermediate

The "focal distance" of the sensor does
mean that when you are detecting print on the
surface of a sheet of paper, the probe must
be spaced some distance away--perhaps l cm.
This spacing must be held absolutely constant;
otherwise you will hear variations in pitch
that are due more to the surface coming in
and out of focus than to changes in reflectivity.

I use my thumb and middle finger to pinch
the probe at its forward end, thus using the
ends of these fingers to space the probe at a
constant distance away from the surface. You
could, if you were inclined to, fit the end
of the sensor with a tubular spacer. However, for the production model, this constituted machining a special part, and we had
all fallen victim to that roadblock before.

Notes on Uses

Both the probe itself and its instructions
(in print and in braille) can be gotten from
the San Francisco Lighthouse for the Blind.*2
These instructions list household and office
uses--checking for lights which are left on,
checking to see that your typewriter ribbon
is working, finding the letterhead on company
stationery, etc. I intend only to supplement
this material in this article and gear it

more toward our high degree of sophistication.

A tape can be gotten from Harvey Lauer at
the VA Blind Center*3 describing the use of
the probe for identifying paper money. This
process is slow; it is good for when you are
truly on your own and perhaps as a parlor
trick. (Sorry, Harvey. It is a good tape,

Uses from the routine to the bizarre, such
as adjusting the flame in your fireplace, are
being documented by the Carroll Center for
the Blind in Newton, Massachusetts*4 and will
appear in their publication, "Aids and
Appliances Review." This compilation will
also list the numerous different brands of
probes which are available.

"Science for the Blind,"*5 sells both
active and passive probes built around their
Audicator, as well as selling instruction
tapes having live demonstrations of their

Manufacturers and employment uses (up
through l978) can be found in the Sensory
Aids Foundation catalog, "Sensory Aids for
the Employment of Blind and Visually Impaired
Persons," available from AFB.*6 (The Smith-Kettlewell units listed there are Bill Gerrey's antiques and are not to be confused
with the unit described here.)

It must be remembered that with the new
solid-state sensors, the light source is
infrared. Therefore, the reflectivity of
surfaces may not correlate with what is seen
visually. For example, glossy black surfaces
can look very reflective indeed.

Very often, you can identify wires by their
reflectivity. Different color wires will
cause the unit to emit different pitches
provided the distance from the sensor is
carefully controlled.
The red and black
binding posts on various lab instruments are
a cinch to identify, and I rarely bother
reaching for a voltmeter to do so.

Though there is no substitute for having
your own adapted instruments which you can
truly read, it is sometimes handy to reach
over to the front panel of an instrument
using a light probe to get a relative indication from its meters. I see this as a
crude shortcut, not a legitimate procedure.
This system absolutely breaks down if a meter
is illuminated from behind. Also, some
meters have a little strip of mirror behind
the pointer which you must either avoid or
carefully trace with the probe; if you do use

the mirror, the probe sensitivity will have
to be turned way down.

I have had occasion to use active light
probes in reading mechanical gauges which
cannot be read with a meter reader. For
example, in a pinch, I can get relative
indications off a visual vacuum gauge used in
repairing player pianos. I much prefer,
however, getting out my trusty electronic
version (see our Vocational Aids Catalogue)
for getting an accurate reading.

If you ascertain that the light probe is
the best way to read a given gauge, you can
fashion an attachment and design a special
probe for this specific purpose. For
example, I could adapt my vacuum gauge by
gluing a pivot and a movable hand right in
the middle of the glass crystal, then
mounting a Spectronics sensor on the side of
the hand so that it can trace the course of
the gauge needle. Finally, an outer disc
containing braille markings can be fitted
around the gauge, against which the position
of the hand is read. In operation, the hand
is turned until a dip in pitch occurs, at
which point the sensor is positioned over the

When it comes to checking your typing with
the probe, there are times when the tiny
sensor on the end of a cable would come in
handy. With practice, such a specially
designed unit can be used to reposition the
paper into the machine so as to continue
typing on the last line.

Incidentally, it is possible with practice
to determine whether the last word typed was
long or short. This is indeed useful in case
an unforgivable dunderhead calls you on the
phone and interrupts your work. With the
probe, for example, you can determine whether
or not the last word you typed was a conjunction or not. Once again, the small sensor on
a cable can be braced against the type fork
and the carriage manipulated to perform this
test more easily.

Circuit Operation and Description

Although I highly recommend that you try
one of the production models (I have bought a
couple myself), this will give you the chance
to make one to your own specifications. For
example, you may wish to preset its sensitivity with a screwdriver adjustment. Our
deaf-blind readers can change a couple of
components to get a low frequency and enough
output to be suitable for tactile indications.

Circuit Operation

The internal light
source, the LED in the Spectronics sensor, is
infrared and cannot be seen visually. The
LED is operated in the conventional way
through a current-limiting resistor.

The oscillator in the unit is a 555 timer
which directly drives the tiny speaker
through a resistor. (A normal-sized speaker
could be used; because of its lower impedance,
it should be driven through a resistor of
perhaps l00 ohms.) The charging current for
the timing capacitor is controlled by a
"current amplifier" transistor, the base of
which is controlled by the phototransistor.

The phototransistor is properly seen as a
current source. The resistor network in its
collector is best termed a variable "current
divider;" the sensitivity control shunts
current away from the network controlling the
base of the current amplifier.

Circuit Description

The negative side of
the battery goes through the on-off switch to
ground. (In the production model, this switch
is a part of the sensitivity control.) The
positive of the battery goes directly to the
VCC line. The cathode of the LED is grounded,
while its anode goes through 390 ohms to the
VCC line.

The emitter of the phototransistor is
grounded, while its collector goes through
two branches to the VCC line: First, the
collector goes through the sensitivity control
(500K rheostat), then through 390 ohms to
VCC. Second, the collector goes through l.8
megohms, then through l megohm to VCC. The
junction of the latter two resistors goes to
the base of the PNP transistor which is the
current amplifier. (This could be any PNP
silicon transistor such as a 2N2907--we used

The emitter of the PNP transistor goes
through 390 ohms to VCC. Its collector goes
through l2K, then through 0.0luF to ground,
with the junction of this resistor and capacitor going to pins 2 and 6 of the 555. Pin
7 of the 555 goes directly to the collector
of the PNP current amplifier.

Pin l of the 555 is grounded, while pins 4
and 8 go to VCC. Pin 3, the output, goes
through the speaker, then through 390 ohms to
VCC. (The speaker used here is Panasonic No.

For our deaf-blind users, the speaker or
transducer can be driven through 47 ohms

instead of 390; then increase the timing
capacitor and/or the l2K resistor in series

with it. Initially, try a timing capacitor
of 0.1uF with its series resistor being
increased to perhaps 39K. It will take some
juggling of these components to suit your
taste and your transducer.

When the frequency is slowed down for
tactile output, the dynamic range around an
intermediate frequency is reduced, making
adjustment of the sensitivity control very
critical. For this reason, you may wish to
use a l0-turn 500K pot in place of the garden-
variety 270dg unit.

The task remains to identify the leads on
the Spectronics sensor. Two leads emerge
from each element in the package. The pairs
of leads cannot be confused, since the
elements are separated by a little mounting
flange. The LED will look like a diode on
your continuity tester; the tester will sing
when its positive lead is on the anode. As
for the phototransistor, proper connection to
your continuity tester will create a system
which acts like a light probe (which it is).
This will occur when the tester's positive
lead is on the collector and its negative
lead is on the emitter (the base of the
transistor is not available).

*1 PDI,
l458 West County Rd. "C",
St. Paul, MN 55ll3,
(6l2) 633-9633.

*2 San Francisco Lighthouse for the Blind,
1155 Mission Street,
San Francisco, CA 94l03,
(4l5) 43l-l48l.

*3 Mr. Harvey Lauer,
Blind Center ll7A,
VA Hospital,
Hines, IL 60l4l.

*4 Carroll Center for the Blind,
770 Centre Street,
Newton, MA 02l58.

*5 Science for the Blind,
P.O. Box 385,
Wayne, PA l9087.

*6 American Foundation for the Blind,
l5 West l6th St.,
New York, NY l00ll.

Parts List

Resistors, W1/4, 5 percent:

  • 4 - 390 ohm
  • 1 - 12K
  • 1 - 1meg
  • 1 - 1.8meg

Potentiometer with switch:

  • 1 - 500K linear, Mouser No. 31CT505
    (Mouser Electronics, ll433 Woodside
    Ave., Lakeside, CA 92040)


  • 1 - 0.01uF disc ceramic


  • 1 - Spectronics Reflective Sensor, SPXl404,
    available from Cetec Electronics, 72l
    Charcot Ave., San Jose CA 95l3l, and
    Force Electronics, 2955 Gordon Ave.,
    Santa Clara, CA 9505l.
  • 1 - PNP transistor, 2N5139 or any small-
    signal, general-purpose unit
  • 1 - NE555 timer chip


  • 1 - Panasonic EAF12R01 (Suffix "A" for
    wire leads, "B" for bent pins, "C"
    for straight pins.) Available from
    Allied Electronics, 1355 McClean
    Blvd., Elgin, IL 60l20.



Whether you are stationed in an
office or taking your turn at manning a
booth, much energy goes into wondering
whether or not a non-vocal person has
approached you. This unobtrusive device
does nothing until someone has been standing
at your station for a few seconds, after
which a quick little "beep" alerts you to his


Whether it's at a cake sale or a university
registration desk, we have all been asked to
put in our time in "personing" the booth.
About every 20th person, you get someone who
simply cannot bring himself to address you
first. This leads to your frequent address
of mirages in order not to miss anyone--
"Hello?" you mutter nervously.

Well now, with this device you can content
yourself with "woolgathering" until a bashful
soul has been caught in your snare, at which
point you can say, "Yes?...Well go ahead and
be that way!" And you can do so with confident abrasiveness.

The system uses a "mat switch" of the kind
used for automatic doors and burglar alarms.
A selection of mats is given at the end of
this article, including good-looking ones for
on top of the floor and switch arrays which
go under the carpet.

Two sections of a 556 dual timer chip are
connected as one-shots in cascade. The first
of these sets the length of time that a
person has to stand there before he gets
caught, while the second sets the duration of
the beep. The beep itself is generated by a
Star Micronics buzzer.

The unit can easily be built into a small
minibox, which is then laid on the desk or
taped up under it. The buzzer can even be
shut up inside the box if the environment is
fairly quiet.

It was necessary to play a trick on the
second one-shot; otherwise they would both
trigger as soon as the switch were closed.
Its timing capacitor is not returned to
ground, as is usually done; it is brought
over to the battery side of the mat switch
so as to put a charge on this capacitor for
the initial state. In this way, the second
one-shot thinks it has already just been
triggered; its discharge pin immediately sets

about discharging the timing capacitor in
order to initiate "another" cycle.

Note that in the one-shot configuration,

the Threshold and Discharge pins are often
directly tied together--something you never
see in the free-running oscillator connection. Thus, in the first section these pins
are tied together, pins 2 and 1 respectively.
However, in the second section it was necessary to buy a little time in the "false post-
triggered state" mentioned above to keep this
section from firing early.


The positive of the 9-volt battery
goes directly to the VCC line, while its

negative side goes through the mat switch to
ground. Pin 7 of the 556 is grounded, while
pins l4, 4, and l0 go to VCC.

Pins l and 2, the Discharge and Threshold,
are tied together; they go through 5uF to
ground (negative at ground). Pins l and 2
also go through l00K, then through a l meg
rheostat (delay control) to VCC. To trigger
this one-shot, pin 6 (trigger) goes through
0.luF to ground, as well as going through a
l00K pull-up resistor to VCC. The output of
this section, pin 5, goes through 0.luF to
the trigger (pin 8) of the second section.

Pin 8 also has a l00K pull-up resistor going
from it to VCC.

Also in the second section, pin l3, the
Discharge, goes through 47K to pin l2, the
Threshold. Pin l2 also goes through 0.47uF
to the negative side of the battery, not
ground. Pin l2 goes through l00K, then
through a l meg rheostat (beep duration
control) to VCC.

Pin 9, the output of the second one-shot,
goes to the control pin, pin 8, of the Star
Micronics CMB-l2. Pin l of the buzzer is
grounded, while pin l4 goes to VCC.

Parts List

Resistors, W1/4, 5 percent:

  • 1 - 47K
  • 1 - 100K

Potentiometers (connected as rheostats):

  • 2 - lmeg single-turn, PC-mount trimmers


  • 2 - 0.1uF discs
  • 1 - 0.47uF disc or mylar
  • 1 - 5uF 10V electrolytic


  • 1 - NE556 or ICM7556 dual timer chip
  • 1 - Star Micronics buzzer, CMB12

Mat Switches

  • (Tape Switch Corp. of America, 100 Schmitt
    Blvd., Farmingdale, NY 11735. Tel: (516)
  • Controflex Switching Mats (on top of carpet):
    • CVP623, 6 by 23 inches with 30 inches of
      connecting cable
    • CVPl723, l7 by 23 inches with 6 feet of
      connecting cable
    • CVP2335, 23 by 35 inches with 6 feet of
      connecting cable
      (The above are 3/32" thick, olive-colored
      vinyl. A 5-lb. pressure closes them.)
  • Under Rug Switching Runner:
    PE30-5, 30 inches wide, 5 feet long. Can
    be cut to shorter lengths. Cable must be
    soldered to wires at open end.
  • Indoor/Outdoor Tape Switch:
    121-BP(1), 121-BP(2), 121-BP(5)...are
    available in l, 2, 5, 8, l0, l2, l5, l8, &
    20 feet (length specified in parentheses).
    9/l6 wide and 5/32 thick, to be secured
    with adhesive No. l05 or in channel No.
    l04. Eight oz. finger pressure for switch
    closure; comes with l8", 22-gauge leads.
    Can be used outdoors on fences.


(The following was gotten from BTP February
1971; it was originally reprinted from "Sound
Talk," a service to the industry from the
makers of Scotch Magnetic Tape, Vol. III, No.
1, 1970.)


Heavily excerpted, this material
covers all the "right things to do" to preserve your recordings. As was suggested in
the original article, pick out the ideas
which you can economically afford and which
do not interfere with your everyday use of
magnetic media. Although this article pre-
dates floppy discs, the ideas presented here
are relevant to them.

The Recording Area

Ideally the equipment room of a recording
studio should approach, as closely as
possible, a "clean room" environment. By
definition, a "clean room" is characterized
by the absence of normally expected airborne
dust and lint. The integrity of this area
should be maintained by periodic cleaning of
shelves and floors. When vacuum equipment is
used for cleaning, the exhaust from this unit
should be located outside the room.

It is doubtful that smoke will contaminate
the tape, but ashes can. Therefore, smoking
should not be allowed directly over the
machines or when handling tape. Food and
drink should also be prohibited for obvious

The equipment area should be such that
reasonable control of temperature and relative humdidity can be exercised. Variations
in temperature should be held within plus or
minus 5 degrees F. of a preselected value,
and variations in relative humidity should be
kept to within plus or minus 10 percent.
Preferably the temperature should be in the
70's with a relative humidity of 40 percent.

When recording on location or at home, it
may be difficult to control the surrounding
environmental conditions. At the very least,
it is important to eliminate the entry of
foreign material into the machine. It is
recommended that the equipment always be
covered during storage, and as much as
possible during operation. Many of the protective dust covers provided by manufacturers
permit their machines to be operated while
they are in place; these should be used when
in an uncontrolled environment.

Tape Storage

The temperature and humidity of the tape
storage area should closely approach that of
the work area. The smaller the environmental
change experienced by the tape, the better
will be its operation and reliability. As a
general rule, a temperature between 60 and 80
degrees F. and a relative humidity between 40
and 60 percent is recommended. If the
environmental conditions of the storage area
vary widely from the recording area, allow
time for the tape to reach temperature and
humidity equilibrium before putting it into

Recording tape, especially cartridges and
cassettes, poorly stored or casually laid on
the dashboard or in the glove compartment of
an automobile, can be damaged by the heat of
strong sunlight. The molded cases used for
some cartridges and cassettes can be
permanently distorted if subjected to high
temperatures. Both cartridges and cassettes
use splices within their tape rolls which can
be affected by heat. The splices may separate, or the adhesive may soften and "ooze"
from the edges of the splice, causing
adjacent layers of the tape to stick together. The exposure of the splice adhesive
will also collect any contamination present
in the case, causing additional problems.

Protection from accidental erasure while in
the storage area is easily accomplished and
is, ironically, of little concern. First of
all, fields strong enough to cause erasure
are just not normally found in an office or
home atmosphere. Secondly, if the tape is
kept as little as three inches away from even
a strong magnetic source, this spacing should
be sufficient to offer adequate protection.
{Editor's Note: With the advent of very flat
loudspeakers, fields from their permanent
magnets are often strong enough to damage
cassettes or credit cards. Keep magnetic
storage media away from your talking clocks,
"Mr. Thin" radios, and light probes.}

During storage, the tape must be enclosed
in a container (original box, plastic case,
tape cannister) for several reasons. One
reason is to provide protection from physical
damage. Another reason is to protect the
tape from dust.

The closed containers should be placed into
storage on edge, so that the reel is in an
upright position. While they may also be
stored individually, lying flat, tape boxes
should never be stacked so high that there is
a possibility of crushing or distorting the
bottom container from the excessive weight of
the stack; this could cause edge damage to
the reel of tape in the bottom container.

For long-term storage, additional protection from dust and moisture can be gained by
sealing the container in a plastic bag. It
is generally considered good practice to
clean the container before using it, so that
dust which has accumulated during storage
will not contaminate the recorder or tape.

Of primary importance is the way the tape
is wound on the reel, since poor winding can
result in distortion of the tape's backing.
A winding tension that is relatively low is
recommended. Three to four ounces per l/4
inch of tape width is sufficient to render a
firm, stable wind on a reel configuration.
This tension, while great enough, does not
result in high pressures within the roll that
could permanently distort the backing. Backing distortion, caused by extreme pressures
within the tape pack, may result if a roll of
tape which has been wound too tightly is
subjected to an increase in temperature while
in storage.

Too low a winding tension can cause difficulty as well. If the wind is too loose,
slippage can occur between the tape layers on
the reel. This "cinching," as it is called,
can distort the tape by causing a series of
creases or folds in the area that has slipped.
When the roll is unwound, the backing will be
wrinkled. When an attempt is made to use the
tape again, the wrinkles and creases will
disturb the necessary intimate contact
between the tape and the head. Because the
tape is repeatedly lifted from the head, the
result will be a series of signal variations.
If the tape is properly rewound immediately
after cinching, there is a good possibility
that the information may be saved.

Some recorders now in use do not have a
method of adjusting winding tension; therefore, care must be taken while operating
these machines. Sensible operation of fast-
forward, rewind, and start controls can
eliminate the sharp stress loading associated
with starting and changing the tape direction.
Tape distortion and "cinching" can be reduced
by allowing a minimum of slack when threading
and starting the machine. It is also good
practice to allow the spinning tape reels to
completely stop before changing tape direction.

Along with proper attention, another important consideration is quality of the wind.
The successive layers of tape should be
placed on the reel so that they form a smooth
wind with no individual tape strands exposed.
A smooth wind offers the advantage of built-
in edge protection. A scattered wind will
allow individual tape edges to protrude above
others. Since there is no support for these
exposed strands, they are vulnerable to

It is sometimes suggested that tapes in
storage be rewound at specific intervals,
such as every 6 or l2 months, to relieve
internal pressures. This may be recommended
for tapes of marginal quality or for those
with other than heavy-duty binders for
securing the oxide coating. For modern tapes
with polyester backing and advanced binders,
this periodic rewinding may not be necessary.

A good practice, however, is to select a
random sample from various areas of the
library for visual inspection. The reels
chosen can be examined for loose windings and
dust accumulation. They should be checked
for rippled edges and other signs that indicate the presence of physical distortion. If
anything is found that indicates a problem
may exist, additional samples should be
inspected to ascertain what percentage of the
library may be effected.

If the above recommendations concerning the
storage environment and the actual preparation for storage are followed, no serious
problems should be encountered, even in long-
term storage.

Shipping of Tapes

There are certain precautions which apply
to the shipment of recording tapes that
should be followed to ensure safety in
transit. Logically, the first consideration
would be the physical protection of the tape
while being transported. Also, since the
tape carries magnetic information, measures
must be taken to protect the reels from
accidental erasure.

The outer shipping container into which the
tapes are placed must have the necessary
strength and rigidity to protect the tape or
tapes from damage caused by dropping or
crushing. While a container which is l00
percent watertight would not be necessary, it
must nevertheless provide a reasonable degree
of water resistance. It should, for example,
be capable of protecting its contents from
being damaged when left on a loading dock in
the rain.

While it is always good practice to secure
the free end of a reel of tape, this is
particularly important when preparing reels
for shipping. A short length of pressure-
sensitive tape is all that is necessary.
{Editor's Note: For many years, plastic
clips have been available which hold the free
tape end and which slide in between the
flanges of the reel and the tape pack. I
consider these devices mean, nasty, and
barbaric, since their very insertion and
their presence as the package is crushed
causes serious edge distortion.}

As far as erasure of the tape is concerned,
laboratory-conducted tests have determined
what would constitute adequate protection
from stray magnetic fields of a magnitude
which may possibly be encountered in transit.
It was found that field strengths within the
tape of 50 Oersteds or less caused no discernible erasure. The average bulk degausser,
on the other hand, produces a field of l500

Sources of magnetic energy to which tape
being shipped might be subjected would be
motors, generators, transformers, etc. These
devices are designed to contain their magnetic fields to accomplish some type of work.
Bulk degaussers, on the other hand, are designed to produce a maximum external field
that is strong enough to erase a tape while
it is still on the reel. It is safe to
assume that field strengths from other
devices would not be more than l500 Oersteds.

Because field intensity decreases rapidly
with distance from the source, the 50 Oersted
point (mentioned earlier as not affecting the
tape) is reached at a distance of 2.7 inches
from a l500 Oersted source. From this it can
be seen that the easiest and least costly
method of obtaining erasure protection is by
ensuring a 3-inch spacing between the tape
and the magnetic source. It is suggested
that tape being prepared for shipping be
packed with bulk spacing material such as
wood or cardboard between the tapes and the
outside shipping container. This magnetically protective spacing can also be justified
because of the excellent protection gained
against physical damage to the package contents.

Recent laboratory tests concerning exposure
of recorded tapes to X-ray have determined
that the recorded signal is not affected by
even severe exposure to this source of radiation. The tests involved a commonly used
recording tape with several different frequencies recorded on it.

The X-ray machine was operated with 200mA
at 110kV for a 6-second exposure time at a
36-inch distance. Testing and measuring the
signal output before and after exposure indicated no signal loss or degradation.

Tape in transit may be subjected to temperature extremes. Temperatures as low as -40F
might be encountered in the cargo hold of
high flying aircraft. A temperature of l20F
could be encountered in a motor vehicle in
the summer sun. It must again be emphasized
that all incoming tape should be allowed to
reach environmental equilibrium before being

Good Operating Habits

The container in which tape is stored is
probably the cleanest space in the recording
studio; of course, this is the reason that
tapes should remain in their boxes until they
are ready for use. To maintain the cleanliness of the container, it should be closed
when the tape has been removed.

The hub is the strongest and most stable
part of the reel. Always handle the reel by
the hub and not the flanges.

When handling tapes, use utmost caution to
ensure that the tape does not become contaminated by fingerprints. Simply stated,
fingerprints are nothing more than deposits
of body oils and salts. These oils will not
attack the oxide-binder system, but they will
form excellent adhesive areas for dust and

Fingerprints on the backing are just as
serious as on the coating because dirt
deposits will transfer from the backing of
one wrap to the coating of the next wrap on
the reel. When a reel has been contaminated
in this manner, the tape deck itself can be
affected and will spread this contamination
to other clean reels of tape that are used
after the dirty reel.

The above gives reason for visually inspecting the tape machine after each roll of
tape has been run to determine if cleaning is
necessary. If the machine becomes contaminated with dust or wear products from the
tape, complete contamination of an entire
roll of tape can easily be the result.
Contaminants can collect on heads and guides
and be dumped along the backing or coating
surface of the next tape. This contamination
will then be wound into the reel under pressure, causing it to adhere firmly to the
surface. Each one of these deposits will
appear as a "drop-out" or group of drop-outs
the next time the tape is used.

Tape contamination caused by fingerprints
can be reduced by remembering not to touch
the tape unnecessarily. Frequent cleaning of
the tape deck will reduce the chance of
spreading contamination from one reel of tape
to others in the library. A cotton swab or
lint-free pad moistened with "Genesolve-D"
(an Allied Chemical trademark) or "Freon TF"
(a DuPont trademark) or similar cleaner is
recommended for cleaning all components along
the tape path. If other types of cleaning
agents are used, they should be given time to
thoroughly dry before loading the tape; this
will prevent damage should the cleaner have
any tendency to attack the magnetic tape.
Accumulation of tape wear products on the
transport can be largely eliminated by using
high reliability tape.

Maintaining reel integrity cannot be over-
emphasized since valuable information can be
lost, not because of tape failure but because
the tape was distorted by a faulty reel.
Empty reels should be thoroughly inspected
and cleaned before winding tape on them for
storage. Reels with hub damage, such as a
plastic burr, or with dirty hubs, can cause
tape distortion exactly as outlined in the
preceding paragraphs.

The effect of a broken or cracked flange is
easily noticed, since the tape will exhibit a
series of nicks or mutilated areas along one
edge, and the cause can be easily detected
because of the obvious defect in the reel. A
bent or distorted reel, however, can also
cause damage to one or both edges if the tape
is allowed to rub against the flange being

Not only can edge damage be caused by a
defective reel and by improper storage, but a
similar type of damage will also occur if any
of the tape machine components are misaligned.
Any of these faults can result in a complete
failure of a roll of tape. Of course, damage
to the edge will result in loss of information on the edge track. Further, however,
the debris generated from the edge damage can
be redeposited onto the remaining surface
across the entire width. An examination of
the edges of a tape that has been damaged in
this manner will disclose an accumulation of
oxide debris.

While edge damage is serious, it is sometimes difficult to ascertain its cause or
even to notice its effect until the damage is
severe. Operators must acquire the habit of
physically inspecting the tape machine in the
area of its guides and heads for an excessive
build-up of oxide or backing debris. This is
generally the first clue that something is
wrong. Excessive drop-outs on an edge track
or a loss of high frequencies may also indicate that an alignment or tracking problem

It is also good practice to observe the
physical condition of the tape. A sure sign
of developing edge damage would be a lip or
distortion on the edge being injured. When
wound on the reel, the effect of this lip
will be cumulative and can stretch the backing. The stretched backing will be rippled
and will not conform to the recorder heads
the next time the reel is used.

If tape in this condition is properly
rewound immediately before being put into
storage, it may be possible to salvage the
roll. If this is not done, the backing will
be permanently stretched and will not recover.
This will result in the entire roll having to
be discarded.

Major Catastrophe

The discussion, to this point, has been
devoted to precautions and suggestions
involving the day-to-day routine use of
recording tape. The final area of concern,
while a remote possibility, is nevertheless
of utmost importance because it affects not
only just a single reel of tape but the
entire recording library. This section will
be devoted to two forms of major catastrophe:
fire and nuclear radiation.

Fire Damage

For a substance to burn, there
must be a breakdown of the organic materials
contained in it. The organic materials in
magnetic tape are the plastic backing and the
binder. To burn, these must first vaporize--
thus increasing their exposure to the oxygen
in the atmosphere--and then rapidly oxidize,
producing light and heat. An ample supply of
oxygen is required to sustain burning.

Since magnetic tape contains no "built-in"
oxidizer, it cannot burn in the absence of
air. Simply stated, its behavior can be
closely compared to the way in which a
tightly wound roll of paper would burn.

While the "self-ignition" temperature of
polyester-backed tape is in the neighborhood
of 1000 deg. F., temperatures below that
point can still cause damage. Polyester film
will shrink 1-1/2 percent at 300 deg. F. and
25 percent at 325 deg. F. Acetate film,
because of its sensitivity to heat, will
exhibit greater shrinkage and backing distortion, and is more susceptible to heat
damage than polyester. If a roll of tape is
heated to the approximate temperatures listed
below, certain effects will be noted when the
roll has cooled.

  • 250 deg. F.--Backing distortion
  • 320 deg. F.--Softening of both the backing
    and the binder with some
    "blocking" or adhesion of
    adjacent layers.
  • 550 deg. F.--Darkening and embrittlement
    of the backing and binder.
  • 1000 deg. F.--Charring of the backing and

When charring occurs, the tape cannot be
unwound from the reel, since it will flake
when touched. The temperature limitation of
present-day tapes is a function of the
organic components, and not a function of the
gamma ferric oxide.

Winding and storing magnetic tape properly
will lessen the possibility of damage in the
event of fire, since tape is a poor conductor
of heat. It is sometimes possible to recover
information from a tape receiving slight fire
damage by carefully rewinding it at minimum
tension. The information it contains should
be transferred immediately to another reel of
undamaged tape.

We recommend CO2-type fire extinguishers
for combating burning magnetic tape. CO2 is
clean, and this type of extinguisher contains
no chemicals or powders that could harm the

{Editor's Note: I just called our Fire
Department to ask if CO2 extinguishers were
even made anymore. The Captain assured me
that they were, but that they are generally
inefficient in fighting fires; although they
are ideal for this purpose and for electrical
fires. He recommended having both CO2 and
powder type on hand, or planning carefully
what kind of conflagration is to be set.}

If water reaches the tape, it will probably
not cause complete failure, but there may be
some evidence of "cupping" or transverse
curvature. The amount of "cupping" will
depend on the quality of the wind, backing
material, and the length of time the roll is
exposed. If the wind is loose or uneven, the water can more easily reach the oxide surface
and the cupping will be more pronounced. The
tape should be removed from the water as soon
as possible--certainly within 24 hours.

After removal, the rolls should be allowed
to dry on the outside at normal room temperature and then be rewound a minimum of two
times. This rewinding will aid the internal
drying and will also help the rolls to return
to equilibrium faster. If moisture is allowed
to remain within the roll, severe "blocking"
(adhesion of adjacent layers) can be the

If a temperature increase is also incurred
while the tape is water soaked, steam or at
least high humidity will be present. This is
more likely to cause damage than water alone.
A temperature in excess of l30 deg. F. with a
relative humidity above 85 percent may cause
layer-to-layer adhesion, as well as some
physical distortion.

Nuclear Radiation

As a general statement,
it can be said that magnetic tape will be
unaffected by nuclear radiation until the
dosage approaches a level 200,000 times
greater than that which would cause death in
50 percent of the exposed population. Radiation of this level (l00 megarep) would tend
to increase the layer-to-layer signal transfer or "print-through" by about 4dB, but
would not prevent information retrieval.

Nuclear radiation at the above level would
also have some physical effect on the tape
coating and backing. The backing will show
significant embrittlement, and it is expected
that the wear life could be reduced by as
much as 60 percent.

It is reasoned that whatever electromagnetic field might result from a nuclear
detonation would not be of sufficient
intensity to adversely affect the tape;
therefore, the threat of signal erasure is
virtually non-existent. The effect of
neutron bombardment would no doubt be limited
to activation of the iron oxide in the
coating. This would produce a radioactive
isotope that itself might become a source of
further radiation, but it is theorized that
such activation would not produce a change in
the overall magnetic properties of the

Radioactive dust or fallout is not capable
of producing the dosage necessary to adversely affect magnetic tape. The recommendations
made earlier to protect the tape from normal
contamination are applicable here.


As can be seen from the above discussion,
when speaking of major catastrophes, heat and
fire damage are considered much more serious.

Under proper storage conditions, magnetic
tape has the ability to retain intelligence
for an infinite period of time; of greatest
importance is the physical preservation of
the medium so that adequate head-to-tape
contact can be maintained.

If any questions about this topic arise,
simply write to:

Product Communications
Magnetic Products Division
3M Company
3M Center
St. Paul, MN 55101

{Editor's Note: In writing, please refer
to the original "Sound Talk" article of 1970;
remembering that it is now 13 years later and
these people have never heard of me.}


The following was submitted by John
Dascenzo, W3IPD, of Fairfield, PA:

"I have always had stability problems when
using a soldering gun, in positioning the
point on the work and then pulling the
trigger. No matter how careful I am, the
action of my pulling the trigger frequently
causes the point to move. I found a simple
solution for this problem.

"First, the trigger of the gun is tied
permanently "on" with a piece of string
(color of this string not important). The
gun is then plugged into a foot switch.

"My foot switch is rather fancy. I placed
a microswitch on the side of a heavy metal
block--about the size of a brick. It is
arranged so that by merely sliding my shoe
slightly, the gun goes on. An AC receptacle
for the gun is also mounted on this block."

Thank you, John Dascenzo. Your point has
good basis in physiology. If we look back at
"Soldering, Part II" (Winter 81), an analogy
is drawn between the wrist and a pulley system, with the tendons being "ropes" which
connect the fingers to their muscles in the

forearm. Applying tension or moving one of
the fingers puts the wrist in a very unstable
condition; uncontrolled movement of the hand
is the likely result.

The following ideas were contributed by
John Lizza of Carle Place, NY:

"Obviously, Nichrome wire in heating
elements cannot be spliced by soldering the
broken ends. Repairs can be made by inserting the broken ends into a crimp lug and
crimping it tightly.

"I do automotive work, and it is often
necessary to note small but critical differences in the voltage on the electrical
system (say from 13V to a high of 15V while
the battery is charging). However, my multimeter only has scales of 0 to 50 volts and 0
to 5 volts.

"I get around this problem by connecting a
zenor diode (for example, a l2.4-volt unit)
in series with my meter while setting its
range to the 5-volt scale. In this way, the
critical changes end up as a 2-volt range at
the bottom of my more sensitive scale.

"To check liquid levels--brake fluid, oil,
water, etc.--use a straw or a piece of tubing
through which you are constantly blowing.
You will know immediately when the other end
becomes obstructed by the fluid."

Thank you, John Lizza. Also, it seems I
missed a major point in describing his work
on circuit boards with the hand in a plastic
bag. He points out that this technique really shines when checking for intermittents
and loose connections. Sorry.


Editor's Note: Here's what us old folks do
to get around not having a scientific calculator.

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Technical Tables Bank provides thermoform
copies of the more than 300 tables in its
collection at prices substantially below
cost. The collection includes standard
tables found in mathematics, computer
science, statistics, chemistry, physics,
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The Pictorial Catalog (print only) of the
collection consists of a written description
of each braille table plus a reproduction of
a sufficient portion of the print table from
which it has been adapted and transcribed to
permit positive identification of domain,
range and number of significant figures.


  • NBA members (1 copy) . . . . . $ 7.50
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Also available from the Book Bank of the NBA
are the following titles on radio information. These titles are listed in their
General Interest Catalog, which may be
obtained for $l.00 from the NBA Braille Book
Bank, 422 Clinton Avenue South, Rochester,
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The following items are available on tape
from The Volunteers of Vacaville, P.O. Box
670, Vacaville, CA 95696. (Catalog also
available on open reel or cassette.)

  • "Electronic Devices and Circuit Theory,"
    Boylestad, et al. Cat. #J-235.
  • "From Five Watts to One Thousand Watts,"
    Mede. Cat. #F-004.
  • "Introduction to Computer Sciences and
    Data Processing: 2nd Ed."
    Schmidt, R., et al. Cat. #I-078.
  • "It's Easy to Use Electronic Test
    Klein & Gilmore. Cat. #A-l62.
  • "Micro-Electronics, Analysis and Policy."
    Lloyd, R. Cat. #E-l68.
  • "Tape Editing."
    Tall, J. Cat. #D-036.
  • "Understanding Radio Electronics: 4th Ed."
    Kaufman, et al. Cat. #J-l29.


I got a note from Joseph Giovanelli informing me of the proper meaning of the root
"lexis" and of my incorrect use in the contrivance "lexicography." "Lexis," as it
seems, means "word," while "biblio" means
"book." This may well explain why books are
not written on polycarbonate (Lexan). It's
Greek to me, Joe.

With regard to the above, I offer the
following two reasons for retaining the name,
"Lexicography." 1} Have you ever noted the
Arctic response from a co-worker of whom you
innocently ask, "Would you please type this
20-page bibliography?" 2} In my years of
experience with products and organizations,
I have noted that there is only one thing
worse than calling it a bad name: changing
it. I hope you all enjoy that column,

I am horrified and incensed at the fact
that several of my suggested questions were
left out of the Evaluation Questionnaire on
the contrived rationale that it should be
objective. These questions are listed below.

H.5. Are the braille cells (please circle
the appropriate response)

Too large, too small, or standard?

I.5. In the large-type version, is not the
print too close to the page?

J.1 Closed-circuit phone jack with insulating washers (please circle the appropriate
mounting hole). {Hmmmm, how'd that get in

J.2 Don't you think that if university
professors were blessed with the Editor's
quick wit that our grades would have been

Say Yes

J.3 Must the Editor's veiled greatness
ever remain tragically o'ershadowed by his
obvious sheepishness?

Yes sir, no sir, three bags full.

See you next Spring.

CORRECTION--"SCA Decoder" (SKTF, Fall 1981)

We forgot to power the RCA CA3089; pin ll
goes to the VCC line, which in turn goes
through a switch to the power supply.

CORRECTION--"Little Go Beep, II"
(SKTF, Summer l982)

In the first mike preamp using a TL072 (top
of page 33), we find, "pin 3 also goes
through 4.7K to ground." No, No! In building this preamp, we discovered that its bias
must be VREF1, just like pin 2. Therefore,
"pin 3 should go through 4.7K to VREF1."

Sorry, a whole battery supply of engineers
missed that one.

CORRECTION--"The Fowle Gimmique"
(SKTF, Summer l982)

Jim Swail points out that an offset voltage
in some LM358's can introduce an error at the

triggering point of the "beep oscillator"--
his unit was triggering low by about 4mV.
(Data sheets on the 358 list offsets of 2mV
nominal and 7mV max.) There are four
possible solutions:

  1. The LM358 can be directly replaced
    with a high-grade op-amp, National LF442ACN,
    whose offset is 0.5mV nominal and 2mV max.
    (Available for $6.l0 from Hamilton Avnet,
    ll75 Bordaux, Sunnyvale, CA 94086.)
  2. For the same money, you can buy
    several 358's so as to hand-pick one with a
    low offset.
  3. An offset adjustment can be built
    into the instrument by connecting the bottom
    end of the calibrated pot through a 500-ohm
    rheostat to the negative end of the battery,
    not to ground. The calibration and offset

    adjustments will interact, however.

  4. One could rebuild the device with
    two single op-amps having provisions for
    offset pots (See the l00K pot in the original
    SK Gimmick, Spring l98l.) With this arrangement, the l meg offset pot in the present
    Fowle Gimmique's oscillator can be eliminated.

Subscription reminder

Dear Subscriber:

Our records show that your subscription to the
Smith-Kettlewell Technical File expires as of
this issue. We very much want to keep you, for
obvious reasons. It is true that we need the
financial support, but we also would hate to
miss our chance of getting nuggets of knowledge
from anyone. Please don't pass us by unduly.

Actually, you are in luck as far as subscription price is concerned. It is true that the
yearly rate has gone up to $l5 for braille and
large print, and $8 for cassettes. However,
in an attempt to simplify our mailing list
situation, we are switching everyone to a
purchase of "calendar years" of the magazine.
This means that your renewal will actually be
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order to shift you into phase with everyone
else. Act now, and you will be getting five
issues for the price of four.

Please make checks payable to the Smith-
Kettlewell Eye Research Foundation at the
above address. Thanks for your support.

Bill Gerrey, Editor