July 1960 Popular Electronics
[Table of Contents]
People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Popular
Electronics was published from October 1954 through April 1985. All copyrights are hereby acknowledged. See all articles from
This is a great example of how Popular Electronics
and John T. Frye used the "Carl & Jerry" series to
teach some basic electronics design principles through story
telling. In this adventure, the guys decide to build a tachometer
from schematics they found in a magazine. They debate amongst
themselves how the circuits works, the best way to assemble
the circuit, component selection, vibration- tolerant mounting,
and how to properly calibrate the tach to accurately display
engine revolutions per minute (RPM).
Carl & Jerry: Tussle with a Tachometer
By John T. Frye W9EGV
"Here are two electronic tachometers we can build for our
car," Jerry said as he spread a magazine and a little yellow
booklet on the bench in front of his pal, Carl. "This one uses
an 884 thyratron powered by a vibrator power supply. As you
can see, it's a detailed construction article, and the gadget
uses a relatively inexpensive 1-ma. meter as an indicator.
"The other one, in this booklet published by Sylvania, has
two 2N233 transistors connected in a one-shot multivibrator
circuit. Power is taken directly from either a six- or twelve-volt
car battery. However, about all we have to go on here is the
diagram and a very limited description. And this tachometer
uses a fairly expensive and delicate 50-μa. meter."
"Do both work on the same principle?"
"Actually, yes. Whenever a selected spark plug fires, the
thyratron is triggered into firing or the multi vibrator circuit
into flip-flopping. Each 'firing' or 'flip-flop' sends a pulse
of current through the meter which has a large capacitor connected
across it. This meter-capacitor combination responds to the
average current produced by the pulses. Since these pulses are
equal in amplitude and are uniformly spaced, the average current
indicated by the meter goes up in linear fashion with the frequency
of the pulses. That means the meter can be calibrated to show
the rpm of the motor."
"I say we build the transistor job," Carl decided, as he
finished looking over the two articles. "We have the transistors
and the meter, and we should know enough about electronics not
to need step-by-step instructions."
"Okay, but before we start, suppose you tell me once more
why we need a tachometer. Remember we resolved that anything
we put on the car had to be functional."
"A tachometer is functional," Carl insisted. "Knowing exactly
how fast the motor is turning over is important in many cases.
For instance, take 'boxwork,' as we hoity-toity motorists call
gear-shifting. There is one proper engine speed for each shift,
and working with a tachometer permits you to find and use those
speeds. Also, we can log the oil pressure for a particular engine
speed and use that as a reference later to see if we're losing
pressure. We can note at what engine speed our generator begins
to charge the battery and use this as a check on the generator's
operation. With a little math that takes into account the rear-axle
ratio and the rear-wheel circumference, we can convert rpm into
mph and check on the accuracy of our speedometer."
"Enough!" Jerry interrupted. "I'm convinced. All that bothers
me now is how we're going to calibrate the tachometer."
"Well, just remember that a particular cylinder of a four-cycle
engine fires only once every two revolutions," Carl pointed
out. "When the engine is turning over at 4000 rpm, our tachometer
will be receiving 2000 pulses per minute."
"I've got it!" Jerry suddenly interrupted.
"Let's get busy and build the thing. Then I'll show you an
easy way to calibrate it."
It didn't take the boys long to collect the parts they needed.
But Carl and Jerry prided themselves on making their electronic
equipment as compact and well-arranged as possible, so they
spent considerable time on layout. Since they realized that
the tachometer would be subjected to intense vibration in the
car, they anchored all parts for the multivibrator circuit solidly
on a small perforated board of insulated material, and then
fastened this board securely inside a small metal cabinet. Two
10,000-volt capacitors, a neon bulb, and a fixed and variable
resistor for attenuating and limiting the high-voltage pulses
from the spark plug were similarly mounted in another metal
box. Phono jacks on the boxes allowed them to be connected together
by a short piece of RG-58/U coaxial cable. Another length of
cable connected the multivibrator unit to the meter, which was
shock-mounted on a bracket designed to clamp on the steering
"Well," Carl said as he surveyed the completed tachometer,
"I guess we're ready to mount it in the car and calibrate it."
"We calibrate it first and then mount it in the car," Jerry
corrected him. "Trot out the sine- and square-wave generator
and connect it to the input of the multivibrator circuit while
I set up the 'scope."
Carl did as instructed, then watched as Jerry ran leads from
the output of the audio generator to the vertical input terminals
of the oscilloscope and connected the 60-cycle test voltage
terminal on the 'scope to the ungrounded horizontal input terminal.
"Here's my idea - double-check me and see if I'm wrong," Jerry
said. "Our 0-50 μa. meter will indicate 0 - 5000 rpm. That
means 48 μa. must correspond to 4800 rpm. This reading should
be produced when the multi vibrator is receiving 2400 pulses
per minute, or 40 pulses per second.
"Our square-wave generator should put out a pulse that will
trigger the multivibrator in the same fashion that the attenuated
pulse from a spark plug does," he continued. "All we have to
do is adjust the calibrating resistor of the tachometer so that
the meter reads 48 μa. when the multivibrator is being fed
a square-wave signal of 40 cps. We can double-check the linearity
with square waves of 30 and 20 cps. They should produce readings
of 3600 and 2400 rpm respectively."
"Sounds okay to me," Carl agreed, "but how are you going
to be sure you have exactly 40 cycles from the generator? The
dial calibration is reasonably accurate, but you can't depend
on it down to the cycle."
"That's where the 'scope comes in. We'll compare the 40-,
30-, and 20-cycle output of the generator with the 60-cycle
line frequency with Lissajous figures. Watch."
Jerry turned on the 'scope and switched on the audio generator,
set for sine-wave output. As he approached the 40-cycle mark
on the dial, the rapidly revolving pattern of interlaced curving
lines slowed down and finally stopped.
"See," Jerry said; "a line along the left side of the pattern
would touch three of the loops while a line across the top would
touch two. That means the ratio of the signal generator frequency
to the line frequency is 2:3 or 40:60."
When the generator was putting out exactly 30 cycles, two
loops of the pattern touched the imaginary vertical line and
only one touched the horizontal line. At 20 cycles, only one
loop still touched the horizontal line, but three loops touched
the vertical line.
Jerry went back to the 40-cycle frequency and switched the
generator over to square-wave output. As he did so, the distorted
pattern began to wiggle, showing that the change in output had
caused the generator frequency to shift slightly. A touch of
the generator tuning knob stopped the pattern again. Jerry reduced
the generator output until the meter indication began to fall
off and move erratically; then he increased the output until
the reading was stationary.
"Okay, now set the calibrate control for a 48-μa. reading,"
he instructed Carl. When this was done and the generator set
exactly for a 30-cycle output, the meter read 36 μa. When
the frequency was reduced to 20 cycles, the reading dropped
to 24 μa.
"Right on the money!" Jerry gloated as he grinned across
at his pal. "The thing is certainly linear over the top half
of the scale at any rate. Disconnect that six-volt lantern battery,
and let's install the gadget in the car."
The multivibrator unit was bolted to the metal body of the
car up under the dash, the attenuator unit was mounted on the
front of the firewall in the engine compartment, and the connecting
coax cable was run through a small hole in the partition. Connections
were made to the rear spark plug and to the cold side of the
ignition switch so that the tachometer would be switched on
with the ignition. When everything was connected, the boys started
the motor. Then they adjusted the variable resistor in the attenuator
unit until the meter gave a steady and unvarying indication
at a constant engine speed, and moved up and down smoothly as
the motor was speeded up and slowed down.
"Well, it seems to be all right, but I still would like to
be sure the indication is accurate at slow speeds," Jerry fretted.
"We both know bottom-of-the-scale meter readings are often less
dependable than those shown in the top half of the scale."
"Maybe so, but since our square-wave generator won't go below
20 cycles, it looks as though 2400 rpm is the lowest engine
speed we know is accurate," Carl observed.
"Wait a doggoned minute!" Jerry suddenly exclaimed, clapping
an open palm to his forehead. "When the engine is running 400
rpm, the tachometer is receiving 200 shots a minute from a single
spark plug. And since all six plugs fire once every two revolutions,
the coil is putting out 6 x 200 or 1200 shots a minute, right?"
"And when the tachometer is receiving 1200 pulses per minute,
it reads 2400 rpm. Can you see where I'm heading?"
"Yeah, I sure can. All we have to do is connect the pickup
of the tachometer to the hot lead from the ignition coil and
adjust the idle until we get an indication of 2400 rpm. Then
we reconnect the pickup to a single spark plug, and if the meter
action is linear we should get a reading of 400 rpm."
"And 400 rpm is very close to the slowest speed we'll need
to read. So if the meter indicates correctly there, we can depend
on it over the whole scale."
The lead from the coil to the distributor was arranged so
that a temporary connection could be made to it. Then Jerry
adjusted the idle screw until 2400 rpm was indicated on the
meter. Next, he used a pair of plastic photography tongs to
transfer the input connection of the tachometer from the high-tension
terminal of the coil to a spark plug.
"What does it read?" he called to Carl. "It might be just
a freckle low," Carl said slowly as he peered closely at the
meter, "but it's so close to 4 μa. that you can't tell the
"Good!" Jerry said with satisfaction.
"We certainly went to a lot of trouble to make sure this
thing was telling us the truth," Carl observed, turning off
"When it comes to test equipment of any kind, either you
have confidence in it or it's no good," Jerry remarked. "The
time a technician takes to make sure his instruments are accurate
is never wasted. Working with a meter whose readings you're
not sure of is like using a rubber ruler to build a house. But
now I've got the connection back on the spark plug, so what
say we taxi around a bit with our tachometer?"
"Be my guest!" Carl said, and he opened the car door for
Carl & Jerry: Their Complete Adventures is now available. "From 1954 through 1964,
Popular Electronics published 119 adventures of Carl and Jerry, two teen boys with a passion for electronics and a knack
for getting into and out of trouble with haywire lashups built in Jerry's basement. Better still, the boys explained how
it all worked, and in doing so, launched countless young people into careers in science and technology. Now, for the first
time ever, the full run of Carl and Jerry yarns by John T. Frye are available again, in five authorized anthologies that
include the full text and all illustrations."
Carl & Jerry Episodes on RF Cafe
- Carl & Jerry: Anchors Aweigh, July 1956
- Bosco Has His Day, August 1956
- The Hand of Selene, November 1960
- Feedback, May 1956
- Abetting or Not?, October 1956
- Electronic Beach Buggy, September
- Extra Sensory Perception, December
- Trapped in a Chimney, January 1956
- Command Performance, November 1958
- Extracurricular Education, July
- Treachery of Judas, July 1961
- The Sucker, May 1963
- Stereotaped New Year, January 1963
- The Snow Machine, December 1960
- Extracurricular Education, July
- Slow Motion for Quick Action,
- Sonar Sleuthing, August 1963
- TV Antennas, August 1955
- Succoring a Soroban, March 1963
- "All's Fair --", September 1963
- Operation Worm Warming, May 1961
- The Crazy Clock Caper, October 1960
- Two Detectors, February 1955
- Tussle with a Tachometer, July 1960
- Therry and the Pirates, April 1961
- The Sparkling Light, May 1962
- Pure Research Rewarded, June 1962
- A Hot Idea, March 1960
- The Hot Dog Case, December 1954
- A New Company is Launched, October 1956
- Under the Mistletoe, December 1958
- Electronic Eraser, August 1962
- Blubber Banisher, July 1959
- "BBI", May 1959
- Ultrasonic Sound Waves, July 1955
- The River Sniffer, July 1962
- Ham Radio, April 1955
- El Torero Electronico, April 1960
- Wired Wireless, January 1962
- Electronic Shadow, September 1957
- Elementary Induction, June 1963
- He Went That-a-Way, March1959
- Electronic Detective, February 1958
- Aiding an Instinct, December 1962
Posted July 22, 2014