July 1960 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
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.
The same is true with his long-running "Mac's
Service Shop" series of techno-dramas. In this adventure, the the two teenagers
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). Being set in
1960, this is one of the first appearances of transistors in circuits rather than
vacuum tubes. Transistors were still very mysterious - and even detested - by many
electronics hobbyists and professionals, so pieces like this helped the newfangled
technology gain acceptance.
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 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 column.
"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
"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 difference."
"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 the ignition.
"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 his friend.
Carl & Jerry: Their Complete Adventures is now available. "From
1954 through 1964, Popular Electronics published 119 adventures of Carl Anderson and Jerry Bishop, two teen boys with
a passion for electronics and a knack for getting into and out of trouble with haywire lash-ups 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
Vox Electronik, September 1958
- Pi in
the Sky and Big Twist, February 1964
Bell Bull Session, December 1961
Cow-Cow Boogie, August 1958
Picture, June 1955
- Electronic Eraser,
- Electronic Trap, March
- Geniuses at Work, June
- Eeeeelectricity!, November
- Anchors Aweigh, July
- Bosco Has His Day,
- The Hand of Selene,
- Feedback, May 1956
- Abetting or Not?, October
- Electronic Beach
Buggy, September 1956
- Extra Sensory
Perception, December 1956
- Trapped in a Chimney,
- Command Performance,
- Treachery of Judas, July
- The Sucker, May 1963
- Stereotaped New
Year, January 1963
- The Snow Machine, December
Education, July 1963
- Slow Motion for
Quick Action, April 1963
- Sonar Sleuthing, August
- TV Antennas, August 1955
- Succoring a Soroban,
- "All's Fair --", September
- Operation Worm Warming,
Santa's Little Helpers - December 1955
Two Tough Customers - June 1960
Transistor Pocket Radio, TV Receivers and Yagi Antennas, May 1955
Stomping, March 1962
- The Blubber Banisher,
- The Sparkling Light, May
- Pure Research Rewarded,
- A Hot Idea, March 1960
- The Hot Dog Case, December
- A New Company is Launched,
- Under the Mistletoe,
- Electronic Eraser,
- "BBI", May 1959
- Ultrasonic Sound Waves,
- The River Sniffer, July
- Ham Radio, April 1955
- El Torero Electronico,
- Wired Wireless, January
- Electronic Shadow,
- Elementary Induction,
- He Went That-a-Way,
- Electronic Detective,
- Aiding an Instinct,
- Two Detectors, February
- Tussle with a Tachometer,
- Therry and the Pirates,
- The Crazy Clock Caper,
Posted September 21, 2020(original 7/22/2014)