Here is an example of just
how far we have come in the realm of electronics. In a world where you can go to Harbor
Freight and buy a digital multimeter with an accuracy of 1% or better for a mere $3,
this article from the July 1958 edition of Flying Models illustrates the dedication that
was necessary in order to outfit yourself with even the most fundamental tools for flying
radio control airplanes. It was part of era where building your own electronic device
was less expensive than buying one prefabricated. Such was the case for the aircraft
radios, too, as evidenced by the number of advertisements in magazines of the day for
As is often true, there are good and bad aspects of building versus buying. Building
gives you the intricate knowledge of how everything goes together and functions along
with actually using the stuff you build. However, the ability to buy your equipment pre-built
leaves more time for honing flying skills. That last statement helps explain the incredible
level of competency exhibited by the young 3-D airplane and helicopter flyers.
APPROXI-METER -There's no need to buy an expensive meter for R/C flying
by Ted Strader
The barest necessities in R/C usually include a milliammeters
in the 0-1, 0-3 or 0-5 mA range to tune the majority of receivers to frequency.
With the basic parts acquired, the R/Cer's next desire is some form of indicator to
at least check batteries.
The Approxi-Meter is designed to turn your basic milliammeters into a simple volt-ohm-milliammeters
(VOM) using parts you may find in your junk box, and if not, components which can be
bought for less than a buck! To keep cost down a tube socket was used instead of a selector
switch for circuit transfers.
When finished you will be able to check three voltage ranges with reliable accuracy;
use the low resistance section to check continuity of receiver and battery leads, relay
points and general low impedance wiring (0 to 2K ohms); use the high resistance section
to check coil windings, resistor continuity, shorted condensers and general resistances
from 2K to infinity.
Diodes can also be checked but in reverse, i.e., contact the cathode end (bar) to
the negative prod and other end to positive prod. If diode is good, little or no movement
will result in the high resistance range. (A commercial VOM is so wired, through its
selector switch as to maintain polarity for this operation).
The sketch will clarify most points - a few notes may 0-3 and 0-5 milliammeters use
the same value resistors; 0-1 mA meters use the resistor values in parentheses and have
the same full scale readings as the 0-5 meters.
All connections are made between tube socket lugs and meter post lugs. The lead from
positive meter lug passes through the tube socket center lug and may be soldered to it
for more support.
batteries in the resistance check circuit will last as long in the Approxi-Meter as on
the shelf if not shorted, and do not necessarily have to be absolutely fresh. The selector
pin end of the negative prod is a small wire brad soldered to the wire and covered with
a short length of neoprene tubing. Prod ends are phone tips, heated and forced into short
lengths of poly tubing after connections are made. These can be altered to suit available
The case was fashioned from thin sheet aluminum. A cookie sheet will supply a good
grade of sheet or you may have a small box into which the parts will fit.
One last note: In case you'd like to invest about 50¢ more you can get 5% tolerance
resistors and make the 0-1 milliammeters more accurate by employing resistors with values
closer to what they actually should be. Substitute 5.1K for 4.7K, 75K for 82K and 0.24M
Users of 0-3 and 0-5 milliammeters can get more accuracy. from the high voltage range
by substituting a 51K for the 47K.
We used 10% tolerance resistors only because the idea was to keep the cost down as
low as possible.
Though hand-built this meter has all of the appearance of a professionally made unit.
A single 0-5 milliammeters is all you need to make this combination Volt-Ohm-Ammeter
for R/C tests.
Posted November 19, 2010