The Approxi-Meter... for When Close Is Good Enough
July 1958 Flying Models

July 1958 Flying Models

July 1958 Flying ModelsTable of Contents

These pages from vintage modeling magazines like Flying Aces, Air Trails, American Modeler, American Aircraft Modeler, Young Men, Flying Models, Model Airplane News, R/C Modeler, captured the era. All copyrights acknowledged.

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 an 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 kits.

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

Approxi-Meter... for when close is good enough, July 1958 Flying Models - Airplanes and RocketsThe 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.

Approxi-meter Schematic - Airplanes and RocketsThe 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 material.

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 50ยข 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 for 0.27M.

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 March 23, 2022
(updated from original post on 11/19/2010)