by Harry Lythall - SM0VPO

I have already mentioned that Operational Amplifiers were developed for use in computers but are in fact misused in analogue electronics. So what did they use before the Op-Amp? Resistors! Here is a simple circuit than can be used to multiply and divide using only resistors.

The circuit is very simple and needs no active devices at all. The battery voltage can be anything you like. I used 1.5 volts. Consider the threre potentiometers drawn vertically, number them VR1, RV2 and RV3 from left to right. RV1 and RV2 are linear taper pots and are calibrated 0-10. RV3 is also a linear taper pot but is calibrated 0-100. Here's how it works:

RV1 divides that battery voltage by a value (5 = 50%). RV2 further divides that voltage (5 = 50%). So 50% x 50% = 25%. RV3 must be set to 25 (25%) to get a zero ballance on the meter. In this example we have used this computer to calculate 5 x 5 = 25. You could have performed a division: 25 (RV3) divided by 5 (RV2) = 5 (rotate RV1 for zero ballance on the meter). The horisontal pot (RV4) in series with the meter is adjusted for best sensitivity as you come close to an answer. Begin with RV4 at maximum or you can bash the poor old meter against it's end-stops.

An ordinary panel-mount potentiometer has a turn of 300 degrees. Calibrate it with a large pointer knob and make a scale measured with a protractor - 30 degrees for every whole number RV1 and RV2, and 30 degrees for every 10 at RV3. You nay want to glue a perspex or plastic pointer under the knob to extend it. The whole computer is dirt cheap but before anyone asks me - NO! You cannot run DOS, Windoze 2000 or UNIX operating systems on it! You can make it more accurate by reducing the value of RV1; a 1K0 pot will give you an accuracy around 1% which is probably better than you can read the scale.

Best regards from Harry - SM0VPO.

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