Jameco Electronics Puzzler Solution: Non-Potentiometer Potentiometer

By Forrest M. Mims III

Design a POT Compliant Pot?

While Leo Smith was pondering how best to design a substitute for potentiometers banned by the Federal Panel of Obsolete Technology (POT), he walked outside to watch the twilight glow in the western sky. Suddenly the security light outside his office switched on and triggered a banned light bulb in his mind that signaled a solution: the POT Pot.

Leo knew that the security light outside his office and the night light inside both employed a cadmium sulfide (CdS) photoresistor to activate a light when the resistance of the photoresistor was increased by the absence of light. He also knew that photoresistors were among the discrete analog components that had not been banned by the POT.

Leo's first thought was to make a passive volume control for the radio project simply by using a mechanical slider that could be rotated to alter the level of ambient light striking the sensitive surface of a CdS photoresistor. This method, however, would be highly unreliable and would fail entirely when the radio was in the dark. Leo decided the best solution would be to install the CdS photoresistor inside a light-tight corner of the radio and illuminate it with a small LED. A simple rotor equipped with a perforated vane would be rotated to alter the light received by the CdS cell, thereby providing a true variable resistor that was not banned by the POT. Leo quickly prototyped the volume control. It worked so well he named it the POT Pot.

Leo Smith's prototype POT POT
The essential ingredients of Leo Smith's prototype POT POT.

Going Further

You can easily experiment with Leo's solution using various LEDs and CdS cells available from Jameco. Leo's first test circuit used a red LED. But since CdS photoresistors are more sensitive to green than to red, would a green LED provide the same results with less current but a slightly higher voltage? These tradeoffs are best addressed with bench tests of various LEDs and a CdS photoresistor.

Another factor worthy of experimentation is finding the optimum outline of the slot in the rotating vane that controls the light received by the CdS photoresistor from the LED. With care it should be possible to design shapes for linear or log response. Keep in mind that the open slot can be replaced by a properly shaped opaque vane.

Back to the Electronics Puzzler >>