Jameco Electronics Puzzler: Voice TransmitterBy Forrest M. Mims III
Conductive foam is the missing componentHow It Works
The image below shows the circuit with the two input leads inserted into a piece of conductive foam plastic. For test purposes, insert the two wires directly into opposite sides of conductive plastic. Pressing down on the foam plastic reduces its resistance and increases the frequency of the tone emitted by the speaker.
The mystery component is conductive foam plastic
Many different resistive sensors can be used to alter the frequency of a tone emitted by a 555 pulse generator. This method is certainly one of the simplest--and it allows electronic musician Joe Rhythm to control the tone frequency with a spare finger or even an elbow.
For practical use, build a sturdy version of the sensor by sandwiching the conductive foam between two pieces of copper-clad PC board or other conductive surface to which wires can be soldered or attached. Hold the sandwich together with nylon 2-56 screws and nuts.