Found in a Raytheon transistor manual from the 1950s, this circuit uses one of the first commercially available transistors, the CK722. The circuit is for using a small speaker as a microphone.
This circuit is from an April 2000 Poptronics. The magazine no longer exists, but the circuit can...
We found this circuit in the Radio Electronics Handbook magazine special edition of 1992. The...
The integrated circuit used in this project is uncommon today, but the configuration can be...