This circuit makes use of a low impedance microphone. The relay is according to the supply voltage which can be between 9 and 15 V. The operational gain is approximately 100.
This 200 kHz oscillator was found on a 90's EDN. It consumes only 2.8 mW and has CMOS compatible...
The circuit is from the 1990 Motorola Linear Databook, but can be implemented with other operating...
This circuit was found in a 1977 Radio Handbook. The MOSFET supports an equivalent. Capacitor C must...