This circuit detects when an audio signal exceeds a certain level or even occurs due to an object falling, a door slamming or other. The circuit is from a Radio Electronics from January 1980.
Suggested by Motorola documentation from the 1980s, this circuit can be implemented with...
This oscillator reaches 50 MHz with an overtone crystal. The circuit is from Electronics Now...
This modulator circuit for low intensity signals is from old documentation. The integrated circuit...