This circuit, which is present in other points of this website, was found in a 90s Hands On Electronics. The circuit activates a buzzer with an oscillator.
This circuit is from the 80's and can be implemented with modern operational amplifiers equivalent...
Placed near a common receiver, this oscillator is intended to generate a beating signal to receive...
This 2000 circuit can be used to match the low impedance of a microphone to the high input...