The operational amplifier used is not common today, but the circuit can be implemented with modern operational ones. The gain is 1000 and the source must be symmetrical.
This circuit was found in Electronics Now. With it the dialing signals of a telephone are...
Found in American documentation, this simple circuit activates a buzzer with embedded oscillator. A...
I found this circuit in an old English magazine from 1979, but it can be easily assembled with...