This circuit was found in the September 1971 Radio Electronics magazine. The buzzer can be replaced by a relay according to the supply voltage. R1 adjusts the sensitivity.
This small blocking oscillator for approximately 1000 Hz was found in an American documentation from the...
An audio signal is applied to the input of this circuit. Depending on the position of the selector...
Using only one LM107 operational amplifier, National Semiconductor suggests the circuit in the...