This relay triggered by voice or by another sound was found in an Elektor from the 90's. The circuit uses a 12 V sensitive relay and has no sensitivity control.
This circuit came out in a 1978 American publication, but can be assembled with transistors BC547 or...
The trigger signal for this circuit is an audio tone. Tones up to close to 100 kHz can be used. The...
This circuit was found in a 70s documentation. The transistor is from that time. L1 has 4 turns of...