This circuit was found in an American documentation from the 80's. The sensor is a special integrated circuit, uncommon in our times. There are modern equivalents.
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...