This circuit was found in Electronics Today International of the 80's. The circuit uses still common components, providing the indication through LEDs.
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...