This sensitive 5-band circuit was found in a September 1995 issue of Electronics Now magazine. The headphones, for greater sensitivity, should be high impedance.
With a strong positive feedback (via a 10 M resistor) we can get the trigger setting for the...
The critical component of this circuit is the transformar. We found it in an 1980s documentation....
This old oscillator using germanium transistors has a high impedance headphone as inductor to...