This tone recognizer circuit that relies on R5 and C1 was found in the December 1994 issue of Electronics Now. The circuit operates on voltages from 6 to 12 V typically.
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...