This circuit for audio signals was found in the March 1990 Radio Electronics magazine. The circuit uses a unusual optocoupler that can be replaced with an equivalent one.
This interesting source of very low voltage for reference finds applications in the...
This sonic switch, sound relay or simply vox uses a medium impedance speaker as a microphone. The...
Using red LEDs with ignition voltages of the order of 1.5 V we can make a multiple voltage...