This circuit rejects frequencies around 1 kHz and can be adjusted using C1. Equivalent operational amplifiers can be used, and the power supply must be symmetrical.
This circuit triggers the LED when the monitored battery voltage drops below 9 V for 12 V...
We found this circuit in a Radio Electronics from January 1967. The SCR is already the C106, which...
This power source with silicon diodes at a time when valves were still used was found in Radio...