This tone control was obtained in a Signetics documentation of the 90s. The integrated circuit can be practically any operational amplifier. The power supply is 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...