This conventional relay light detector circuit uses a 741 as a voltage comparator. The sensitivity adjustment is made in R1. The supply voltage depends on the relay used.
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...