This interesting constant current source circuit was found in the September 1989 issue of Radio Electronics magazine. The power supply must be dual.
This circuit triggers the LED when the monitored battery voltage drops below 9 V for 12 V...
The gain for this circuit is given by the ratio between R2 and R1. The circuit is suggested by...
Only for reference this is a circuito f a commercial receiver of the year 1958. The circuit uses...