This circuit was found in the September 1971 Radio Electronics magazine. The buzzer can be replaced by a relay according to the supply voltage. R1 adjusts the sensitivity.
The circuit presented may use any general purpose transistor. The indicator is an LED and...
The operational amplifier used in this project no longer exists, but the circuit can be adapted to...
This car battery monitor was featured in the January 1988 issue of Radio Electronics magazine. The circuit...