Here is a simple circuit in which an LED will continue flashing as long as the voltage of the battery to be monitored is above the set level.The transistors Q1 and Q2 are wired as a complementary astable multivibrator.The circuit can operate from a voltage range from 6V to 12V. The voltage level at which LED has to stop flashing can be set by adjusting the preset R4.When the battery voltage reaches the threshold the flashing frequency decreases and when the voltage goes below the threshold the LED becomes OFF.The diode D1 is used to compensate the changes in the base emitter voltage of Q1 due to temperature changes.
Circuit diagram with Parts list.
- Assemble the circuit on a general purpose PCB.
- Batteries from 6V to 12V can be monitored by this circuit.
- The diode D1 must be a Schottky diode.