Simple Battery Monitor Circuit

This simple Battery Monitor lights an LED when the battery voltage drops below 9 volts. It is an ideal add on circuit to monitor the charge level in 12 volt miniature batteries used in Portable devices or Alarm systems. In the standby state, LED remains off.

Working of the circuit is based on the base biasing of transistor T1. When the battery voltage is above 9 volts, base-emitter voltage will be same. This keeps both T1 and LED off. When the battery voltage reduces below 9 volts due to consumption, base voltage of T1 drops while its emitter voltage remains same since capacitor C1 is fully charged. At this stage, base of T1 becomes positive and T1 turns on. Capacitor C1 discharges through the LED and it lights.

Simple Battery Monitor Circuit


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  • Jim Keith

    There is nothing special about the CL-100 transistor other than its age and lack of availability. I would use the common 2N3904, but almost any small NPN transistor should work.

    While the LED will light as battery voltage decreases, I doubt that it will be bright or stay bring for any length of time–since it is simple, try and see.

    • Circuitdud

      Not likely to get a response at this date but…..
      Hmmm..not sure why even a “dead” 12 V battery would not keep the LED going brightly, in fact without a resistor in the circuit I would think it would burn out rather than go dim.

  • Brian

    What would the radio shack equivalents be for this circuit? I don’t have any transistors for T1 in my inventory, but I do have a radio shack down the block. Also what would I have to change to make this work for a 24v system where if the voltage is below 24v the light comes on? Thanks

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