Indicator for rechargeable battery

I present to you a circuit of very small dimensions, designed with a few SMD components; easy to implement in narrow spaces, important for some electronic projects. The LED D2 illuminates when the voltage applied to the circuit input drops below a prescribed value with the help of the semi-adjustable RV1. It polarizes at the base Q1, under normal conditions, it being open and blocking Q2 by the voltage C-E lower than the voltage for opening the B-E junction assembly Q2-R3-LED D2. If the voltage drops below the prescribed value, Q1 locks, and Q2 is correctly polarized at the base by R2 and the LED signals red, which means low battery.

The circuit is calibrated with the help of an adjustable voltage source and a voltmeter, connected to the input, and the corresponding regulation of the RV1 is mandatory.

Have fun with it !