Low battery indicator

Some rechargeable batteries have a rule that they cannot be discharged below a certain voltage. The circuit provided here can indicate that the terminal voltage of a 12V rechargeable battery has dropped to the termination voltage. The indication method uses a flashing LED to express it.

The comparator chip LM319 (IC1) is designed as the core of the circuit. LM319 is a high-speed dual comparator chip. This article only uses one of the comparators. The 1.2V reference voltage of the comparator is generated by the bandgap voltage zener diode D1 (LM385), which is added to the comparator's non-inverting input pin ④. The voltage output by the voltage divider R2, R3 and VR1 is sent to the comparator's inverting input pin ⑤.

At the beginning, if the battery voltage is sufficient, the voltage of the inverting input pin of the comparator should be higher than the voltage of the non-inverting input pin, and the output of the comparator is at a low potential. IC2 is a multivibrator composed of a 555 chip. It does not oscillate when the reset pin ④ is at a low potential. Since the ④ pin of IC2 is connected to the output of the comparator, IC2 does not oscillate when the battery voltage is normal, and as a result, LED1 does not flicker.

If the battery voltage drops to 10.5V, the voltage of the comparator's inverting input pin can be adjusted by VR1 to be slightly lower than 1.2V. As a result, the comparator output becomes high, IC2 is activated and starts to oscillate. At this time, LED1 flashes, indicating that the battery voltage has dropped to the specified limit and needs to be charged immediately before it can be used again. Both IC1 and IC2 work with the 5V DC voltage output by IC3.