Low Cost Battery Condition Indicator

This design combines power-on and low-battery indication, can operate with any battery voltage up to 15V, has very low current drain (2mA or less) and costs less than $3.50 with new parts. When the battery voltage is above a predetermined minimum, power on is indicated by what appears to be a steadily lit LED. In fact, the LED is being pulsed by a free-running relaxation oscillator formed by IC1c, one gate of a 4093 CMOS quad Schmitt NAND. The frequency of this oscillator should be at least 50Hz, so that it appears to be continuously on while at the same time drawing far less average current than a steadily lit LED.

Circuit diagram:
The series resistor for the LED needs to be selected for each battery voltage, to limit the current to a safe vale or you could use a fixed resistor and a series trimpot for flexibility. Low battery voltage is indicated by the LED pulsing at around 1Hz. The battery voltage is monitored by transistor Q1 and trimpot VR1. Once the voltage at its base falls below 0.6V, Q1 turns off and Q2 turns on to enable the 2-gate oscillator formed by IC1a and IC1b, which runs at 1Hz. The pulses from this oscillator are inverted by IC1d to gate the LED oscillator on and off. Calibration can be done with a variable bench power supply set to the lowest battery voltage you will accept. Power up the circuit and adjust VR1 until the LED pulses once per second.