Battery Low Indicator

EE 59 March 1989 "BATTERY LOW" INDICATOR by J. Ruffell Today there are innumerable pieces of equipment that are powered by batteries, both dry and rechargeable. In many cases, it is difficult to determine whether those batteries are still fresh or fully charged or if they need replacing or recharging. Here is a small circuit that monitors the battery voltage and gives an audible warning when that voltage becomes too low. The indicator described here is small enough to enable it being fitted inside the battery-operated equipment, such as a portable shaver or receiver. It draws a current of not more than 1 mA, so that it does not noticeably increase the load on the battery. Circuit description The circuit is based on two opamps that are housed in Type TLC272 chip. Opamp Al, connected as a comparator, compares the battery voltage, applied to the inverting input via potential divider R1-(R3 + P1), with a reference voltage of about 4.7 V that is applied to the non- inverting input. Owi...