Low Battery Detection Circuit
Voltage Supervisors are devices commonly used to shut down a microcontroller to prevent brown-outs from causing havoc in the microcontroller’s memory.
They can also be used as a simple circuit to detect a low voltage condition on a battery.
To give an example a single 1.5V Alkaline AA battery has limited usefulness after it has reached 1.1V.
( Image used is from Energizer datasheet https://data.energizer.com/pdfs/EN92.pdf )
In our example circuit we’re going to go with four AA batteries in series, which would start out at 6V, and then select a voltage supervisor that will tell us when the batteries are getting down to around 4.5V.
To calculate the current limiting resistor for our supervisor with a 4.55V threshold and a diffused red LED with a 2.1V Forward Voltage:
( Threshold voltage - LED Forward Voltage ) / LED Current
4.55V - 2.1V = 2.45V
2.45V / 0.008A = 306 Ohms
The threshold voltage chosen can vary depending upon the amount of warning time needed before the battery is dead.
Through hole TO-92-3 package voltage supervisors are becoming harder to come by, to help widen the perspective options for through hole prototyping here’s a SOT-23 adapter breakout board, and some mounting clips to create a panel mounted LED indicator:
|Capital Advanced Technologies SOT-23 Adapter Breakout Board Part Number: 6103|
|3mm and 5mm Bezel Mounts for Panel Mounting LED Indicators|
The voltage supervisor selected needs to be Open Drain/Open Collector Active Low type that can sink enough current to power the LED. To conserve energy the LED current limiting resistor value can be experimented with to the highest resistance value that still allows the LED to be seen.