Auto Power Off (APO1)
An automatic power cutoff circuit for 12V systems that prevents battery overdischarge in mobile and solar applications.
One of my favourite projects—I did the entire thing from start to finish: component selection, schematics, simulation, layout, testing, and board fabrication.
APO1 is an automatic power-off circuit for 12V automotive systems. When the battery drops below 12.5VDC, it cuts power to connected devices after a configurable time delay. This prevents rapid on/off cycling when voltage hovers around the threshold and provides a convenient way to leave equipment running briefly after turning off the car.
I use it with my APRS station in my car.
Design
A PIC microcontroller handles the logic, enabling very low power operation with user feedback. A button allows manual on/off control or forced-on mode (no automatic shutoff).
A precision voltage reference and comparator circuit trigger the transition at exactly 12.5VDC.
Specifications
- Two 9 amp peak outputs (with heatsink)
- 3 amps sustained without heatsink
- First output turns off after 1 minute of low voltage
- Second output turns off after 20 minutes of low voltage
I made 10 prototype units at about $17 each and personally use three of them.