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complete 2013 hardware

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.

The Problem

If you’ve ever left radio equipment running in a parked car, you know the sinking feeling of coming back to a dead battery. Car batteries don’t like being deeply discharged—it damages them permanently. But turning off the radio every time you park is a pain, especially for APRS trackers that you want to run continuously.

The APO1 solves this by automatically cutting power when the battery drops below 12.5VDC. That’s low enough that you get good runtime, but high enough that the battery isn’t damaged.

How It Works

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.

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. The threshold had to be precise—too high and you’d lose power unnecessarily, too low and you’d damage batteries.

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

The two-output design lets you prioritize devices. Critical equipment (like an APRS tracker that you want to run as long as possible) goes on Output 2 with the 20-minute delay. Less critical equipment goes on Output 1 and shuts down first.

The Build

I designed the schematic, simulated it in SPICE to verify the voltage detection behaviour, laid out the PCB, and had prototype boards fabricated. Then tested each unit to verify the voltage threshold was accurate.

Made 10 prototype units at about $17 each. I personally use three of them—one in each vehicle with radio equipment.

Usage

I use it with my APRS station in my car. The radio can run for hours after I park, tracking position and relaying packets, then automatically shuts down before killing the battery.

See Also