Category: Electronics — Status: Complete

One of my favourite projects, mostly because I did the entire thing from start to finish, from component selection, schematics, simulation, layout, testing and board fabrication (the fabrication was done elsewhere, but I did all the routing and placement). It is a simple (reasonably) auto-power-off circuit for use with 12V automotive systems. The idea is that once the battery drops below 12.5VDC it should cut off power to the devices, after a time delay. This time delay prevents rapid on/off cycling of the device when the voltage conditions are thresholding around 12.5VDC (a precision voltage reference and comparator circuit are used to trigger the transition). It also provides a convenient way to leave something on for a few minutes after turning off the car. I use it with my APRS station in my car.

The design uses a PIC microcontroller to handle the logic of the device and allows for very low power operation and some feedback and user interface design. You may press a button to turn it on or off, or hold the button to force it on with no automatic off. (There is no forced-off mode)

It has two 9 amp peak outputs when it has a heatsink. Without a heatsink, it can only sustain around 3 amps of current. The first output turns off after 1 minute of low voltage, the second after 20 minutes of low voltage.

All in all, I made 10 of these devices as my prototype (costing about $17 each) and make personal use of three of them. The photos below show an assembled device. When I get the chance, I’ll put up the schematic and board design and so forth.

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