[aprssig] APRS in-dash display

[Jason KG4WSV]

On Sat, Jan 16, 2016 at 11:24 PM, Greg D via aprssig <aprssig at tapr.org>
wrote:

> 4.  The Pi will boot itself when the car turns on, but I need a way to
> automatically safely power down the Pi when I turn the car off. Any ideas
> on this?
>

Monitor system voltage - there's a clear difference when the engine is
running (specifically when the alternator is on) and when the system is on
battery only.  IIRC that's going to be in the vicinity of 13V or 13.5V.
Use a voltage divider (maybe add a zener for spike protection) to feed this
to an ADC on the Pi (it does have ADCs, right?) to monitor this voltage,
and shut down based on the voltage drop.

You could hit scenarios where the thing shuts down before the accessory
voltage goes off and it leaves you in an undesirable state.  I have a
circuit (small AVR) that turns my D700 off or on depending on voltage (the
D700 is wired directly to the battery).  The AVR current draw is so small
it doesn't hurt to leave it on all the time, at least for my daily driver.
You may think about something similar.

I'm not familiar enough with the Pi to offer any more specific suggestions.

-Jason
kg4wsv
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