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help me design a time delay circuit for car install!

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Senior Member
Sep 12, 2012
I've had several nexus 7s installed in my own and friend's cars loaded with timur's, lineageOS, auotodroid and such. Of the two remaining, I drive mine daily so get enough charging time to keep the tablet charged enough to avoid complete depletion. The other nexus installed in a friend's car, however, only gets driven a few hours a week these days - meaning it would get discharged fully every now and then.

Some reddit entries suggest to add a double throw switch to toggle between ACC (normal operation) and constant hot (manual emergency operation). Well I thought maybe you could add a timer relay off ebay and let the tablet charge for a couple of hours, and then break the circuit to stop draining the car battery when the tablet is most likely fully charged. This would allow me to set off the switch once I am leaving the car and leave it to charge unattended. Surely a minimal drain for a day or two won't kill the car battery, but I am assuming there will be times when she won't drive for a whole week or so.

What I am wondering is, if I set it up this way, I (or whoever happens to be driving) would HAVE to remember to switch it back to normal opeartion when coming back to the car next day. Is there a better way to do this, say after it is done charging (a set amount of time has passed), it resets the switch position to ACC automatically? Better yet would be if it does the whole process all on its own, like so:
1) runs off acc while engine is running
2) switches to constant 12v when engine is off
3) switches back to acc after a preset amount of time (2hrs for now)