This project needs a power supply. Since this is all going in my car, I'm going to use your standard 12 volt cigarette lighter as a power source. However, it is always powered, and I don't want the devices connected to it to drain the battery when the car is off. Since I also don't want to tap into my car's ignition relay, I'm going to detect engine activity by monitoring voltage changes. While the car is off, the cigarette lighter will measure about 12.3 volts. When the car is running, the voltage will be about 14.6. I can detect this voltage change with an Attiny85, which will control a relay that controls power to the connected devices. I could have not used the Attiny, but I like the flexibility and ease of use it provides.

I also want to be able to use the system while the car is not running, but I cannot reliably detect when the car is set to ACC. To get around this, I have added a button that can be used to enable the power supply for 10 minutes at a time. I could have used a simple toggle switch, but I am forgetful a piece of trash who has managed to lock their keys in their car multiple times. (always carry a spare in your wallet)

This video shows basic operation. The red led is lit when the relay is activated, and vice-versa. The button is currently set to keep the relay on for ~10 seconds for testing purposes. Within the next few days, I'm going to get this all soldered up on some protoboard and get it in some kind of enclosure. Need to keep things pretty and organized somehow.

Warning: feature creep may be starting to set in