You can add a timer using a capacitor, transistor, and a variable resistor but no voltage sensing. If you want all that then spend the $69.99. Using a simple relay costs maybe $10 and you don't need voltage sensing or time delay IMO. Why would you need it? In a vehicle I used to won I ran a 12 volt buss from the battery back to a small board loaded with fuses and a large relay. I had access to both constant and switched 12 volts this way. When I started the vehicle power was switched to three radios, a radar detector, and a stereo amp. When I turned the key off they all shut down. If i wanted to listen to a radio without the vehicle running I moved the ignition key into the ACC. position and the relay pulled in turning everything on. I can't really think of a reason to have your radio turn off after you shut off the ignition and exit the car, walk halfway across the yard .............
...runna pile-up on 6m SSB