12v Shutdown Delay

[K7VZ]

I was thinking of wiring a radio in my Jeep that is powered through a relay connected to the ignition switch. I want the radio to turn on and off with the vehicle. My question though, how would I add a shutdown delay circuit. What I'd like to have it do is leave the radio on for 5 minutes after the ignition has been turned off. Do I just add a capacitor and timer circuit to the relay? Has anyone done something like this before?

[KD7WCD]

You mignt look at this APO3 Automatic Power Off Switch by West Mountain Radio. http://www.westmountainradio.com/product_info.php?products_id=apo3_pp

Or just put that second battery in and run the rig off of that and then if the radio runs it down by accident you are not stranded.

[K7VZ]

Sweet! That's what I wanted. I knew I couldn't be the first one that has thought of this.

[N7TLW-WQGX468]

Some thoughts:
It's a nice box, but from the description, it looks more like it's a low-voltage shutoff switch. I thought the idea was for

...the radio to turn on and off with the vehicle.
If the idea is to

leave the radio on for 5 minutes after the ignition has been turned off.
there are a couple of ideas that come to mind.

Do I just add a capacitor and timer circuit to the relay?
An RC circuit would be a simple way to do it, but you would want definitely want to use it to control a relay or a transistor.
A 555 timer in monostable (one-shot) mode would give a more precise time.

Has anyone done something like this before?
I used to use a spare automotive defroster control module. This module had a built-in timer that would automatically turn off after a fixed time. I enabled the timer manually, but you could use a one-shot timer triggered off the ignition switch. Be sure that the module uses a timer; some of the early ones used a bimetallic switch instead.
You might also consider shutting off your relay while the starter motor is running to prevent voltage spikes from killing your radio.
Just a few thoughts....

[KD7WCD]

I think the idea behind it is that it starts its timer when the voltage drops once the engine is off and the alternator is no longer providing power. Once that drops off then the timer starts and turns off the power if it dose not come back up to the prescribed voltage. The nice thing with this set up is that it dose not have to monkey around with tying into the ignition system and it is more or less plug and play.

I know I saw an article in QST at some point not to long ago for a DIY project to do the same kind of a thing but have not been able to find it. If I do I will post it. It would be easy enough to build a timer/relay that could tie into the ignition system.

[W7TXR]

I have an idea though I've never tried it and also could be totally useless because I'm not familiar with 2007 thru current Jeep - Mine is 2006 Wrangler and does not have this function.

First: On later vehicles, the car (Jeep?) radio does not turn off when you turn off the ignition key until a set timeout or you open the door. If your Jeep does not do this then reading further down is a waste of time.

Idea: If it does have this type of delay, I'd guess you could splice into the radio power and use this voltage to turn a relay off when the Jeep radio turns off. Your 12 volts for the ham radio (assuming it's directly from the battery or other high current source) is routed thru the relay so that when the car radio shuts off after time out or when you exit the vehicle, opens (turns off) the battery 12 v to the ham radio.

My 2 cents....... TXR

[N7TLW-WQGX468]

I think the idea behind it is that it starts its timer when the voltage drops once the engine is off and the alternator is no longer providing power.
Ah, you're right. I didn't read carefully enough:
"When supply voltage drops below the Automatic Power Off Switch set point and stays there for longer than the time limit."
Important point, "for longer than the time limit."
I do like that it is simple, doesn't require anything but the power gozintas and gozoutsas.

[N7TLW-WQGX468]

I have an idea though I've never tried it and also could be totally useless because I'm not familiar with 2007 thru current Jeep - Mine is 2006 Wrangler and does not have this function.

First: On later vehicles, the car (Jeep?) radio does not turn off when you turn off the ignition key until a set timeout or you open the door. ....
Idea: If it does have this type of delay, I'd guess you could splice into the radio power and use this voltage to turn a relay off when the Jeep radio turns off. Your 12 volts for the ham radio (assuming it's directly from the battery or other high current source) is routed thru the relay so that when the car radio shuts off after time out or when you exit the vehicle, opens (turns off) the battery 12 v to the ham radio.

My 2 cents....... TXR
another 2 cents worth having.