I love the fact that I can turn the radio on in my 2001 A4 without needing a key in the ignition. And I love the fact that it turns off automatically after about 30 minutes (or is it 60? I forget now).

Is this a function built into the head unit, or into the car's wiring itself? Can I take advantage of this feature with an aftermarket head unit?

 

It's a feature of the stock head unit.

With the stock radio you have two main power wires: constant power (basically a direct connection to the battery) and switched power (when the key is in the ON position).

Most aftermarket head units only power up and stay on when they receive switched power. Some may turn on when a button is manually pressed but this is still a head unit feature.

Here's what you could do: add a relay in to the switched power wire going to the aftermarket head unit, so that the relay will turn on whenever the key is ON or whenever a switch or something else energizes the relay. Would be pretty easy to do.

But this wouldn't turn off the head unit automatically, would need to be done manually (by turning the switch off).
So then you could build a timer circuit to automatically turn off the power after a certain time period, but this gets a little more complicated.

 

My aftermarket head unit will not turn on without turning the key. But I'll easily sacrifice that feature for the plethora of features I get with my aftermarket one.

 

There's no reason other than non-knowing that you couldn't build up a circuit to give this feature back! I never had a stock radio in my 2001 and wasn't aware of this feature.

I have a Nexus 7 tablet as a "head unit" / entertainment / nagivation / internet / etc. experience in my A4. It uses the factory switched power wire and this kinda bothers me because sometimes I want the tablet to stay on after turning the engine off, but turning the key OFF starts the turn-off sequence.

Anyways, I'm thinking about adding a relay to the switched power line like I mentioned in my previous post, but using the "key removed" signal wire from the ignition switch to turn the tablet off when key is removed (instead of key OFF).

 

I've never found myself wanting to turn my radio on when the car is off. Don't see when I would use it either so I'm perfectly fine how it is currently.

My last car had the same feature but I also replace that with an aftermarket stereo, so for 4 years I've never found myself wishing I could turn my stereo on without turning the key to on. That's me though.

 

Thanks for your opinion. It's completely different from mine and did not really contribute to answering my question, but it was very exciting to read.

Thanks! That was actually helpful. I ended up buying this product:



Combined with a Funk switch (which I have room for in my dash), I can wire it up to turn on power to the entertainment system for XX minutes while the car is off. Using a relay, I can also wire it to cut off power to that timer while the car is running, which cancels any existing countdown and keeps the Funk switch from doing anything. This completely retains the original functionality of the OEM head unit.

Another side benefit to this is the ability to charge my phone for XX minutes while the car is off, since the USB hub is wired into the entertainment system.

That's what I'm doing as well.

 

In case you haven't seen my Nexus 7 build thread, and are interested:
Nexus 7 in 2001 A4, fully loaded!

 

That double din box is perfect! I've been looking for a simple double din-sized enclosure for a while, and all I kept coming up with are overpriced carputer enclosures with features I don't need. The box you're using there looks perfect. And $7.49 is certainly the right price Now I know to look for something called a "project box" instead (never heard that term before).

 

Would you be at all interested in a white plastic shell (to support the Nexus 7) like I used in mine? I have a bunch of materials to do it, but I only have a Nexus 7 2012-version for mockup.

 

Sure, That would be great!