How can I actuate a relay coil from a headphone jack's internal SPDT?

Question

I want to plug my iPod which in turn supplies power to an external audio amp while sending iPod audio signal to same amp.

Is it possible to wire an 1/8" (3.5mm, mini) stereo jack in such a way that when the connector is inserted, a relay coil is actuated that powers on a car audio power amp? I have a 12V DC relay but how can I use the jack's internal switch to actuate the coil when the headphone jack's switch is essentially backward?

This is for a boat. The only solution I can come up with is to have the relay coil normally actuated but I think this would eventually drain the battery (I haven't done the math on that).

I have this Radio Shack version headphone jack with the internal SPDT but the switches are not isolated from the audio signal and are N.C.: http://www.radioshack.com/product/index.jsp?productId=2103451 Surely designed this way so that the audio signal is passed to the big-speaker-amp when the headphones are disconnected.

(this is where the images were to my minijack diagram and my relay diagram...oh well...I'm a new user and this is clearly spam...)

Answer 1

You want jack "C" at the bottom of this link: http://en.wikipedia.org/wiki/Phone_connector_%28audio%29

The tip and sleeve switches are electrically isolated from the plug so signal is transmitted uninterrupted.

There are 1/4" jacks here (http://www.minute-man.com/acatalog/1_4__Stereo_Jacks.html) that do this. I'm sure they have 1/8" jacks that do the same thing, but I didn't look.

I was looking for the same thing, but for switching inputs based on whether the plug was inserted in the effects return on a tube guitar amp. Can't be puttin' that signal level into the common when someone has it in their hands!

Answer 2

I don't believe this can easilly be done using just the jack.

You may be able to do something whereby you monitor the resistance across the terminals to determine if it's open-circuit, or look for any form of signal coming out of the jack, but that would all take constant power out of your battery and not give you the automation you're after.

You should be able to do it using the IO connector on the bottom of the iPod, however. This has both audio and power connections. (pinouts here).

You could use the power output (usually used to power things like FM transmitters) to switch the relay on via a transistor.