Detecting audio over an RCA cable, and tripping a relay

[signal15]

I put in a multi-zone audio system and it's got a paging function.  If 12v is applied to the paging terminals, all zones switch to Input 1 and play whatever is coming into that input.  I've got an Android phone with Vera Alerts on that input for announcements.  Right now, I have to add a line of code that trips the relay for a few seconds, and then plays the alert.
 
What I'd like to do is to have something that detects audio coming over the RCA input, and then sends 12v to my paging terminal.  
 
Is there anything out there that will do this?  If not, how exactly would I go about building something (preferably with the ability to hold the trigger for a configurable amount of time after the audio signal goes away)?

 

[neillt]

Is it 12 volts or a contact closure?  I know you said 12v, but I just wanted to make sure.

 

[signal15]

Is it 12 volts or a contact closure?  I know you said 12v, but I just wanted to make sure.

 
It's 12v dc.  So, if I could find something with a relay on it, I could just wire 12v to the relay.  
 
I found this thing:
http://www.quasarelectronics.co.uk/Item/cebek-pm-11-audio-signal-controlled-relay-switch-module
 
Looks like Smarthome had a couple of things also, but they are discontinued products.

 

[JimS]

What you are looking for is called VOX (voice operated switch or transmit).  It is used in ham radio (among other things) so you don't have to push the transmit switch.  It should be pretty easy to find a circuit diagram or kit.  A search for "vox" and a few qualifiers like "voice operated switch" turns up lots of things.  One is:
 
http://www.rainbowkits.com/kits/vox-1p.html
 
Even shows the complete circuit ..
 
Typically these switches keep the contacts closed until the audio stops with a small delay so it doesn't drop out between words.

 

[wkearney99]

And bear in mind that a simple VOX switch will miss the first few seconds of whatever you're using a source. So don't depend on hearing whatever it detected from the absolute beginning.  For that you'd need something actually buffering and playing back the sound, which would likely be a lot more complicated/expensive setup.

 

[signal15]

And bear in mind that a simple VOX switch will miss the first few seconds of whatever you're using a source. So don't depend on hearing whatever it detected from the absolute beginning.  For that you'd need something actually buffering and playing back the sound, which would likely be a lot more complicated/expensive setup.

 
I can program my alerts to have an alert tone/sound before the message (which I have already done).  So this might be a non-issue.

 

[kevkmartin]

I use these for audio detection and switching. They have a 12v out trigger (status).

http://www.speakercraft.com/products/installation-accessories/electronics-control-accessories?page=shop.product_details&flypage=flypage_sc.tpl&product_id=190&category_id=18

 

[signal15]

I use these for audio detection and switching. They have a 12v out trigger (status).

http://www.speakercraft.com/products/installation-accessories/electronics-control-accessories?page=shop.product_details&flypage=flypage_sc.tpl&product_id=190&category_id=18

 
I bought one of these from MCM.  It arrived on monday, and it works *great*.  The only thing I noticed is that it slightly reduces the signal passing through it.  I have an android phone plugged into input B that does my voice alerts for HA.  When listening to music, if a voice alert triggered, it was way to quiet, even with the volume all the way up on the Android device.  I ended up having to run the Android device into a cheap $20 mixer first and crank the level there.  That mixer amplifies the signal enough so I can get my voice alerts to the same volume as the music.