Auto-sensing relay CV - how to?

I would like to make a sort of auto sensing relay circuit to switch between input. The situation is that I have a pair of RCA jacks on the back of a unit, and a 3.5mm on the front panel. I would like the audio from the rear panel RCAs to be default, but when a signal is fed/jack is plugged in to the front input it automatically switches to that source.

This could done with a switching jack for the front panel, however I would like to avoid dragging the rear input signal all the way to the front panel to pass through the switching contacts, so I am thinking of a relay to switch between the two inputs, in the middle of the unit to minimise distances.

I think the auto switching could be done with a comparator on the front panel input, which outputs a CV for the relay when the signal is above the threshold. The only problem I could see with is the relay inadvertantly switching back during a short pause in the audio, but this could be resolved by a 10 second or so delay for the relay switching back. Ideally I would like it to auto switch when the jack is plugged in, but I'm not sure of a way to this without running a CV through the jack?


Thanks

KISS... a switching jack is very simple are generally lower distortion than electronic switching,.

JR

http://sound.westhost.com/project38.htm

Run the signal into an opamp. Rectify the output which will drive a transistor to control the relay. Put a capacitor between the rectified signal and ground before the base resistor. The cap will be charged with the signal and discharge through the base resistor and keep the transistor on when there is no signal. But don't forget after removing the jack the same time delay will remain before the RCAs are activated.

Here is an example with a fet. Replace the 0.1uF cap after the rectifier with a largish electrolytic, put a trimpot in series with it to set the discharge time.

http://www.discovercircuits.com/DJ-Circuits/audiodetect1.htm

OK thanks for the quick response and good links, I think I will re-evaluate how much I really need this as well and then decide, an extra 6 inches of cable can't be that bad!

I would use a switching jack to control a relay. That way you can keep the default audio path short and just bring the control voltage to the front. Or if you don't have a switching jack, use a normalled balanced jack where the ring is grounded and normalled to the base of an NPN relay driver transistor and a pull up-resistor to B+. When nothing is inserted, the base sits at 0V and when inserted, the normalled connection to ground is broken and the base is pulled up to B+, switching the relay.

put a small toroid coil on the input line, this will cancel extraneous fields, when the music comes thru the toroid, amplify it then rectify it, which can all be done with a dual opamp, then feed the output to a latching relay, use any transistor on the output of the last opamp stage (dc) to pull in the relay,

then have a reset sw on the front panel to unlatch the relay, with led indicators for the high end model,

let me know if you need the circuit,

I was thinking about doing this on a preamp. The method I had in mind was similar to that used in stomp boxes - let the TS jack in a TRS socket complete the circuit.

So your relay would get the +5V (or whatever), but the coil's -ve terminal would be connected to the Ring on a TRS jack socket. When you plug a jack in, you establish the voltage drop across the relay required to switch it to the jack input.

I wouldn't personally bother using any sort of comparator based signal present detection... far too complicated for my liking!

Matt