70V Audio Sensor

Thread Starter


Joined Aug 23, 2011
So I am working on a system that requires feedback from several relays to let me know when the relays are latched. Mostly, it is just sensing when 24V has been passed through the NO contact on the relay and that is pretty straightforward. The hard part that I am comming to you all for help with is one of the relays, a DPDT, has a 70V audio signal along the line.

Unfortunately, just knowing when the relay has latched isn't enough. I have to know if there is any audio on the line. So basically, I need a VOX type circuit that gives me 5V or 0V when there is audio and changes when there isn't audio to 0V or 5V respectively.

A little about the 70V audio: From what I have measured with a Multimeter, the AC voltage is 140V and when the amp is turned down, the voltage and current decrease although I do not know the relationship.

I would like this circuit to be able to sense down to a whisper along the lines. The circuit's output will then be put into a GPIO pin running on 5V logic.

Another constraint is that I have very limited board space and height and no ability to use through hole parts. And as always cheaper is better.

Thanks in advance!