I would like to use my audio interface to trigger a mains relay, as soon as the audio interface is turned on. Unfortunately, the interface turns on electronically, as opposed to using a mechanical switch. (I need to hold a button for 3 seconds to turn it on. Annoying but c'est la vie).
Therefore, I figured using the S/PDIF output of the audio interface to trigger the mains relay, by cascading two relays: one relay triggered by the S/PDIF output, which in turn triggers the mains relay.
So with the audio interface turned on and nothing playing, I measured the S/PDIF output on the scope and Vmin and Vmax are around -700mV and 500mV respectively, yielding appx. 1.2Vp-p and about 4mV Vac measured with an AC multimeter.
Unfortunately, I reckon 4mV is too low for triggering any kind of relay.
For raising the voltage, I looked at level shifting, but I it's problematic because as demonstrated by the Vmin/max, S/PDIF is a differential signal which makes things trickier, and besides, as far as my limited knowledge goes, I think level shifting needs a higher base voltage anyway.
Finally I thought about amplifying the S/PDIF signal with a MOSFET but I've no experience with those and I've no idea how to go about it.
I prefer to avoid something like an Arduino.