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.

  • \$\begingroup\$ AC p-p voltage is Vmax - Vmin, not 4mV. It's a high frequency signal, apparently too fast for the multimeter. So this should be doable. \$\endgroup\$ Dec 11 '20 at 16:35
  • \$\begingroup\$ @BrianDrummond thanks for the quick reply, I updated the text, 4mV was measured with a multimeter. As I wrote Vp-p is about 1.2V \$\endgroup\$
    – pilau
    Dec 11 '20 at 16:37
  • \$\begingroup\$ SPDIF signal is at audio samplerate speeds with manchester coding. I wonder if a series capacitor with a diode would give you a voltage good enough to enable a mosfet. \$\endgroup\$
    – Jeroen3
    Dec 11 '20 at 16:52
  • \$\begingroup\$ Do you maybe have an example circuit? \$\endgroup\$
    – pilau
    Dec 11 '20 at 17:14
  • \$\begingroup\$ S/PDIF on a coaxial interface is not a differential signal. It should just be nominally 1Vpp signal with no DC component. \$\endgroup\$
    – Justme
    Dec 11 '20 at 18:06

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