Detecting 240volt AC and operating emonTH digital input


I am using an emonTH to monitor domestic hot water temperature. I now want to add monitoring the Thermostat switch operating. I can separate the cores to and from the thermostat. Is there a non contact way of monitoring the wire from the thermostat such that it can signal a digital input. High when voltage present, low when no voltage present. Ideally battery powered. At worst I could put in a mains relay that is tricky as there appears to be no neutral in the wiring. I am guessing it is very low current.

Here’s an idea - but no promises that it’ll work:

If you can get hold of an input on the emonTH processor, then a low value capacitor (i.e. a few turns of insulated wire wrapped around the high current wire on the load side of the 'stat) might pick up enough voltage to show up (digital or analogue) - but you’d need to do something in software to get a steady signal, and determine how big a capacitor (how much wire to wrap around) you needed.

If you’re not counting pulses, that’s an obvious input to try.

I’m not sure if it was on here or elsewhere but I’ve come across a neon and an LDR taped together before now.

[Not strictly contactless I know, unless your thermostat already has a neon! but once a neon is fitted there is a level of isolation there.]

And it would be OK to earth the other side of the neon - or, if you don’t need to distinguish between 'stat closed and circuit isolated, wire the neon across the 'stat contacts.

Thanks, I will look at sketch and experiment.