Looking for a wireless sensor that can read on/off state of a low voltage circuit | Home Assistant | Page 1

tiny compass Dec 26, 2025, 9:08 PM

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Hi, I've been wanting to monitor the activity of the humidifier on my home's central heating system. I don't need to control it; I just want to monitor the on/off state of the solonoid (circuit) which releases water into it. At first I thought maybe I could use one of my cheap Aqara door sensors and that it might get triggered by the electromagnetic field, but that didn't seem to work.

Does anyone know of any products which could work similar to one of these door sensors that could monitor the on/off state of a 24v circuit? I was hoping to find something in a similar form factor as these Aqara door sensors, because they are tiny, the battery lasts forever, and I've already got a Zigbee2MQTT bridge set up to feed into HA, but any suggestions are welcome; I'm just not really sure what to search for to get myself in the ballpark.

Thanks!

gusty nexus Dec 26, 2025, 10:22 PM

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If you have a 24V reference/supply voltage close by, you could also use something like a Shelly to monitor if the circuit is switched on. it wouldn't even require a battery and you could take it one step further by controlling the humidifier with the Shelly and/or measure its power draw

tiny compass Dec 26, 2025, 10:38 PM

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Hmm, yeah, I guess that would probably make more sense... I have some Shellys I use for lights. Personally I'd prefer not to introduce a point of failure to my humidifier by effectively turning the Shelly into an inline switch, when I have no need to control the humidifier's water supply independently of the furnace. I'm not sure off the top of my head whether I can, or how I would wire it so that the Shelly can monitor it without controlling it. I'm guessing it's possible though.

gusty nexus Dec 26, 2025, 10:44 PM

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I mean you don't have to control the humidifier with the shelly

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just take the 24v from the humidifier as input to the switch, the report the result to HA

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no need to wire it to the output/in series with the humidifier. if you're worries about the point of failure, just wire it in parallel

#Looking for a wireless sensor that can read on/off state of a low voltage circuit