Voltage presence sensor needed

I’m looking for a sensor that will monitor for the presence of voltage (120VAC).

I have an outdoor wood boiler, and by detecting if there is voltage along the circuit, I can create automations for when there are issues. I need to monitor for voltage at four different locations in this circuit. The circuit ends up going to a neutral through a fan, so it can’t be “confused” by this.

I attempted to use a Shelly Plus i4. This kinda worked, until the last place where I’m measuring (a fan). Due to there being a path to the neutral through the fan, it constantly showed as energized, regardless of whether it was or not. Relays would not work here either, since the primary coils all go to neutral also.

Due to location, it would be preferred to be WiFi-based.

I think a workaround would be four relays where the secondary coils are connected to a door sensor’s reed switch. When the primary is energized, it would complete a circuit indicating the door sensor would be “closed”, indicating the presence of voltage. If I can’t find something to handle four channels, I will probably end up going this way.

I’m hoping that someone knows of something that would work for this purpose. Ideally, it would use one WiFi connection and have at least 4 channels for indicating the presence of voltage.


Edited: adding boiler schematic

I want to know if there is power available on:

  1. High Temperature Aquastat - lower output indicates high temperature condition
  2. High Temperature Aquastat - upper output feeds to the light/boiler switch
  3. Light/Boiler switch - upper output to Boiler Fan Aquastat
  4. Boiler Fan Aquastat - upper output

Technically you can sense voltage only between two wires. In practice if N is grounded you can sense just Live wire. If you have just neutral wire to sense , you must go with current sensing. Try to illustrate your circuit little better, there are so many ways of sensing if something is “on or off”.

We need a schematic, other ways it could be a long time guessing.


If you just need an I/O you can use ESP board with an optocoupler, there are many solution that you can build yourself but I believe the easiest would be using an optocoupler like Amazon or Aliexpress.
On market these boards exists in 120VAC version I found this 120 VAC version.
The logical output will go to your ESP or other microcontroller that can be connected to your HA instance.

An ESPHome code example that should work in this post..

A similar solution is in this post, just using Tasmota instead of ESPHome.

Alternative is measuring current using ESP and CT Clamp (CT Clamp Current Sensor — ESPHome)
CT clamp and voltage measuring device are available also commercially example Shelly.

Please mind you are dealing with VAC be careful and if you are not sure recheck everything and if your are still not sure ask to someone that can help you.

You don’t need to sense voltage. I have a Sonoff Basic running ESPHome. If it’s getting power, it is online.


If the status is “connected”, you have power at the outlet.

Backstory: All of my outdoor outlets are chained to a single GFI at the breaker panel area. When we get a hard rain, the GFI trips. The Sonoff Basic just lets me see the status in an automation.

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Maybe I am misunderstanding the issue, but if you are into shellys - why not use a shelly mini 1PM in those 4 locations - they will give you a voltage readout as an entity.

Then you have not connected it correctly. Sensing needs to be in parallel with the fan, not series. Please do this:

Also please consider getting a qualified electrician to perform this work. There is no room for mistakes.

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Thanks to everybody that has responded so far! :+1: :grinning:

Several people asked for a schematic, so I added it to the original post. It really is that simple - power goes to a high temperature Aquastat, then to a switch (where it either turns on a light or allows the rest of the boiler to operate), then to another Aquastat, where if the temperature drops to a level it energizes the blower fan. When the water has heated up enough, the Aquastat removes the power from the blower fan. I’ve indicated the Aquastats as a SPDT switch, since that is essentially what they are.

I want to detect if power is present at the high temperature line, and the upper output (as shown) for the two Aquastats and the switch.

This sounds like what I need, especially the 120VAC version (with 8 channels). Still need to connect it to ESPHome, and put it in a case for holding all the parts. Doable, but I don’t have a 3D printer to make a custom case.

This sounds even better. It looks like I would need one for each channel that I want to monitor this way. Too bad the Sonoff 4ch Pro R2 wouldn’t work - there would only be one “connected” for all four switches on it.

I’m only into Shellys because I thought that the Shelly Plus i4 would work.
Though I do have some plans in the future for using a Shelly Plus Uni…

Yeah, I’m doing all of these in parallel.
I completely built my house, including all of the wiring, so I know that I can handle this.