Hardware for Multiple Line Voltage Sensors?

I have a system (hot tub) I’d like to monitor. It has several components (pumps, heaters) which run on 240V AC. This is in the US, 60 Hz. I’m not really interested in controlling them, but I’d like to know when these various components are active.

For other projects, I’ve had good luck with two different methods, but neither is ideal for this situation. First, I modified a normal Visonic Zigbee door sensor by replacing the reed switch with a pair of wires attached to a relay (contactor) powered by line voltage. My other solution was to go directly to the GPIO pins on the Raspberry Pi running HA from a set of four relays to monitor the various components in my home heating system.

I’d rather not have to set up four or more sets of Zigbee door monitors and relays all next to each other. It seems inelegant, wasteful, and a bit of work to keep replacing batteries. And the devices I want to monitor are too far from the RPi running HA to connect wires to the GPIO pins. I don’t really want, and shouldn’t need, to run yet another RPi.

In an ideal world, I’d like a Zigbee or WiFi device which can sense voltages on four or more different wires. I’d rather not introduce any new protocols or integrations, and it needs to be local, with no cloud dependencies.

I’m hoping somewhere, someone has already invented this. Or maybe the great minds on this forum can come up with a more elegant solution.

How are you with electronics?
ESP8266 or Arduino w/Ethernet reporting via MQTT. ESPHome can likely do this on the ESP8266, as well as Tasmota.

An optocouplier (4N35) on each 240V input, with a bridge rectifier in front. Series resistor of about 12K, 2W or higher or power the LED. Output side collector to +V through 4.7K pullup, emittor to ground, I/O pin of processor to junction of resistor/collector.

One of those on each 240V signal you want to monitor. Keep them isolated away from the low volt stuff. I do this exact thing for my air compressor.

Technically, that’s not outside my skill set. In practice, I’m a bit lazy. I was trying to avoid much of that effort, as well as any new protocols (I don’t currently use MQTT.)

It may come down to that, if there’s no easier or more off-the-shelf answer.

Some of the Shelly devices might adapt to your project. With each Shelly 3EM, I believe you can monitor current draw on three circuits. Having a passive couple to the circuits seems a good route to go. You would have to do some HA sensor work to get your indicators, but the upside is you get realtime power usage. https://shelly.cloud/products/shelly-3em-smart-home-automation-energy-meter/

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Maybe this?

This is good video on how it works.


The energy meter stuff is very interesting, but I also stumbled on some tutorials for using DS18B20 temperature sensors with an ESP32. That was another project I had in mind for the hot tub. Maybe I can do it all with one device tucked into the mechanical spaces of the spa enclosure. Power isn’t a problem, and it’s close to a WiFi router.

I see myself going down an ESP32/ESPHome rabbit hole. Not sure I should thank you or not :wink: