Reading another devices temp sensors - possible?

Hi folks!

I have a solar water heater for my pool. It has two temperature sensors attached to a control unit. The control unit has access to AC power - and the pump used to pump the water through the solar cells is connected to this controller.

The controller reads the pool temperature, then the roof sensor, and then if there is enough of a difference and the pool has not reached max temperature it will start the pump to heat the water. It’s basically identical to this unit:

I want to read the two sensors whilst leaving them connected to this unit. Essentially piggy backing these connections and hooking them to an esp32 or esp8266, then publishing them over MQTT.

I want to know - is it feasible to connect the sensors to my own device? IS there some principle I’m not aware of that makes me reveal my n00b status?


It looks like it’s using hard wire transmit from the sensor to the unit. If that’s the case you’d need to find a serial out on the head unit. Then you could use a esp/arduino whatever to read the serial and send it over mqtt.

It is highly possible to just read the inputs from the sensors, but at what voltage is the logic coming through. serial would be so much easier.

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Thanks forsquirel

I might unplug it and have a look. Any pointers on reading serial output?

It also dawned on me - why the heck do I need this controller? I could put an ESP* unit here, read those values directly from the temp’ probes using the ESP - publish to MQTT and then use HA to calculate the difference and decide to switch on a relay for the pump or not. I could even replicate the Summer (always try to heat) and Winter (cycle pump each day for 3 minutes) modes in HA.

I think I know which is easier! The only catch would be the load on the relay that sends power to the pump. I’d have to watch that…

reading serial, well, you have to have serial headers present on the device.

You could possibly just roll your own device completely with an ESP if you wanted to go that route. also, there are relays that work with arduino/esp that will switch 110/220.

I’m working with 230-240v AC - but it’s the amperage of the pump motor that would be the main concern… I could see some relays getting cooked. Might have to be careful…

I might look at some sonoff devices and see what they are rated for…

How did you go with this Brendan, im thinking of doing the same thing, and i know it has now been a few years ago for you, do you have any insights for me?


Nope - I chickened out on many levels:

  • Controlling a pump with a Sonoff seemed like it be just too much load for it
  • All of this apparatus is under timber stairs - not water proof, and not fire resistant
  • A controller is worth $300. I’m just not that ready to gamble.

I still want a solution to get me those temps though!