Checking temp on solar collector and triggering a relay in basement

My next project is going to be putting four 30 tube solar collectors on my house. My plan is to turn my pump on when the temperature of my collector is hotter than the temperature of my holding tank. To do this I need a way to monitor the temperature and then flip a relay to provide power to the pump.

It would be nice if checking the temp was simple as running some cat 3 from my basement (where the pump is) to the roof (where the collectors are) and then attaching a temp sensory to the end of it to take my temp reading, but I doubt it'll be that easy. My goal is maintenance free, but I'll entertain anything you guys can think up.

That would be a standard industrial solution ...
There are a couple of caveats,

The sensor is probably going to require a high impedance input so hanging it on a long cable carrying a voltage based reference is going to be noisy and probably not work well.
you will of course need two sensors tank and panel ...

In industry these would generally be referred to as transmitters, a module that houses the sensor, converts its output to something useful and calibrated which the control system can use.
4-20mA loops are very common, good for long distances and far less susceptible to noise than voltage based signals.
Because the output is current based small changes in cable impedance do not affect accuracy, the voltage necessary to drive a specific current may alter but the current will not.

It depends on the accuracy you want to achieve ... If you use screened twisted pair you may well get away with driving a PT100 from a constant current source and reading the voltage at the source, cable resistance will only be a tiny fraction of that.

Have you considered driving the pump with PWM based on the temperature difference that way you keep the panel as cool as possible at all times and the overall temperature of the system stable, no slugs of hot or cold water flying about the place.
I think it would probably give you some efficiency gains.


I know it's a simple question and I don't mean to waste your time. Googling things like "arduino long temp sensor" doesn't come up with a whole lot.

I'll look into all the information you provided. One other thing I found when people are looking for long temp sensors is using something like the DS18B20. If I can get 40' within 5 degrees F, I'd be a happy camper.


There are no problems with long cables with low-cost digital temperature sensors like DS18B20. CAT5 is typically used. Take a look on the ArduinoInfo WIKI: http://ArduinoInfo.Info

There is also a link to connecting multiple sensors on the same wire, so you COULD easily monitor more than one part of your collector or even a separate small black-plate to "anticipate" the solar heating.

It also shows individual chips and waterproof versions.

Information on power control, relays etc. here:

Thanks a bunch. I'll give one of those low cost digital thermometers a try and see how it goes.

One other thing I found when people are looking for long temp sensors is using something like the DS18B20. If I can get 40' within 5 degrees F, I'd be a happy camper.

The nice thing about the DS18B20 is that being digital, if it works at all over that distance, you'll get considerably better accuracy than that.