We have a heating system that uses an ntc resistor to measure water temperature. I would like to replace that ntc resistor by a fixed one (low enough to disable heating) temporary on a 5 minute schedule, or any other.

It's to prevent the warm water using up all the heat and not leaving any for the central heating of the building. We got fooled by the contractor and a proper upgrade would now cost several thousands, so we just want to trick the system. ;p

The arduino is not supposed to measure anything, just to connect one of the two resistors to the heating system.

Is an arduino the right choice for that? If yes, which model? What extra elements would I need? Transistors? Relays? They shouldn't have too much own resistance to not influence the measuring of the heating system when the NTC is connected.

Just a simple connection of the extra resistor would work too because the resulting resistance would be lower and still indicate "water is hot enough" to the heating system.

Any help is welcome, even if it is not arduino based.

1 Answer 1


It sounds like what you want is a small SPDT relay (maybe a reed relay).


simulate this circuit – Schematic created using CircuitLab

Turn on the GPIO, the MOSFET (or NPN BJT if you choose - increase R1 if you do) turns on, which energises the relay. The relay then switches between the NTC and the resistor being connected into the circuit.

  • For the benefit of the OP, I suggest you clarify what M1 is - and suggested part numbers or specs.
    – Nick Gammon
    Oct 15, 2016 at 22:53
  • 1
    @NickGammon That portion of the circuit is the standard relay switching circuit. There are a brazillian tutorials and examples citing parts and specs online. Why replicate it needlessly?
    – Majenko
    Oct 15, 2016 at 22:55
  • cool, think that is what i need! would you use the arduino or is it overkill in that simple scenario? 5 mins on, 5 mins off, repeat .. Oct 16, 2016 at 0:57
  • Arduino is a good choice.
    – Majenko
    Oct 16, 2016 at 8:35

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