Therm RTD

Therm RTD

Therm RTD is an addition to the Therm PID Controller family, with support for RTD temperature sensors. RTDs (or Resistance Temperature Detectors) use a coil of fine wire made from a material (usually platinum, copper or nickel) that has a very predictable temperature coefficient of resistance (or change in resistance as temperature changes). RTDs are generally more accurate and stable than thermocouples, and have a much greater range than thermistors – although they can tend to be more expensive than both.

The hardware and functionality of Therm RTD is very similar to the original Therm; the biggest change is the replacement of the thermocouple chip with a MAX31865 chip to support RTDs instead. This chip also supports thermistors; however, the reference resistor must be replaced with a larger one before using higher resistance thermistors. The OLED display, USB port, and solid state-relay driver are left unchanged, but the external power input has been removed. Removable mounting feet have also been added to assist in installation.



The new hardware comes with a firmware update that is compatible with both models. The menu system has been improved, with more extensive reboot and reset options. Issues with non-volatile memory have been fixed, allowing settings to be saved across reboots. Temperature display resolution has been increased for both C and F, and PID calculations are now performed using 1/10th degree precision. Source code for both the firmware and the hardware are available on the Protofusion repository.

Future updates are planned to add software-selectable reference resistors so that both RTDs and thermistors can be used without hardware modifications. Temperature profiles with support for multiple setpoints are also in the works to facilitate more complex temperature control applications.

Matthew is a computer engineer who enjoys designing hardware for embedded control systems. His hobbies include photography, gardening, coffee, and anything DIY.

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Posted in Projects, therm
8 comments on “Therm RTD
  1. shashi says:

    dear sir ..,

    M shashi form india m looking PIC series LCD pid temperature controller with timer kindly support me .

    Best regards


    Couple of questions:
    -What is the PCB tool you are using here? Seems they are not Eagle files.

    -Have you built any boards up you are willing to sell?

    • Matthew Reed says:


      We primarily use Eagle for PCB design here at protofusion. You can find the Eagle files for this design at under the therm-rtd branch.
      I’m sorry, but we are currently not selling this device. We will be sure to post on the blog if we ever do end up listing any for sale.


  3. Andy says:

    Hey Matthew,

    Just trying to build the therm-ng firmware for an RTD sensor – looking at the code, if you change the sensor type const to be “MAX31865_RTD_SENSOR” it doesn’t build at it’s looking for the max31865.h include rather than the max31856.h. Not sure if it needs a different library added or is that just a typeo (you mention the max31856 should work for both thermocouples & RTDs above).


    • Matthew Reed says:


      The therm-ng firmware does not have support for the MAX31865 chip that is found on this version of therm. The older therm firmware repo ( contains the library you’re looking for, and is the correct version of firmware for the therm-rtd. The therm-ng firmware currently only supports the MAX31856, and uses a different processor than the therm-rtd design, but it should be fairly easy to add in the MAX31865 library if that’s what you’re looking for.

      Hope that helps!

    • Andy Lyon says:

      Ah that makes sense – thanks Matthew. Is there a way to download the original therm firmware? Looks like downloads are disabled for that repo currently.


    • Hey Andy–I just enabled downloads on that repo so you should be able to grab a zip of the firmware.

    • Andy Lyon says:

      Amazing, thank you Ethan! Boards are all ordered – wish me luck 🙂

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