How to connect to a PT100 RTD

This tutorial is intended for users of Evolved Horizon – a software for acquiring, visualizing, storing and analyzing sensor data from Arduinos. If you have any questions contact us at In order to use Evolved Horizon™ to collect data from a PT100 RTD, the signal needs to be boosted using an amplifier such as the Adafruit MAX31865 breakout board, which is then connected to the SPI port on processor board where you are running Evolved Horizon™. This requires a small amount of assembly, which will be described here. This amplifier works with 2, 3 and 4-wire RTDs.

The board comes with two 2-pin terminal blocks for connection to the RTD, and a pin header for connecting to the processor board. These need to be soldered together.

The Adafruit MAX31865 components

Terminal block and pins soldered on to board

The MAX31865 board comes configured for 4-wire RTDs, which have two wires connected to each side of the thermal element. Similarly coloured wires come from the same side, and should be connected to adjacent slots on one of the terminal blocks. For example the first terminal block (F- and RTD-) could get a red wire in each slot, and the second terminal (F+ and RTD+) block could get a black wire in each slot.

For use with 2 or 3-wire PTDs, jumpers on the board need to be changed. See Adafruit’s datasheet for more information.

Jumper leads are then connected from the pin header to the processor board. Ground and Vin are connected to corresponding pins on the processor, the 3v tap is ignored, and SDO, CS and CLK are connected to whichever pins have been assigned to these functions on the processor.