This project reads 4 x PT100 temperature probes and controls 8 relays (4 heating and 4 cooling) the board mounts a Raspberry Pi which it provides filtered power for and acts as an I2C slave device for the RPi to control, advanced monitoring of the RPi activity will allow a full reset of the RPi should it become unresponsive.
The analog side is based on the Microchip AN687 design with an analog multiplexor to select the PT100 channels. Two precision calibration resistors are also integrated in to other channels in the mux allowing automatic temperature calibration down without any manual calibration at around 0.3 degC (errors due to variations in channel on resistance in the mux)
The system is Completed and tested.
Please see attached documentation and firmware
Copies also at rodyne.com