The cased protoype showing the balmy conditions inside my house, summer 2003.

The Sensor

This uses the combined humidity and temperature sensor SHT11 from Sensirion.

The SHT11  is in a SMD package, while the SHT71 is a SIL 4 pin version.

The specifications for both are the same.

The SHT11 sensor, soldered to some SMD proto board

The double sided PCB has been designed to allow the more easily obtainable handy 8pin DIP carrier version to be used.

The sensor contains a 14 bit analog to digital converter for temperature conversion resulting in a maximum resolution of 0.1 C spanning -40 to 123.8, while the humidity sensor uses a resolution of 12 bits (0.03%RH) with a range of 0 to 100%RH. Note that there is a maximum deviation of 3% between 10-90% RH.


The controller project is made up of three parts. Click on the title to view the text

  • Controller board    reads the sensor, updates the LCD display, reads the rotary encoder, switches the outputs.
  • DC Output board    this houses three switched MOSFET 10A outputs
  • AC Control board  gives you the option of housing the power transformer and one switched AC output. If the AC output is used, this takes the place of the heater DC output.

If you just require temperature and humidity readings, the controller board can be used as a stand alone concept.


This page copyright Colin Barnard 2004