Digital Thermostat with LED Temperature Display

I needed to replace two old, unreliable thermostats for controlling the heating and cooling for a large garden shed. Commercial basic digital thermostats are available quite cheaply, but some lack the ability to control heavy loads or have the extra features that I require for saving energy when the door is often left open or to indicate temperature being out of range etc. I like the PIC18F1320 microcontroller used in my previous project - so decided to use it again in a very similar design to drive three multiplexed LED displays and read the temperature from a Dallas/Maxim DS18x20 "1-Wire" digital sensor.

The microcontroller communicates with a Dallas/Maxim "1-wire" digital temperature sensor (in fully powered mode). It has been successfully tested with DS18B20 and DS18S20 types. The choice of sensor makes no difference to the temperature display resolution here. If a DS18S20 device is used,the software uses the "Count_remain" variable to increase it's resolution to 0.1 deg centigrade. The controller drives the three (high efficiency) LED single digit common cathode type display segments through series 510ohm resistors to help limit the current to remain within the 25mA per pin drive limit, together with the multiplexing action which effectively reduces the average current. The multiplex is the same "Charliplexed" circuit as used in my previous project - this saves two pic port pins for other uses. Using the on-chip oscillator again frees-up the two pins normally used for connecting a crystal to be used as ports instead.