Universal Thermostat

This circuit came about because of the dissatisfaction regarding the operation of the thermostat in a refrigerator. When using the built-in thermostat, it turned out that it was necessary to reduce the temperature setting in the summer in order to keep everything cold, compared to the setting in winter. This is probably as a result of a temperature sensor that is mounted too close to the cooling element, which means that phenomena such as thermal leaks and the average temperature in the fridge are not sufficiently accounted for in the control loop.

Resistors
R1,R2,R3,R5 = 4kO7
R4 = 2kO2
R6 = 10k
R7 = 33
P1 = 10kO preset
Capacitor
C3 = 100nF
Semiconductors
D1,D2 = 1N4001
T1 = BC547
IC1 = 7805
IC2 = PIC16F628-04/P (programmed, with software # 080090-11)
Miscellaneous
X1 = 4MHz ceramic resonator
S1...S3 = miniature push button
K1,K2 = 2-way pinheader
K3,K5 = 3-way pinheader
K4 = 16-way pinheader
DS1820 and 3-way ribbon cable
LCD with 2x16 characters
PCB #080090-1 from www.thepcbshop.com