Continuously scan port 1 of 8051 microcontroller, which connected with temperature sensor. If the data entered from sensor through P1 is greater or equal to 25, then send high active signal (1) to port 2 bit 5 (P2.5), which connected to relay (electrically operated switch) of a cooling conditioner to turn it on. If the entered data is less or equal to 20, then send low signal (0) to P2.5 to turn the cooling conditioner off. Make a delay of about 5 sec for each situation (in the delay loop use NOP and DJNZ considering that the time to execute one instruction is 1 cycle = 1 s).