The figure below shows the schematic of a Digital Thermostat. The output of LM35 is digitized by the A/D. The LM35 is a temperature transducer IC with an output of 10 mV per ‘C. This 10 mV corresponds to 1/2 LSB and the circuit resolution is 2 ‘C if the Vref/2 is set for 2.56 V. The resolution become 1’C if the Vref/2 is reduced to 1.28. The A/D will be more sensitive to noise if the Vref/2 is getting lower.
By holding either of the set buttons closed, the desired temperature is set. While either button is depressed and for a short time after it is released, the SCC80C451 programming could cause the desired 9set) temperature to be displayed. The ambient temperature could be displayed at other times.
SCN8051 internal register is place to store the set temperature. By writing anything at all to the A/D with pot pin P10 set high, the A/D conversion is started. The digitized actual temperature is compared with the desired temperature, and by clearing setting port pin P12 the heater is turned on or off. We can use another port pin to turn on or off an air conditioner, if we desired.
If common anode LED displays are used, the display drivers are NE587s. Of course it is possible to interface to LCD displays as well. [Source: NXP Application Note]