Controller Circuit

The schematic of a Digital Thermostat is shown in Figure 16. The A/D digitizes the output of the LM35, a temperature transducer IC with an output of 10 mV per °C. With VREF/2 set for 2.56 V, this 10 mV corresponds to 1/2 LSB and the circuit resolution is 2 °C. Reducing VREF/2 to 1.28 yields a resolution of 1 °C. Of course, the lower VREF/2 is, the more sensitive the A/D will be to noise.

The desired temperature is set by holding either of the set buttons closed. The SCC80C451 programming could cause the desired (set) temperature to be displayed while either button is depressed and for a short time after it is released. At other times the ambient temperature could be displayed. The set temperature is stored in an SCN8051 internal register. The A/D conversion is started by writing anything at all to the A/D with port pin P10 set high. The desired temperature is compared with the digitized actual temperature, and the heater is turned on or off by clearing setting port pin P12. If desired, another port pin could be used to turn on or off an air conditioner.

The display drivers are NE587s if common anode LED displays are used. Of course, it is possible to interface to LCD displays as well. It is explain in the figure of circuit schematic in below;