Basic object of this project is to control one Heater and one Fan to maintain constant temperature in an environment. For this we are using ATMEGA8L micro-controller as the main controller and LM35 for Temperature Sensor. One 16X2 LCD is attached with the micro-controller to display the Set Temperature Value and Present Temperature Value. LCD will also to display the status of Heater and FAN. For Heater we are using one Relay to control 230VAC Heater Element and we have attached on 12VDC Brush less DC FAN directly with the board.
Whenever the system is switched ON, LCD will display “SV” (for SET Temperature Value) and “PV” (for Present Temperature Value) at the left side of the display. On the right side we will display the status of FAN (as “FN”) and Heater (as “HT”). We have interfaced three switches with the micro-controller. Now to change the “SV”, we have used two switches as UP switch and DWON switch. One another switch is left unused for further development of this project. Now on pressing UP switch, value of SV will increase and DOWN switch, value of SV will decrease.
To do the main function of the project, i.e. controlling of FAN and HEATER, we use three compare functions in our software. To overcome the problem of frequently ON/OFF of HEATER and FAN, we assume Hysteresis of 10C, i.e. HEATER or FAN status will be changed if PV is out of +_ 10C. So, if PV is greater than +_ 10C of SV then FAN will be on to cool down the environment and if PV is bellow +_ 10C of SV then HEATER will be ON to increase the temperature of the environment. And when PV is within +_ 10C of SV, HEATER and FAN will be OFF. This is to note that in a moment either HEATER or FAN will be ON.