PC Based On/Off Timer makes the circuit ON and OFF according to the ON time and OFF time. This is the most useful PC Based ON/OFF timer which uses a simple program written in C language and interface circuit using monostable multivibrator as shown in the circuit diagram. It has industrial as well as household applications. In addition it is very cost effective and most suitable for those who have a PC at workplace or home. In the program, you have the provision to enter the time the time in minutes as well as in seconds. Then it will ask for ON time and OFF time to control the appliance. Accordingly appliance will remain ON and OFF.
About the Circuit :
- The circuit for interfacing the PC’s parallel port with the load is very simple. It uses two ICs, one is for triggering and other used for monostable operation. The glowing of LED D3 indicates the triggering of monostable multivibrator designed using IC2. Whereas glowing of LED D2 indicates the ON/OFF condition of relay. Transistor T1 is used as a relay driver. The program code is written in ‘C’ language and compiled using ‘Turbo C’ compiler. When the program is run, it prompts the user to input the time duration in seconds or minutes to control the appliance. After entering the required timing, press any key from the keyboard. Suppose you input the total duration as‘t’ minutes, of which ‘on’ and ‘off’ durations are ‘t1’ and ‘t2’ minutes respectively. The program will repeat the on-off cycle for t/(t1+t2) number of times. After completion of the total time, to repeat the cycle, you will have to reset the time in the program to activate the circuit.
- The program uses two bytes for storing integer data type. So when input is given in terms of seconds or minutes, it can hold 65,535 seconds or 18 hours at the maximum. The sleep( ) function in the program is used to hold the appliance in ‘ON’ and ‘OFF’ periods as entered by the user against prompts. The sound function is used to give a beep during ‘ON’ condition of the appliance.
- When program is executed following messages are displayed on screen: “This is the program of ON/OFF Timer” “The circuit provides delay in seconds and minutes” “Press any key to continue…”
- After pressing any key the program gives following messages: “Enter your choice to set the time duration. Enter time duration in minutes (press m). Enter time duration in seconds (press s)
- After pressing ‘m’ through keyboard, the program asks, “Enter the total time duration (in minutes): Enter the time to keep the circuit ON (in minutes): Enter the time to switch OFF the circuit (in minutes): Press any key to start the program…” After pressing any key, the program generate control signal at port-2. The circuit is triggered and relay is on. While circuit is ON the program gives message as: “Your circuit is ON” After ON time is over, the relay becomes OFF and the following message is displayed on screen: “Your Circuit is OFF”. If the total time duration is greater than the sum of ON time and OFF time, then the above cycle repeats. The time is entered in seconds then also the above messages displayed, only change is that in time duration is entered in seconds. At the end program displays the message as: “Your program completed successfully.”