all about wiring diagram

Timers are very useful both for industrial applications and household appliances. Here is a PC-based timer that can be used for controlling the appliances for up to 18 hours. For control, the timer uses a simple program and interface circuit. It is very cost-effective and efficient for those who have a PC at workplace or home. The tolerance is ±1 second.

The circuit for interfacing the PC’s parallel port with the load is very simple. It uses only one IC MCT2E, which isolates the PC and the relay driver circuits. The IC prevents the PC from any short circuit that may occur in the relay driver circuit or appliance. The glowing of LED1 indicates that the appliance is turned on. Transistor BC548 is used as the 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 ‘x’ minutes, of which ‘on’ and ‘off’ durations are ‘y’ and ‘z’ minutes, respectively. The program will repeat the on-off cycle for x/(y+z) 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.

PC-Based Timer Circuit Diagram



The program uses two bytes for storing integer type data. So when input is given in terms of seconds or minutes, it can hold 216–1=65,535 seconds or 18 hours at the maximum. The sleep() function in the program is used to hold the appliance in ‘on’ or ‘off’ condition for the ‘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.

EFY note. The source code and executable file of this program have been included in this month’s EFY-CD.

Anyone who has designed circuits using the 555 timer chip will, at some time have wished that it may be programmed for longer timing periods. Timing lengths larger than a couple of minutes are tough to succeed in as a outcome of part leakage currents in massive timing capacitors change into vital. There is then again no reason to opt for a basically digital resolution just yet. The circuit shown here uses a 555 timer in the design but nonetheless achieves a timing interval of as much as an hour! The trick right here is to feed the timing capacitor now not with a constant voltage but with a pulsed dc voltage. The pulses are derived from the un smoothed low voltage output of the facility provide bridge rectifier.

The power provide output is just now not referenced to earth potential and the pulsing full wave rectified sign is fed to the bottom of T1 by the use of resistor R1. A 100-Hz sq. wave signal is produced on the collector of T1 as the transistor switches. The positive 1 of 2 of this waveform charges up the timing capacitor C1 by manner of D2 and P1. Diode D2 forestalls the cost on C1 from discharging through T1 when the sq. wave signal goes low. Push-button S1 is used to start the timing duration. This methodology of charging uses moderately low component worths for P1 (2.2 MΩ) and C1 (100 to 200 µF) however succeed ins timing periods of as much as an hour which is much longer than a regular 555 circuit configuration.