CIRCUIT DESCRIPTION AND WORKING:

The circuit comprises decoder, inverter, latch circuit, and relay driver sections. The circuit, excluding relay drivers and relays, is powered by a 5V DC regulated supply. Relay drivers and relays are driven by a 12V DC regulated supply. Each relay is rated 12V,200-ohm.

The circuit for switching on/off 6 loads is shown in Fig. 2. For more than 6 loads, you can add more ICs in a similar way as shown in this circuit. IC 74LSI54 is a 24-pin, 4-to-16 line decoder IC. This IC is designed to accept four inputs and provide 16outputs. Input addresses Al through A4 to pins 20 through 23 of IC1 and IC2 (IC 74LSI54) each are given from the data lines of the computer parallel port.

In this circuit, only pins 2 through 9 of the parallel port are used, where data lines D0 through D3 form the 4-bit data input and D4 through D7 are address-select or control lines for the circuit. Pins 18 through 25 are shorted to ground. Data lines D0 through D3 are the input addresses for IC2, and data linesD4 through D7 are the input addresses for IC1.

When Enable pins EI and E2 (active low) are high, all the outputs go high irrespective of the address inputs (AI through A4). Enable pins EI and E2 of ICI are grounded and its output pins YO through YI5 are connected to Enable pins of the respective decoder ICs.

Initially all the data inputs (D0 through D7) are low. Thus, except Y0,all the outputs of ICI and IC2 are high. The output Y0 of IC2 is not used, for the reason that when all the input data is low, none of the outputs of IC 74LSI54 is used for switching the loads. Suppose, out of eight input data lines, D0 is high. So, except Y1, all the outputs of IC2 will be high. YI is then inverted using IC4 (IC 74LS05). The output of IC4 at pin 2 is given to pin 3 of IC7 (lC 74LS74). IC 74LS74 is a dual D-type flip-flop used for latching the data.

With an active rising edge of the clock pulse (CP-1 or CP-2), the data input will be locked in IC7 through IC14 until the next rising edge of the input clock pulse. The outputs of ICs 74LS74 are given to relay driver ICs ULN2803 (lC-15 and IC-16), which, in turn, drive the relays. The relays are used to switch on/off the appliances.

Fig 2. Circuit diagram of the project

SOFTWARE :

The program to control the appliances is written in C. it is compiled using Turbo C compiler. The flow chart of the program is as follows

Fig-3: Flow chart for Implementation of Program

Fig-4: Flow charts of the functions Used