An ordinary AC switchboard contains separate switches for switching ‘on’/’off’ electric bulbs, tube lights, fans, etc. A very simple, interesting circuit presented here describes a feather-touch switchboard which may be used for switching ‘on’/‘off’ four or even more devices. The membrane or micro-switches (push-to-on type) may be used with this circuit, which look very elegant. By momentary depression of a switch, the electrical appliance will be ‘on’/‘off’, independently. To understand the principle and de-sign of the circuit, let us consider an existing switchboard consisting of four switches. One live wire, one neutral wire, and four wires for four switches are connected to the switchboard, as shown in the illustration below the circuit diagram.
The switches are removed and the above-mentioned wires (live, neutral, L1, L2, L3, and L4) are connected to the circuit, as shown in the main diagram. The circuit comprises four commonly available ICs and four micro-relays, in addition to four micro-switches/membrane switches (push-to-on type) and a few other passive components. IC 7805 is a 5-volt regulator used for supplying 5V to IC2 and IC3 (7476 ICs). These ICs are dual J-K flip-flops. The four J-K flip-flops being used in toggle mode toggle with each clock pulse. The clock pulses are generated by the push-to-on switches S1 through S4 when these are momentarily depressed.
When a switch is momentarily depressed,its corresponding output changes its existing state (i.e. changes from ‘high’ to‘low’ or vice versa) . The outputs of flip-flops drive the corresponding relays, in conjunction with the four relay driver transistors SL100. The wires earlier removed are connected to this circuit. On the switch panel board, the micro-switches are connected, and under the board the connections are wired as suggested above. Relays RL1 though RL4 are 9V, SPST-type micro-relays of proper contact ratings.
The circuit may be expanded for six switches by using one more IC 7476, and an IC ULN 2004 which has an array of seven Darling-tons for driving the relays. So two more micro-switches and relays may be connected in a similar fashion. This circuit can be assembled on a general-purpose PCB and the total cost should not exceed Rs 300. It is suggested that the circuit, after assembly on a PCB, may be housed in a box of proper size, which may be fitted on the wall in place of a normal switchboard.
Author :D.K Kaushil - Copyright : EFY mag