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Switch Module Circuit Description
With the dimmer module described previously, the triac is triggered just once in each half cycle of the mains. Although the trigger current is about 20mA, it only occurs for a brief period so the average current consumption of the circuit remains low.

The Switch module is designed to drive inductive and capacitive loads, as well as resistive. Therefore the triac ideally needs to be driven continuously for the complete mains cycle. The current required for this would be 20mA per module, giving a total of 80mA if four switch modules were fitted. This sort of current cannot be obtained from the basic mains derived power supply arrangement used, so a method of reducing the average current consumption is needed.

The solution is very similar to that employed in the remote control handset. A fast chain of brief pulses are used to drive the triac, such that when the triac switches off (due to the load current passing through zero) it is almost immediately retriggered.

Referring to the circuit diagram in figure 4, U301:A is configured as a free running oscillator at a frequency of about 4KHz. R302 and D301 set the mark-space ratio so that the output is high for 20uS and low for 230uS. When pin 6 of U301:B (ON) is high, the pulsed signal is passed to the triac via Q302 and Q301. The triac therefore receives 20mA trigger pulses while the average current consumption is under 1mA. C303 decouples the supply and supplies the brief current pulses, while C302 and L301 are EMI suppression components.

U301:D and U302:B form the zero-crossing detector. The mains is squared by U301:D, the output of which is a 50Hz square wave. R306 and C304 filter out any high frequency noise on the mains that might otherwise cause erratic triggering. U302:B is a D-type flip-flop, the Q output of which latches to the logic state on the D input when the CLK input receives a rising edge. Thus when the D input changes state, the change is transferred to the Q output on the next falling zero-crossing point. The circuit ensures that the load is only switched at the zero-crossing point to minimise interference, and that the load always receives an equal number of positive and negative half cycles to avoid magnetising the core of inductive loads.

If LK301 is connected to LK302 the circuit is in momentary mode. The control input is then passed directly to U302:B, so that the load is only powered when the remote control button is pressed.

When LK303 is connected to LK302, U302:A is bought into the circuit. By connecting the NOT-Q output to the D input the device acts as a divide-by-two circuit. Thus the output (Q) changes state each time the CLK input receives a rising edge. The result is that the load is switched on by the first button press, off by the second etc. C305, R305 and D305 ensure that both halves of U302:A are reset when the power is switched on, so the circuit always starts with the load turned off.

The circuit around U301:C is the touch sensor input. The high impedance 50Hz signal caused by grounding J306 via the human body, are rectified by D310 and smoothed by C306. This signal is sufficient to drive the high impedance input of U301:C. The output of this circuit is combined with the remote control input (J305) by the diode OR gate (D306, D307 and R307). R310 and R311 limit the touch sensor current to about 25uA, which is so low that it cannot be felt. For safety the two separate resistors must not be replaced by a single component.

Handset Construction
The circuit is constructed on a single sided PCB (see figures 5A and 5B), which is designed to fit into the recommended case. The four corner holes may need to be enlarged to accommodate the fixing screws, and the two corners closest to the LEDs may need to be chamfered slightly as shown.

The PCB construction is generally straight-forward, but the following points should be noted. C1 and C2 must be laid flat against the PCB as shown, and may be held in place with a small amount of suitable glue (eg Evostick) to stop them rattling. The ICs may be fitted in sockets if desired, although these were not used on the prototype. The links should be made with 24SWG tinned copper wire or component lead off-cuts.

The four switches must be fitted on the solder side of the PCB. Depending on the type of switches used, it may be necessary to raise the edges of the switches closest to the LEDs slightly away from the PCB to ensure they are operated positively by the buttons fitted into the case. The three LEDs protrude through holes in the case so it may be easier to fit them after the case has been drilled. One hole should be central, and the others 10mm either side. The hole diameter is 5mm. The LED cathode is normally indicated by a flat on the side of the body.

The battery lead should be soldered to the terminals shown, with the red wire to the positive terminal and black to negative. The lead is then knotted around one of the PCB mounting pillars in the case, when the PCB is being fitted.

The case is supplied with button assemblies for one. two and four button systems. Normally the four button assembly would be used, but if you are building a one or two channel system you could use a suitable button assembly and omit the appropriate switches on the PCB. The PCB is fitted into the case with the screws provided. If the screws are too long they can easily be shortened with a large pair of wire cutters. Finally fix a thin piece of foam into the case to prevent the battery from rattling, and screw the back in place.