Here’s a dimmer that, besides being touch-controlled, also has a setting memory that enables it, for example, to turn the lighting on at the level you had set last time it was turned off. The project uses a specialized IC, an LS7534 from LSI Computer Systems, available from Farnell, among others. This IC is powered directly from the domestic AC powerline, which is dropped using capacitor C3 in order to avoid any thermal dissipation.
The power switching element is a triac, turned on at the zero crossing of the mains via the synchronization information conveyed to the LS7534 via R1 and C2, and turned off after a larger or smaller part of the sine wave so as to be able to adjust the brightness to the required level. The touch pads are connected to the UP and DOWN inputs via two high-value series resistors, which for safety reasons must not be either reduced in value or replaced by a single resistor of equivalent value. Note here that the values of pull-up resistors R4 and R5 can be adjusted between 1 MΩ and 4.7 MΩ in order to adjust the sensitivity of the touch control.
Choke L1 is a conventional toroidal type intended to reduce the interference radiated when the triac turns off, in conjunction with capacitor C1. For safety reasons, it is vital that the latter, along with C3, should be class X2 types, intended for direct mains operation. The triac can be any 400 V, 2–4 A type. You just need to take care to pick a type that is fairly sensitive, with a trigger current of no more than 50 mA, otherwise, the LS7534 won’t be able to trigger it properly. Although on the circuit diagram we have shown the maximum lamp power as 200 watts, it’s possible to go above that, but in this case, the triac will need to be fitted with a heatsink, which will make the project bulkier. If the project is not built into an electrical wall box, you must be sure to choose an insulating case, since in the absence of a transformer, the whole of the circuit is at AC line potential and any accidental contact with it could be fatal.
Using the dimmer is very easy, but requires you to make the distinction between long or short contact with the touchpads. When the light is off, a short touch (typically 34–325 ms, according to the datasheet) on UP makes the lamp light gradually up to the maximum value reached last time it was turned off. When the light is on, a short touch on DOWN makes the lamp slowly go out.
A long touch on UP (typically longer than 334 ms) gradually increases the brightness up to maximum, beyond which it has no further effect. A long touch on DOWN reduces this same brightness down to a minimum.
