Circuit : Matthew Hewson
Email :matthew.hewson@ntlworld.com

Description
This project is intended for controlling mains powered disco lighting, although there are many other possible uses for controlling almost any mains appliance. It will control five seperate channels at up to 3 Amp per channel. This could be increased by designing another PCB with wider tracks than the PCB included with this design. I could have used relays to control my lights, but triacs seemed like a more sensible solution as they react quicker and are totally silent as they contain no mechanical parts. The low voltage control side and the mains side are totally isolated, as opto-couplers are used to fire the triacs. I have not included any schematics as this project is very simple, expecially due to the fact I have included a PCB.

Notes:
There isn't really very much to explain about the circuit. It's designed to switch the neutral side of the mains, although a few minor modifications could change this. When current is passed through the low voltage side of the optoisolator (through a resistor) a small LED inside the isolator illuminates. On the other side of the optoisolator is a detector circuit, and when this detects light from the low voltage side it will allow current to pass through the high voltage side. This applies a current to the gate of the triac and allows it to conduct. This means current can flow through the load you have connected to the circuit, through the triac and down to neutral.

Below is the PCB design along with an annotated copy showing component placement: Construction:
A few notes when constructing the circuit. Firstly, make sure you coat ALL mains tracks on the PCB with a large amount of solder. The tracks on their own cannot handle the current and need an increased area for the current to flow through. Not only that, make sure you use appropriate size cables and fuse the entire unit at no more than 10 Amps. The triacs are semiconductors, and when switching a current of 1 Amp or above they will get hot. You MUST attach a heatsink to all the triacs so that they don't overheat. The larger the heatsink the better. If you plan on switching inductive loads (like I had to) you need to use snubberless triacs. Standard triacs are only suitable for resistive loads such as light bulbs. The triacs I used were also isolated tab, which means that you don't need any form of isolation between the triac and the heatsink. Be aware that on a standard triac one of the legs will be connected to the casing of the triac, and therefore I always recommend isolated tab. I got all the components for this project from rapid electronics. The fuses I used on the PCB are not standard, they are a miniature fuse manufactured by a company called 'littelfuse'. You might not be able to find them anywhere else. It's up to you how you case the project, but it's best to use a well earthed metal box. You should also use an RCD on any home-brew projects that interface to the mains.

Below are some pictures of my light controller: REMEMBER - Electricity is very dangerous. You SHOULD NOT attempt this (or any other) project that uses mains electricity unless you are confident in what you are doing. Neither I or the webmaster can accept responsibility for any loss or damage caused by use of the information contained within this page. Having said that, the project should be totally safe if constructed properly.