Circuit : Matthew Hewson
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
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
Below is the PCB design along with an annotated
copy showing component placement:
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
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.