UV Light Box

Probably the most indispensable tool for making printed circuit boards, is the UV light box. There are a couple of different methods for transferring of a PCB image. The most common homemade method, seems to be the toner transfer method, judging by the number of sites describing it. However, I use the photo lithography method and I have had great results with it so far, so no need to change.

I am not going describe the process in detail, since there is allot information on it elsewhere on the net. In a nutshell it consists of using a photo resist spray, this is a spray that is sensitive ultra violet light, to coat a copper clad fibreglass board. A transparency, containing the printed circuit image, is then placed on top and the board is then exposed to ultra violet light for a couple of minutes. The UV light degrades the coating in areas where it was exposed to the UV light, making it easily removable in a developer liquid like caustic soda. It leaves behind the coating in a pattern matching the circuit image, and exposes the bare copper in places where there are no tracks. An acid solution is then used to eat away the exposed copper, leaving the neat circuit pattern behind, protected from the acid by the photo resist coating.

The ultra violet light source is key to this process and since a suitable light source is not available as an off the shelf product, I decided to make a light box for this purpose. Now, at this point I should mention that this is my second attempt at building a UV light box. The first one was a monstrosity if a box over a meter long, half a meter wide and half a meter high! Inside was a dual fluorescent light fitting, fitted with two 8 Watt UV tubes. This light box worked well and I used it for a couple of years. But it took up way to much space and was cumbersome to setup and use. Through trial and error, I found that it required an exposure time of 8 minutes, for descent results. The biggest draw back of this box, was that it only allowed me to make single side circuit boards. This was fine then, but as my projects increased in complexity, so did the usefulness of this solution decrease.

I needed something that could illuminate a board from both the top and bottom simultaniously, in order to avoid complex re-allignment of the top and bottom layers in between exposures. Secondly, it need to fit on my desk, since working in the garage is a pain. So I settled on a box built from MDF from the local hardware store and UV leds. The end product lloks as follows:

It contains two sections of 54 LED’s each, one for the top and the other for the bottom. It runs off 12V DC and draws about 1.2A. I found that an exposure time of three minutes gives good results. Keep in mind that UV light is extremely bad for your eyes and you should always take the necessary precautions operating equipment like this. In order to avoid unnecessary exposure to harmful UV rays, I added a micro switch in the lid that will switch the light source off when the lid is lifted.

It is worthwhile mentioning that the dimensions of the box and the spacing of the UV LED’s are key to it’s success. LED’s have a conical light pattern that varies depending on the type and model of LED. Consult the data sheet of your model, before doing anything. The trick is to get as much overlap as possible, between all the neighbouring LED’s to avoid getting ‘shadows’ that will ruin the exposure. Some basic trigonometry will be helpful in determining the spacing between LED’s.

The LED’s I used (WW05A3SPE4-B) has a viewing angle of 30 degrees. My box’s inside dimensions worked out to 300mm (W) x 200mm (L) x 110mm (H).

The working area is a normal piece of windows glass, 4mm thick. It is important to get glass that is free of any UV coating that will shield the UV rays. Obviously it has to be spotlessly clean, since any imperfections will show up in the exposure. I use normal methalated spritis to give a good wipe down, every now and then.

The circuit for driving the LED’s is fairly straight forward. The LED’s are grouped in three’s and connected in a current regulator configuration. The schematic for one of the two sections can be found here.

Soldering so many components can really be a pain in the neck.

The circuit was constrcuted using vero board. The soldering took about two nights, lots of components. I used two seperate peices of vero board, that I joined using solder joints in a couple of strategic places.

Using solder joints to join two pieces of vero board together

Once the circuit construction was completed it was time to test everything before final assembly. At this point it is worthwhile remembering that UV rays are harmfull to your eyes, take precaution and use common sense.

To hold the vero board circuit in place, inside the box, I used 10mm stainless steel stand-off’s that was glued to the wood. Make sure that the surface is free from dust and grease and only ginger tighten the screws. I found it easier to first drill the holes and screw the stand-off’s to the vero board, before glueing and positioning it in the box. This way there is no hassling with alignment. Very important, do not use allot of force when screwing the stand-off’s to the vero board. The glue will not form a bond strong enough to withstand the torque forces if the screws have to be undone.

Be sure to put some weight ontop of the vero board while the glue sets. The vero board might have a slight bend to it, caused by the heat from soldering the components. Putting a weight on it will assist in getting the glue to dry with the stand-off’s firmly pressed against the wooden surface.

The glass panel was recessed into the bottom half of the box, by cutting it a few millimeter bigger than the actual opening. I used a Dremmel with routing attachment, to mill away a small section of the edge of the box. This ensures that the glass fits in snuggly and flush with the opening. This provides safer working platform with no risk of cutting ot injuring yourself with the sharp edge on the glass.

I hope that you found this article helpfull and even draw some inspiration to build your own UV Light Box. It is a fun project and a very worthwhile investment that makes DIY printed circuit manfacturing just a bit easier and convenient. I might add an automatic timer circuit in future, but it is quite straight forward to to just time it manually.

Some of my projects

I decided that my first real post will be a brief overview of some of the projects I keep myself busy. I will try to write a more detailed article on each in the future. This article is meant to at least put something up here as a start and also to get myself more familiar with the process of publishing this way.

I have never been much of a writer and English is not my first language, so please bear with me. Hopefully I wont bore you to death in the process.

Firstly, I enjoy tinkering with embedded software. I have always been fascinated by the interaction of software with the physical world. The fascination probably started in the good old days of dial-up modems and the characteristic chirping noise they made to establish a connection. This was hardware and software in action, working together and using sound to get a job done. My young mind was blown and I just had to satisfy my curiosity into this technological wonder world.

In order to get software to interact with the outside world, you need a circuit off course. I make my own printed circuit boards as you will notice. I am getting better at it. I even made my first double sided pc board recently.

This is a USB serial converter / infrared receiver and transmitter. It is the first board that I made using surface mount components. I am quite pleased with the result. It produces a board that requires fewer holes to be drilled, I think. I still need to drill holes for the vias, for which I use a 0.6mm drill bit. The software is still a work in progress though.

I recently pushed my manufacturing and soldering skills to a new limit, with this breakout board that contains an iPod connector.

The pitch of the iPod connector is 0.3mm! This breakout will come in handy in trying to interface iPod accessories. Notice the marks where I cut the traces connecting the ground pins.

Some of my more primitive projects involved white LED’s meant for illumination. We enjoy camping allot and these projects were meant to experiment with new ways LED’s can be applied in the outdoors.

This one is a LED light with a dimmer switch as well as an adjustable day/night switch to activate the light automatically after dark. It works surpisingly well, I just need to redo the board layout into a more suitable format for packaging.

It uses a dual 555 timer packaging, one timer implements pulse width modulation for dimming the LED’s and the other timer is configured as a one shot, to activate the pulse width modulation using a light dependant resistor as input. The LED’s are grouped in groups of 3, each group driven by a constant current regulator circuit to avoid thermal runaway.

The second LED project is simple LED strip with a 555 timer providing a push button on/off circuit.

Here is a lead acid battery charger I built, to charge a 6V LA battery I used for a line follower robot I built long ago. The charger worked well, but was a beast when it came to generating heat. It got hotter than hell and I had to position it in front of a normal household fan running at full speed to keep it from overheating.

That’s it for now. Im hoping in my next article, I will be able to show you some of the equipment I had to make, in order to refine my pcb manufacturing process. More on that later though…

It’s here!

I finally got round to creating a space where I can share my thoughts and hobbies with the world.

I am an electronics hobbyist and the title of this site refers to the idea that electronics are powered by smoke, because without the smoke, there is no magic.

If you came here hoping to find magic tabaco to smoke, sorry to dissapoint.