With a few items salvaged and picked up in car boot sales you can make an effective PCB exposure box for less than half the price of the cheapest commercial boxes. I have made four of these units (some double sided) yet still use the first one I made nearly ten years ago!
I have decided to update my old battered box with a snazzy new one and thought it would be a good idea to share my experiences.
Step 1: Basic Materials
A metal clad tool case (Maplin, B&Q, tool shops - see photos).
Perspex sheet 3mm thick and the size of the inner rim of the lip of your box (see photo later).
M3 Nuts, bolts and washers (20 or so will cover it).
Black card sheet (A3 size, Staples).
2x UV-A tubes - not blacklight tubes, you need to get the right ones (Rapid, CPC, Specialist).
Tube "U" clips to fit the tubes - metal sprung type.
Mains rated switch (Maplin, Rapid, CPC etc...).
Mains flex 3M or so, 3 core 5A (to use whole and stripped into conductors).
Heatshrink tubing slightly larger than the wiring you will use (inner conductors of mains flex).
3A Terminal block or "chocstrip"
Mains strain relief cable gland (Maplin, electrical supplier) to fit your mains cable.
Various short screws, pan head and countersunk self tapping.
2 small blocks of wood 1"x1"
Mirrored card/plastic or tin-foil.
Crimp tool for ring terminal.
Small flatblade screwdriver
Cross blade screwdriver
Long nose pliers.
Knife or scalpel.
Step 2: Strip Out the Toolbox!
Step 3: Mount the Ballasts
Each ballast is capable of driving up to 13 watts. We are going to use two 8 watt tubes so we need two ballasts (you may find one which drives both tubes but you will need to modify the wiring to accommodate this).
Leave enough room around your ballasts to get the wiring in. Your ballasts may have different mounting lugs but these had two which were offset in each corner. Mount using your M3 fixings.
Step 4: Mount Your On/Off Switch
Drill carefully from outside in, the thin covering of your toolbox tears easily. You may have to cut some of the box lining out to get your switch to mount properly. I found using a nice sharp scalpel helps.
Step 5: Install Mains Cable and Wire Up the Ballasts
Warning: because your box is metal it needs a good earth, bolting the earth directly to the casing is best but as the box is made from sectional panels so you may have to provide multiple earth bonds. Think safety - I use a 30mA RCD with pretty much everything with exposed metal parts be it double insulated or not.
At this point you can wire up your ballasts too using your spare stripped mains cabling conductors. The ballasts I have feature spring contacts which trap the wire. Wire up as per the wiring diagram with the mains feed to them in parallel. Leave enough wire to reach to your tubes in the main compartment of the box.
Step 6: More Wiring!
Install the bases as close to the right hand side of your box as you can so all the working parts are in a neat confined space.
Step 7: Encase the Gubbings!
You can glue the divider in but I reccommend just pushing it in so it can be removed for repairs if you need to.
Step 8: Install the Tube Bases and Mirror
Cut up eight pieces of choc-strip connector and screw them onto the tube pins (unless your posh and have tube connection clips!) this makes wiring to the tubes easy.
Place the tubes down on the mirror and shuffle them around until they are in line and leave enough room at either en of the choc-strip to fasten the wires in.
With a small marker pen, mark where your tube clips are and then drill and bolt the tube holders down into position.
Step 9: Wire Up Your UV Tubes
Clip your tubes down into place and give the whole box a good vacuum out as we are about to seal it.
Step 10: The Initial Test!
DONT look directly at the tubes, UVA is very bad for your eyes! Both tubes should light and within 10 minutes be at full brightness. The workshop I tested mine in is fairly cold but the tubes were very bright in a short time.
If all is well, switch off and on to the next stage...
Step 11: Sealing the Whole Lot In!
Lay your perspex over the card and with a sharp scalpel cut around it. You should now have a piece of black card the same size as your perspex!
Taking a smaller rectangular shape (in my case a cutting mat) and have a look at where your tubes are in the box. Cut a smaller rectangle out of the card, this is where you will expose your PCB so it is best that the cut out is central to both tubes.
Tape the card down around your cut out edges so the card doesnt sag in the box.
Step 12: Nearly Done!
In each corner of the box, in line with the top of the sponge lining, screw your wooden blocks in as low down on the block as you can without splitting it (use a small pilot drill at low speed).
Your perspex should sit flush onto the blocks and not bend or warp.
Place your perspex on and slowly drill a pilot hole down through into the blocks. Use a short, pan head screw to fix your perspex down to the blocks, dont go too tight or it may crack!
Step 13: Final Testing...
I suggest leaving it on for an hour or so to make sure that nothing is going to fail or melt!
I made a sticker to place on top to remind me to switch off when opening the lid. UVa isnt very eye friendly.
Step 14: Thats It!
You may need to diffuse the light using some milky acetate or white transparent plastic as sometimes it gives over/under exposure on the board edges but not too bad. Exposure time has a sweet spot of around 4 mins using standard board.
You should have a nice neat and portable exposure unit. There are several improvements you could make to your unit such as:
Interlock switch so the lamps come on after the lid is closed.
An IEC mains inlet so you can disconnect the mains lead.
Maybe white tubes on another switch to help with alignment?
You could also make this a double sided exposure unit with another box.
This box literally cost me no more than £30 to make. The nearest commercial equivalent is £150 so quite a saving!
I hope you have success with this box if you use it, you can make really good PCBs with the correct equipment and good quality fibreboard.