This is an Instructable for a homemade UV exposure box that can be used for making PCBs.
Parts that you will need:
1x ATmega8 microcontroller
2x TIP31 NPN Power transistor
1x Voltage regulator 7805
1x voltage regulator 7812
1x 5v buzzer
2x 33uF capacitors
1x 32,756 kHz quartz crystal
1x 8x2 LCD
3x push buttons
1x power supply for laptops
1x power socket
a few cooling fans
UV leds from ebay
wood for the box
2x opaque white glass
2x normal clear glass
Step 1: UV LEDs
One can get a lot of UV LEDs from ebay: link
I realise that I probably did not need so many, but I used two boards, almost A4 in size with about 350 evenly spaced LEDs per board. One board will be used for the top exposure and the other for the bottom.
The most important thing to remember is that all these leds will require a bit of power. One can use the following calculator online to determine resistor values and led configuration: link
A UV led has a forward voltage drop of about 3.2V and should work at about 20mA
At the bottom of the page, after the calculation, the total current drawn from the supply is calculated. This is important because you now know the voltage output and max current output for the power supply. Also bear in mind that the TIP 31 transistor can handle a collector-emitter current of 1.5A, meaning that each board of leds cant go over that.
Also at such high currents, the transistors as well as the led boards get quite warm, so cooling fans are a good idea: link
Another point is that the leds focus on a spot, so you need to keep the leds quite far away from the exposing pcb (about 12cm). To make the distance smaller and to ensure a good, evenly diffused light, I used white frosted glass.
Another option is to use very high power UV leds, but I discovered these after I got my normal 5mm high-brights: link
Step 2: Control Board
Attached are the files with the board design and the .hex file ready for flashing to the microcontroller.
The +16V socket is the input from the power supply whereas the +12V is an output for the cooling fans.
PIN C0 is reserved for future use.
Step 3: The Enclosure
The idea for this box is to place the pcb that is to be exposed with the transparent mask with the pattern between two glass sheets, like a sandwich, and to slide it into the box like an envelope.
I quite like this idea, because the box is then a sealed unit, and LEDs don't have the tendency to need replacing for quite a while anyway.
As always, planning is everything. Maybe draw the lines where the grooves will be and the holes before hand and see if everything fits before you start cutting. Take into account space needed for the power supply and the fans.
If you dont want cables getting caught on the glass as it is moving in, then you can hide connecting cables in a groove that will be covered by another piece of wood.
If the wood is too thick for the LCD of the push buttons, then mill out a depression with a router.
The microswitch is placed at the back where the glass with the pcb is inserted. This is to make sure that the pcb is fully inserted.
Dont forget side holes on the sides in the top and bottom compartments for the warm air to escape (if all fans are blowing inwards)
Step 4: Final Operation
For a normal photo resist, this box is able to expose the pcb in 45 seconds to 1 min:
I also use a solder mask layer (also from ebay) that needs 30 min of UV exposure for curing:
A video of the UV box in action will be added at a later time when I make my next pcb.
I hope that this instructable was useful to you. This is my first instructable, so any feedback would be appreciated for future instructables.