RF Circuits

Ultraviolet (UV) light boxes can be pretty expensive, some costing upwards of £100. I don't have that kind of money to throw towards making a few hobby circuits, so I decided to build my own out of cheap parts (namely eBay-bought UV LEDs). I thought I'd share with you all my experience.

What is a light box?
A light box is used in a process called photo-lithography to transfer an image from acetate (overhead projector film) onto printed circuit board. This is how to draw all those little lines and circles on printed circuit boards, so that you can stick all the resistors, capacitors and microchips on.

Photolithography consists of the following steps:
1 - Design a circuit on the computer, print the circuit to acetate (OHP film)
2 - UV photo-expose the acetate onto a specially prepared blank circuit board (this is what the light box is for)
3 - Throw in some chemicals
4 - All the copper you don't want on the circuit board melts away, leaving behind some pretty designs

(If you want to know more about photolithography, read the Wikipedia entry here)

Anyway, onto my light box, and how to build your own. You will need the following components:

• A box - I recommend a £3, 3-litre Really Useful Box® (Maplin: N35BG)>. Keep in mind this is see-through, and if you buy better UV LEDs than I did you may get a bit of a tan


• UV LEDs (50 or so) - I bought mine off eBay at £4 for 100, just search around


• Resistors - I used 8x 2W 390R resistors. UV LEDs draw a fair amount of current so I split my LEDs into groups of 6 and used a resistor for each group. YMMV with different LEDs (see the maths section at the end to calculate what you need)


• Some plain matrix board - This is just strip board without the copper strips - Bought from Maplin, thought I can't find the code on their website


• A switch, a wall-jack socket (ie. the socket for a 12v power adaptor), some solid core wire

You will also need a soldering iron, solder and a drill.

My light box varies from traditional light boxes because the UV source in mine is in the lid, and the circuit board sits in the base, face up. Traditional light boxes have the UV source in the base, then a sheet of glass to place the circuit on top of, and then a lid. I like to be different ;).

The first step is to figure out how many LEDs you will need. My box gave about 12cm in height from the LEDs to the bottom of the box. With a viewing angle of 25 deg, this meant that I got a circle of about 5cm at the bottom. Just to be safe, I placed my LEDs ~4cm apart. (See the end of this post for all the complicated mathematics)

Next step is to figure out what size to cut your matrix board, so it will comfortably fit in the lid. Mine was about 18x12cm. This still left gaps around the edge (because the lid is irregular around the edges), though as long as I keep the circuit in the middle it comes out OK.

Draw yourself a grid to lay out the LEDs. Try to make this regular so you get even exposure.

Once you have the grid, you should drill 4 holes around the corners of the board. Make sure you avoid the LEDs, and try not to place it in the middle of a line of LEDs, as you will be running wires down that line. Drill 4 corresponding holes in the lid of the box to mount the PCB.

Now push the LEDs through the holes in the matrix board. Remember to lay them with the polarity facing the same way. You can either place them with the two legs horizontally or vertically. This will depend on how you are grouping them together, try to choose a way which will make wiring easier.

(To hold the LEDs in place for soldering I used a little spot of superglue - don't do this - superglue is pretty toxic and the vapour released during soldering can't be good!!)

When wiring the LEDs I placed them in parallel. All of the cathodes (negative legs) can be joined together. The anodes (positive legs) need to be joined together in groups (the size of which we decided earlier). Each group gets its own resistor, then the other side of the resistor is joined to the +ve supply.

Take some wire and strip the plastic coating off it. Cut it into lengths to join the legs of each LED in each group, one wire down each side of the LED. Solder these wires on and your LEDs will be held in place. You can just about see what I mean in the picture, as well as my ingenious use of Lego blocks :)

Now take some more wire, measure out the length you will need to join all the negative tracks together. Strip a little of the black plastic off the end. Measure the distance you will need to travel to the next track and use a knife to cut the plastic here. Slide the rest of the plastic along a bit, and you have a gap to solder on to the next track. Repeat for all the tracks.

Place the resistors on the board and solder one side to the positive track. Repeat the procedure above to join the other side of the resistors together. You now have a circuit board full of UV LEDs. To test it out, use some crocodile clips or pieces of wire, and connect to the positive and negative rails (make sure you connect to the red wire which is before the resistors! you don't want to connect straight to the LEDs and blow an entire group of them!) If all works, you can move on to the next stage.

Drill a hole in the lid for a power switch an the wall jack socket. Wire these to the board, and then screw them into the lid.

Finally, you need to find some posts to hold the board slightly away from the lid. I used little pieces of Lego since they were all I had to hand.

Once it is all screwed together, you have your very own UV light box! I drew up a little test board and printed it on some acetate. I exposed it initially for 4 minutes, but because of my cheap LEDs it didn't work. You will have to use trial and error to find your optimum exposure time, but mine is about 12 minutes (yes I know this is a loooong time).


Here are some photos of the box in action, and the first few circuits I've made.





All the complicated maths stuff

Resistors
Resistors come in different power ratings - 1/4W, 2W, 4W, etc. You will probably want 2W resistors, as these are good compromise between size and power. That being said, you will still need a bunch of them.

Power rating is calculated using the formula P=IV, where P is the power, I is the current, and V is the voltage. I used 2W resistors, at 12V, so by the equation I=P/V, I can draw 2W / 12V, or 0.16666 amps (160mA). This means a maximum of 8x 20mA LEDs.

LED Spacing
To figure out how far apart your LEDs should be, you need to know two high-school maths principles - trigonometry and the square inside the circle.

First step, figure out the radius of the circle at the bottom:
You should use trigonometry (the tangent of the angle by the height of the box - dont forget to divide the angle by 2!)

The equation is: Radius of circle = Height of box * tan(Viewing angle /2)

Now if you are arranging the LEDs in a neat grid, you will need to know the size of the square inside the circle. This is found using Pythagoras' theorm, the hypotenuse being the diameter of the circle.

The equation is: Width of square = sqrt(diameter² / 2)

This will tell you how far apart your LEDs can safely be to avoid blackspots