Photo Etching: Part 1 - UV Light Box

Before I get started, I first need to say that I haven't forgotten about part 2 of my 'Canine Direction Finder'.  The receiver is on breadboard and all that remains is to build a directional antenna. So, watch this space.

In the mean time, I thought I would add this article I wrote a while back about making your own PCBs using photo etching. Starting by creating your own UV light box using UV LEDs.

Creating your own printed circuit boards is not as difficult or expensive as you might think. In this article, I describe how you can use pre-sensitised boards and a home made light box to create your own boards.

Introduction

Photo etching uses a transparency with an image of the PCB to be created printed onto transparency film, that is then placed over copper-clad board that has been pre-sensitised. These boards are not much more expensive than plain boards. The board is then exposed to UV light through the transparency film.

The board is then put into a tray of developer and the image of the PCB tracks will become visible on the board just like an old fashioned photograph being developed.

Next, the board is etched in a chemical that dissolves the copper except where it is protected by the photographic image of the PCB tracks.

A Light box

WARNING: ULTRAVIOLET LIGHT IS DANGEROUS IT CAN DAMAGE YOUR EYESIGHT. WHEN ASSEMBLING THIS PROJECT DO NOT LOOK DIRECTLY AT A LIT UV LED. IF YOU HAVE TO CHECK THAT THE LEDS ARE WORKING, THEN POINT THE BOARD AWAY FROM YOU AND LOOK AT THE LIGHT REFLECTED ON A PIECE OF PAPER. YOU CANNOT SEE IT, BUT THIS LED PANEL PRODUCES VERY BRIGHT UV.

Rather than buy a light box I decided to save some money and have some fun by designing my own. To start with the light box will have no timer, but rather the exposure that normally needs to be about 30 seconds will just be timed by hand. The first job of the light box will be to create a PCB for a simple timer to control our exposure more accurately.

Traditionally, UV light boxes are built using florescent UV tubes. These are relatively expensive and the tubes have to be replaced, so I decided to use a large array of UV LEDs sourced from eBay as a lot of 100 at a cost of about USD 20 - by far the biggest single outlay for the project.

The light box itself is going to be housed in an 7x5 inch index card box and the copper clad board and photo mask transparency will be sandwiched together in a cheap clip-style photo frame.

Power is supplied by a 12V wall-wart power supply.

You will need

84 (buy 100) x 400nm (at least 3000 mcd) UV LEDs

28 x 56Ω 0.5W resistors

Strip board 15cm x 10 cm (roughly 6” x 4”)

6” x 4” photo clip frame

7” x 5” index card box

12V 1.5A ‘wall-wart’ power supply

2.1mm power socket

When buying the LEDs, it does not really matter whether they are 5mm or 2mm diameter, but do ensure that they are 400nm wavelength and bright enough (at least 3000 mcd). 

The LEDs will be mounted in the top of the index box and the copper clad board will be a good 6 inches away in the bottom of the box, and so the angle of the LEDs is not critical.

The LEDs are arranged in sets of 3 LED in series each with their own current limiting resistor. These series sets are all arranged in parallel on the strip board in a hexagonal pattern.

You will need to make breaks in the copper track of the strip board. The easiest way to do this is by hand using a drill bit. Put the bit at the hole where you want to make the cut and twist it between forefinger and thumb, just enough to break the track. 

There are a lot of LEDs and resistors to solder into place. Start with the linking wires and then the resistors and finally the LEDs.

Since there is a significant risk of accidentally creating a short-circuit, I found a good way of working to be to solder up a batch of three LEDs and then make sure that they lit before moving onto the next set.

Hold the board away from you or upside-down and shine the LEDs onto a sheet of paper and look at the sheet of paper when testing them. Although they do not look bright, these LEDs are emitting a lot of ultraviolet and could easily damage your eyesight.

Make a hole in the index card box for the power supply and attach the LED board to the top of the index box. I used strips of self adhesive velcro to attach the board to the box, but you could also use self adhesive pads.

So there we have it, our own light box. The next step is to try out some test exposures and learn how to etch some boards.

Part 2.