Photo Etching: Part 3 - Make a Timer

Now that we can create our own circuit boards, lets improve our UV Light Box, by adding an automatic timer to it.

The PCB is designed in EagleCAD. A free version is available for non-commercial use. It is not the easiest software to use, but you will soon get the hang of it with a bit of patience. The Eagle project files are available here.

We will use the famous and much used 555 timer and a variable resistor to control the length of the exposure. We will also have two indicator LEDs. A red one to indicate that power is supplied to the Light Box and a blue one that will only be illuminated when the UV LEDs are lit.

You will need the following parts:

1 x NE 555 time

1 x 8 pin DIL IC socket

1 x MΩ Log potentiometer

2 x 470Ω 0.5W metal film resistor

1 x 33KΩ 0.5W metal film resistor

1 x 100µF 16V electrolytic capacitor

1 x Red 3mm LED

1 x Blue 3mm LED

1 x FQP7N10 N-Channel MOSFET 1

1 x Push to make switch

The schematic diagram for the board is shown below, along with the board itself.

The board design is included in this zip. Its also shown below, but not to the correct scale.

Print off the timer board design onto transparency film and expose it in the same way as you did for your test strip, using the exposure you decided on in the previous section.

Make sure that you put the transparency on top of the board the right way up. I.e. it should have the rougher side on which the image was printed to the top, as the PCB layout shown at the end of the document is from the perspective of looking through the board from above and so we need to reverse it. 

When the transparency is on the board in a position for printing, the text should read correctly rather than be mirror image.

Once the board is etched, you will need to drill the board using a 1mm drill bit. Use the diagram below to identify the position of each component. Solder the components into place, starting with the lowest components first.

The potentiometer is attached to short wires to make it stand far enough away from the board to fit into our enclosure. Depending on the design of your index box, you may not need to do this. Similarly, wires are soldered between the termination points on the circuit board and the switch. This also applies to the connection to the UV LED Array and the power socket.

Note that you could use screw connectors for J1, J2 and J3 on the board, but I just soldered leads directly to the board. 

Note also the two red jumper links that are required.

Test the board connected to the power supply and the switch using just the blue LED, before fitting everything into the box and connecting up the UV LED panel.

Line up the completed board against the inside of the box and mark the position in which to drill the holes for the potentiometer, the push switch and the LEDs. The collar of the potentiometer and the snug fit of the LEDs into their holes is used to keep the board in place.

After it is all assembled, we need to calibrate the timer. I used a paper label onto which I marked the times, including the optimal time for my set of equipment of 30 seconds. 

A stop watch and trial and error was used to calibrate the dial.

Conclusion

It is beyond the scope of this article to go any further into PCB design. The author uses the free (for home use and small boards) Eagle CAD software. This excellent software takes a little getting used to, but is very powerful and will attempt to automatically create board layouts from your schematic diagrams.

Have fun, but above all, be careful - almost everything in this project is dangerous!

About the Author
These are my books. Click on the image below to find out more about them.

Photo Etching: Part 2 - Exposure and Etching

We have a functioning light box, so, now its time to do some etching!

WARNING: IN THIS SECTION WE ARE USING POISONOUS CHEMICALS, THAT CAN ALSO BURN. WEAR PROTECTIVE GOGGLES AND GLOVES WHILE HANDLING THESE CHEMICALS.

ALWAYS FOLLOW THE INSTRUCTIONS INCLUDED WITH THE CHEMICALS EVEN WHERE THEY DIFFER FROM THE INSTRUCTIONS BELOW.

You will need:

• An assembled light box from Part 1 of this blog entry
• Small single-sided pre-sensitised copper-clad board 100mmx75mm
• Craft knife
• Steel rule
• Scissors
• Sodium persulphate etch powder
• Universal photo-etch developer
• Disposable latex gloves
• Safety goggles
• Empty bottles to contain the mixed chemicals (not the bottles should be clearly labelled POISON - DO NOT DRINK)
• A bucket
• Developer trays, I use recycled plastic food tubs

Everyone learns by making mistakes. Every exposure box is likely to have a slightly different intensity, and need different lengths of exposure. Also your inkjet printer, transparency film and copper-clad board are all likely to be different to mine. Although there is actually quite a wide margin or error, we need to try and find the optimal exposure for our set of equipment without wasting too much copper clad board.

So, to avoid frustration, just accept that you are not going to get anything except knowledge out of your first few dollars worth of copper-clad board and transparency film.

We are going to start by making a test strip to determine the best exposure for our printer / board / lightbox combination. This is a technique taken from ‘good old fashioned’ film photography. The idea is that we will have a test pattern printed onto transparency film that we will partially cover up with the first exposure and then gradually move the card to expose more of the board for the subsequent exposures. 

But first, let’s print the test strip onto transparency film.

Printing

First of all, we need to create a test strip pattern. The test strip below can be printed directly by printing the last page of these instructions onto transparency film. This film used to be used extensively for presentations on over-head projectors. 

I use an inkjet printer, but if you have a laser printer then you can also get transparency film for them. Be sure to get film for your correct type of printer. You cannot put inkjet film into a laser printer and vice-versa. Any major office supplier will sell transparent film. 

We need to print as boldly onto the transparency film as possible to ensure as little UV as possible leaks through to the coper clad board. To do this, make sure that you set:

the media type to ‘Transparency’

document type to ‘Monochrome’

quality to ‘Maximum’

The transparency sheets are relatively expensive, so I tend to print at the top of the page and then cut the top of the film and use the remainder for the next board, getting three or four boards out of one film.

When you have printed the film, try to keep it clean until you are ready to use it, as any little specs of dust will end up as copper.

A Test Strip

Firstly we need to cut up our sacrificial copper clad board into test strip sized pieces. You can do this with a craft knife and metal rule. Score the board top and bottom, where you want to cut it and then snap it over the edge of a table.

You will notice that the copper side of the board has a thick layer of light-proof plastic covering. This will need to be removed from the test strip before exposing it.

Here are the steps for creating your test strip.

1. Mix water into the developer powder according to the instructions on the packet. Mixing them into a storage bottle. If the chemicals are designed to work at a higher than room temperature then place the bottle it in a bucket of hot water to bring them up to temperature.
2. Do the same for the etch solution.
3. Trim the transparency film to be about the same size as the copper-clad board.
4. Disassemble the clip frame
5. Peel the protective layer off the board and place is sensitized side up on the base of the clip-frame
6. Place the test strip over the board
7. Fix the glass lid of the frame back into place.
8. Place the whole photo frame into the bottom of the light box.

Now we can begin making the exposures.

We are going to make four exposures, for the first three covering successively less and less of the test board. 

The sequence is:

1. Cover sections D, C and B with a thick piece of card (on top of the glass) and expose for 40 seconds.
2. Move the board to cover just the sections D and C and expose for 20 seconds
3. Move the board to just cover section D and expose for 10 seconds
4. Remove the board completely and expose for 10 seconds

Developing

By this time your chemicals will be at the right temperature and we can develop the board. If you are not quite ready to develop yet, then it does not matter, as long as you cover the board to prevent any further exposure from ambient light. Turning it upside-down is sufficient.

Put on your protective eye-ware and latex gloves and place the board copper side up in the tray that you are going to develop in.

Pour some developer onto the board enough to cover it with about half an inch of developer. Then gently rock the tray back and forth. You should soon see a picture starting to develop. Do not remove the board from the developer until the test strip image is clearly visible and well defined on the board. It is much easier to under-develop the board than to over-develop it, so if in doubt leave it in the developer a bit longer.

When its ready, it should look something like this:

When you are happy that the board is ok, then remove it and wash it in the water in your bucket. Pour your developer back into its storage bottle ready for next time.

Etching

Either use a separate tray from the developing tray or clean your developer tray very carefully, then place the board in copper side up again and pour the etchant over it. Again a depth of about half an inch is ideal. The etchant solution will gradually go blue as it dissolves the copper that is not protected by the photo-resist image, so it is much easier to see what is going on if you don’t fill the tray right up.

You may have to fish the board out of the etchant form time to time to check on it.

Almost immediately, you will notice the exposed copper areas to start to go pink. You know when the board is ready when all the pink areas have gone. The two pictures below show a board that is not quite ready yet followed by a board that is fully etched.

When the etching is complete, put it back into the bottle and put your chemicals away safely out of the reach of children. Especially the etchant, which turns an attractive blue color when used.

Wash the board again, and you are done!

Assessing the Test Strip

An example test strip is shown below.

On the left had side of the board, 10 seconds was not enough as most of the copper is still there. On the other hand 80 seconds was too much as the UV clearly went straight through the printed mask. 

40 seconds was still a bit much, but 20 seconds was just about right. Echos of Goldilocks there!

In the final part of the blog entry we will use our new toy to create a PCB for a timer to control the the UV exposure box.

Now I call that bootstrapping!

Part 1. Part 3.

About the Author
These are my books. Click on the image below to find out more about them.

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.