"Just for the fun of it": 2010

Tuesday 28 September 2010

UV Light Box

Makes it much more fun to make printed circuit boards.

When I began working with electronics again after a long break, I quickly realized that I needed a UV light box to produce my circuit boards.

Breadboards and test prints are good for prototyping, but at a certain point is it necessary to have a more robust and portable version.

When I make printed circuit boards I use positive photo printing. The layout I print on transparencies on my ink-jet printer. To get good coverage I need to print layout twice on the same sheet.

Update:

The old ink jet printer gave up and have now been replaced with a new colour laser printer. I no longer need to do double printing to get a good result. That saves oceans of time, because the old ink jet transparency needed to dry totally between prints. That could easy take a couple of hours with the ink I used, even using a heating source to speed it all up a little.
The first test prints on the new printer have been quite successful. I was a little sceptic when I saw the first transparencies though - they looked quite weak, but to my surprise the result was surprisingly good. This circuit board only took a couple of hours to get to this stage. The circuit board obviously still needs the drilling.

The UV light box is made from 9mm MDF (Medium Density Fibreboard). The light source is a 8W UV-A lamp that I have taken out of an old handheld face browner I had lying around from my more vain days. After some experimenting I found the optimal exposure time to be about 25 minutes. So of course I needed a timer circuit.

The first version was a simple timer, with the familiar 555 as the main component. It worked OK and served its purpose. But I wanted to be able to get an indication of the time left, so I decided to build a new timer using CMOS technology. This time with the proven 4000 series.

The cover is locked with a small snap lock.

The UV light and reflective parabola.

The parabolic reflector is made from ordinary kitchen aluminium foil glued to thin cardboard. The UV bulb is placed in the focal point of the reflector. A piece of perspex is used as the top. On the inside of the lid, I have placed some adhesive rubber foam to keep the PCB in good contact with the layout of the film.

The initial timer was build around the well known 555 timer.

Running.

There is drilled a small hole in the lid and a piece of clear plastic is inserted in the hole. That way I can see if the lamp is lit. I suspect that it will give up one day.

UV lamp was purchased somewhere in the mid 80s. Fortunately, I almost never used it, so with some luck should be able to use it for quite a few cycles before it burns out.

The wire can be curled up for easy storage.

The finished UV light box. Now with a digital timer build with the good old 40XX series CMOS.

The new timer from can be set from 1 to 99 minutes.

Saturday 5 June 2010

Circuit Board Etcher

Small portable circuit board etcher.

After having had a several years long break from working with electronics I have finally got started again. I started with building very small projects build on experimental board. Quite soon though, I wanted to make proper circuit boards. It is not really a big job to make your own circuit boards. It just requires some good working space and the proper equipment. Living in a flat in the centre of Amsterdam, space is sparse. So I have been creating my circuits on the kitchen table. But mixing photo print developer and etchant with food production is not an ideal situation.

So to keep things a little more efficient and less space (and time) consuming I wanted to build a device that would do the etching without me having to stay constantly over the whole etching process. And more importantly; when finished etching it is easy to clean and store the etcher in the cupboard.

The Etcher

The etcher has been build entirely from materials I had lying around. Well almost - I bought the power plug so I could avoid having to sling around with a fixed wire. This also makes the etcher easy to store in the cupboard when finished etching.

From another project I had a few pieces of 4mm MDF over. Looking through the kitchen cupboards I found a transparent dishwasher proof food container with lid. The stepper motor and the cog are from an old HP Ink-Jet printer. I never got to use the motor because of its quite low step resolution. Fortunately the stepper motor is quite strong, taking its size in to consideration. The only special component is the stepper driver IC. I had a couple of the driver ICs in my component drawer from some experiments I had been doing some time ago. The rest of the components are all standard components.

The stepper motor has a resolution of 48steps/revolution.

The etcher will tilt the etchant container slowly up and down with the help of an eccentric wheel attached via a cog to the stepper motor. The eccentric wheel is, like the box, made from 4mm MDF. It was meant for testing purpose only, but turned out to work quite well. I polished the rim with fine sand paper and gave it a couple of layers of floor varnish to make the surface both extra smooth and hard. The eccentric wheel only needs to lift a couple of hundred grams so the wear and tear is quite limited.

With my initial build I mounted the eccentric wheel directly to the cog on the stepper motor. This made the whole etcher vibrate quite a lot and made it very noisy. I tried to run the stepper motor in half step mode and that reduced the noise to a quite actable level. After doing a few etches though I decided that the noise was too much anyway. So I mounted an extra cog between the stepper motor and the eccentric wheel. That also meant that I could increase the speed of the stepper motor and thereby reducing the vibrations and make it nearly silent.

The initial eccentric wheel mounted directly on the stepper motor. This caused the whole etcher to vibrate and be quite noisy.

Using a cog between the stepper and the eccentric wheel allowed me to increase the speed of the stepper and thereby reducing the noise to a minimum.

The etcher box is build from 4mm MDF I had lying around. I glued the pieces together with good wood glue. I attached a piece of piano hinge to the back and lid of the etcher to allow both the tilting of the etchant container and to give access to the circuit board.

Under the hood.

The Circuit

The circuit diagram below contains stepper motor driver with speed control, the temperature control and power supply.

Circuit Diagram

Stepper Driver

The stepper motor I have used is actually a 6 wire bi-polar motor but I use it here as a 4 wire uni-polar motor. The reason for that was basically that I had a driver for that configuration lying in the drawer.

The stepper motor driver is build around an H-Bridge driver IC - MC3479. The stepper motor has a quite low step resolution (48 step/rev.) and it is rather noisy if run at a low speed.

The clock input of the stepper motor driver is created by a simple oscillator build around the well known CD4093. This is a quadruple 2 input NAND gate with smith trigger input.

The speed can be adjusted so that the etchant slowly waves over the circuit board to be etched.

Heat Control

I am using natriumpersulphat for etching. This etchant works best at a temperature between 45°C and 50°C. To heat the etchant to the optimal etching temperature, some kind of heating element was needed.

My initial idea was to use some length of constantan wire for the heating element. But that gave me a couple of challenges to overcome. Firstly; my local electronics store only sells constantan wire in big roles with several hundred meters of wire and costing nearly €60. I would only need a few meters so I thought it was a little over the top. Secondly; I would have to build some kind of mechanical frame to hold the constantan wire.

Another idea was to use some power resistors. That would be a cheap solution, but again the mechanical build would be bulky and unpractical. So after a few calculations and experiments with a circuit board I decided to try it out.

The heating element is simply made from a piece of 100 x 100 mm thin circuit board with one long single track. The total track length is around 5.5 m. The track width is 12mil (0.012”) that gives a resistance of the track just around 12 Ohm. The heating element is glued to the etchant container with heat resistant epoxy glue to give a good thermal connection.

The heating element glued to the bottom of the etchant container with heat resistant epoxy glue.

The heating element is connected to the box with a short length of wire and a plug. This makes it possible to detach the etchant container for rinsing, cleaning and storage.

View of the etcher with power cord, stepper motor and connection to the heater.

The whole unit is fed from a 2 x 9V 1.5Amp ring core transformer. Again; I had it lying around from another project that had been shelved. The transformer is therefore also slightly under dimensioned - but I didn’t want to go buy a new one - they are quite pricy. Because of the under dimensioned transformer the speed of the stepper is slightly slower when the heater is on because of the voltage drop.

The heater gets fed from the total 18V. The rest of the circuit and stepper motor is fed from one 9V winding only.

The temperature is controlled by a very simple circuit build around a second gate of the CD 4093 smith trigger. It uses a voltage divider consisting of a 15k NTC resistor and a 10k trimmer. The midpoint of the voltage divider is fed to the input of the smith trigger which drives a transistor that switches the relay off when the desired temperature is reached. This is a very simple solution but in my setup it works like a charm. It keeps the etchant temperature just around 47°C.

The NTC resistor is mounted under the etchant container and is spring loaded so it stays in good thermal contact with the heater. So I do not measure the temperature of the liquid but the temperature of the heater itself. I used a normal thermometer, while doing the adjusting, to measure the temperature of the liquid.

The etchant container sits on a piece of 5mm thick foam. Mainly to keep the noise to a down, but it also ensures a good thermal contact between the NTC resistor and the heating element. It also makes the container sit quite sturdy. In the initial build it had a tendency to glide around, like when a mobile vibrates on the table.

The 'spring loaded' NTC resister from the top and bottom.

The heating element can heat 1dl of etchant from room temperature to 45° in about 15 minutes. If preheating the etchant, the heater only needs to keep the temperature to the desired temperature while etching.

I usually pour the etchant in the etcher and start it at the same time as I start to expose the circuit board in the UV light box. In the time it takes to expose the circuit board the etchant has reached its optimal temperature. It now only needs to be kept the right temperature by the heating element. I use a light box build around an 8W UV-A lamp hacked from an old face browner I bought years ago in my more vain days. See this post. The developing of the photo print takes about 25 minutes with the mentioned lamp so that leaves plenty of time to raise the temperature of the etchant.

Etching a circuit board

Here you can see the progress of the etching process.

The circuit board immediately get a pinkish colour.

The etching is in progress.

Just a few minutes left.

The finished circuit board. Normally I do not use hatch pattern, so this was kind of an experiment.

Circuit Board

Below is the circuit board. You can download the corresponding Eagle files here .

Work Bench

Let the chaos be gone.

A place where I can keep my electronics and do my experimenting - and still have a tidy living space.

Ever since I was a young lad, around 12 years old or so, I wanted to own a bureau or chatol, as it is called where I come from. I think my fascination came from a bureau , I saw in an old antique shop, in the town I grew up in.
That, of course, was a proper bureau. It was made from beautiful dark wood. It had loads of drawers and doors and of course some secret doors and drawers too.

At that age I started working with electronics as a hobby. Being four brothers in the same house, meant that there were quite a bit of sharing spaces. So a bureau would be perfect for all my experimenting. I never got around to get one, but coming from quite a creative family, I decided finally to try to build my own simplified version. Now I only hope my old childhood dream finally comes to good use.

Well I haven't aimed to build a it like the one I saw in the shop. First of all, it would be way too big and way out of my comfort zone. The content of my toolbox is limited to a nail file and a pencil sharpener (more or less). So I have to, and want to, keep it simple. At least for this - my first piece of furniture.

It is just supposed to be a place where I can work with electronics and be able to literally close the lid when taking a break, without having to clean everything up.

Living in an Amsterdam flat, in the middle of the old city, space is very limited. So every corner has to be used to its optimal, without making it in to a total clutter of course.

As you can see on the images below, one drawer section can rotate outwards to give more leg space when sitting working. The total width of the chatol is 110cm. I obviously wanted plenty of drawers on both sides. The drawer section on the right side is 30cm wide and full depth, 40cm. This fits an A4 with room to spare. The swing drawer, as I have decided to call it, is 40cm wide and 30cm deep. This gives 50cm of leg space. That is enough while sitting, but when getting up you will bump into one of the drawer sections. So by swinging one drawer away while working, gives just that extra space needed. I could have made a drawer on wheels, but that would make the construction unstable.

Here under is a model I made from 4mm MDF. I wanted to check if my design was stable enough. The full scale version is made mainly from 18mm MDF.

This small model is actually amazingly strong. I’m not sure if this is a scientific correct approach. But I have trusted my instincts and started the build.