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Silicon Chip Article

 

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A UV Light Box For Making PC Boards

Even though SILICON CHIP publishes most PC board patterns and/or has them available for download, making your own PC boards has for many been put in the "too hard" basket. Here's one reader's way of producing commercial-quality PC boards at home. He starts off by building an exposure light box with timer.

by Robert Scott

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I have been using Autotrax* 1.61 to design PC boards for my own creations for a few years now, ever since it became available at the right price (free!). Before that I used Easytrax* and way in the past I used Bishop Graphics tapes and pads.

LIGHT BOX CONTROL TIMER

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Fig.1: the light box controller is built on two PC boards and this circuit diagram is split in two vertically, each part containing the contents of one of the boards. They are joined by two short cables, one 4-way and one 12-way, which plug into connectors 1/3 and 2/4 respectively.

That at least got me a PC board artwork. Now the challenge was to convert that to a PC board.

I tried using "PressnPeel", a photo-sensitive film which transfers a toner direct to the PC board surface using a hot iron. This then acts as the resist for etching.

However, despite the glowing reports I’ve seen on this product on the ’net, I found it had its limitations.

First, the blank PC board must be extremely clean for the toner image on the film to stick to it. Second, if the PC board artwork is quite a bit larger than the iron then it is hard to get the blank board up to the correct temperature all over for the toner to stick again.

Quite often you would pull away the film only to be left with a result where, Dalo pen in hand, you would have to repair the pattern as best you could.

It wasn’t a very satisfactory situation and to make matters worse, PressnPeel at a retail level adds quite a lot to the finished board cost.

 

Pre-coated boards

 

I had been looking for a source of relatively cheap, photo-resist coated blank board and found it in "Kinsten" positive acting photo-resist coated PC board.

Kinsten coated PC boards are available in both SRBP and fiberglass, single or double-sided and in a variety of sizes, from several sources – I obtained mine from KALEX (718 High Street Rd Glen Waverley Vic 3150. Ph 03 98020788). They can also supply via mail order.

Also available from Kalex are the 8W UV lamps used in this project at $9.75 each plus GST. The developing solution for this resist is available too. While it appears to be just plain old sodium hydroxide (NaOH; caustic soda), it is actually sodium metasilicate, mixed at 50g per litre of water. A 50g pack will cost $2.50 plus GST.

The good news is that I have heard sodium hydroxide works just as well. The bad news is that I have not been able to get sodium hydroxide anywhere down here in Tasmania yet so I cannot verify if the above is true.

Editor’s note: at SILICON CHIP we have been producing one-off PC boards using Kinsten pre-coated blanks for some years (in fact, we published a feature on it in March 2001) and had heard exactly the same thing.

We can confirm that properly diluted sodium hydroxide will develop Kinsten boards perfectly. Too strong a solution and the whole image washes straight off. Too weak and nothing happens. Experimentation is a wonderful teacher. Incidentally, we found that sodium hydroxide is not difficult to obtain from specialist chemical supply houses here in Sydney.

OK, with the availability of the blank board and suitable UV lamps the next step was finding a way to transfer the computer-generated PC board pattern to a transparency through which the Kinsten coated blank boards could be exposed. The idea is to have as high a contrast as possible – black blacks and clear "whites".

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Fig.2: Light Box mains wiring. The two PC board modules control two sets of two 8W fluoro blacklight tubes, as shown here. Incidentally, with suitable mains insulation, these modules could also be used as a general-purpose timer.

The problem with most printers, especially printing onto transparency film (eg, overhead projector film) is that the blacks are anything but. Hold one up to the light and you’ll see what I mean.

If you are very accurate, to some degree this can be alleviated by using two sheets. I get very good results from two toner-coated transparencies from a laser printer stuck together with thin double sided tape. I haven’t tried inkjet transparencies or even know if this is possible with inkjet. I find a good HP or Canon laser printer such as the LaserJet 4 or Cannon LBP 1260 does the job admirably.

I have one of each of these; even the LaserJet II or III will do. These can be obtained quite cheaply second-hand and refurbishing the cartridge is quite easy, even if rather messy.

Editor’s note: inkjet prints can be just as good as, if not better than, laser prints. However, the problem of non-black blacks still exists. Incidentally, great results can be achieved by printing onto plain bond paper – with an appropriate increase in exposure time.

 

Exposing PC boards

 

The Kinsten coated boards are exposed by shining UV light through the artwork transparency. The clear part of the transparency "softens" the emulsion on the PC board, which is then "developed" away with the sodium hydroxide solution mentioned earlier.

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Figs 3 & 4, the component overlays for the Exposure Controller (top) and the display board/timer controller (bottom). CON5 should be a 3-way terminal block, as shown.

Two problems exist. One is to keep the PC board pattern transparency in intimate contact with the board so that there is no light "scatter", causing break-up of tracks. Even the thickness of the film itself can cause problems, so the image on the film should always be on the PC board side, ie, "emulsion to emulsion."

The second problem is to keep the amount of UV exposure constant in both time and strength, so that results are consistent.

Various methods of exposure have been tried over the years – including using the very high UV content of sunlight. But this highlights problem two – the sun’s strength varies according to time of day, cloud cover, latitude, pollution levels, etc!

The answer is to use a dedicated light box. With a timer, the exposure could be set. With pressure applied to the transparency, the two parts could be held together properly.

I thought I would see if a light box project was feasible. First thing? Check the net!

There appeared to be a lot of info but only one with anything like what I was looking for. It consisted of a PIC16F84 programmed as a timer with a basic circuit displaying on 7 segment displays. While it held promise, I believed that with redesign of the firmware for the PIC and particularly the hardware would make it much better.

 

Outline of the project

 

The electronics side of the project consists of two PC boards, each 120 x 64mm. One is for the timer lamp control and power supply, the other the timer control and display panel.

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There are some differences between these photos and the final version - specifically the mains connector, the fuse type, suppressor capacitor and the relay.

One of these is mounted on stand-offs on the underside of a folded aluminium chassis, which also contains the fluorescent tube ballasts and starters. The other is mounted on the side of (and through) the lightbox so that its LED displays and setting pushbuttons are all accessible and viewable from outside.

On the top side of the chassis are mounted the eight "tombstones" which hold four 8W NEC fluorescent "blacklight" (UV) tubes. These are not like the deep purple (almost black) blacklight tubes you see in clubs and discos. Instead, these are described as "actinic blue" and appear white when off but are very strong in UV as well as visible blue light when on.

This chassis is secured by screws in a wooden box, outside dimensions 360 x 120 x 100mm, which has a 6mm glass pane located in a channel in the sides of the box, which places it about 25mm above the fluorescent tubes. There is a hinged lid on the box which has a piece of 6mm foam covered with felt glued to its underside. When the lid is locked closed, the foam and felt force the PC board (and the transparency underneath it) hard against the glass pane.

This ensures that the blank board and the transparency have intimate contact with one another so that the image on the transparency accurately transfers to the blank PC board.

 

The circuit

 

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Below: the completed Light Box with its plywood base removed. The second PC board is on the left side.

Fig.1 shows the wiring of the exposure lamps, ballasts and starters, under the control of the timer PC board.

Power is switched to the fluoro tubes via a mains-rated relay, under the control of the PIC and switching transistors.

The four UV tubes are arranged in two identical parallel circuits, shown in Fig.2. Each one consists of two lamps, two starters and a ballast all in series. The starters are the 4-20W (more sensitive type) for the lower level currents involved with 8W tubes.

It’s a little unusual to have two tubes share one ballast, so a word of explanation might be necessary.

When power is applied, both starters will arc and close due to the internal bimetallic strip. The tube heaters will heat up and the inductor (ballast) will build up a 50Hz varying magnetic field. When one of the starters cool down and open the magnetic field round the inductor will collapse causing a somewhat large EMF to be developed across the inductor. This will appear across the open starter and its associated tube.

The gas inside the tube will ionise and the tube will strike. Once any fluorescent tube strikes, the voltage dropped across it due to current flowing through it is much reduced. If the other starter then opens induced EMF across the inductor again will strike the second tube.

All this happens rather fast and both tubes should be glowing within a second or so.

Sometimes both starters open nearly simultaneously and the startup strikes occur together. This type of circuit is possible with low wattage tubes as the distance between tube heaters is small compared to say, a 36W standard lighting fluorescent and the voltage drop is small.

Is Ultraviolet light dangerous?

From time to time warnings appear about the dangers of UV light. Even as we go to press, UV tanning salons have been implicated in at least one recent death through melanoma (skin cancer).

From the outset, let’s state that staring at any light, especially intense light, is not good for the eyes. Very bright light, especially if strong in ultraviolet wavelengths in particular, is known to cause eye discomfort and damage.

Ultraviolet light is generally regarded as having a wavelength from about 200 to 400nm (nanometres). This is further divided into three sub-bands, UV-C, UV-B and UV-A.

UV-C (200-280nm) has the shortest wavelength and is often used as a germ killer or steriliser. It is regarded as dangerous stuff! Anything which emits UV-C usually has interlocks to prevent accidental exposure to the eyes or skin.

UV-B (280-320nm) has a longer wavelength and is considered less dangerous but exposure can redden and possibly burn the skin and may cause damage to the retina.

UV-A has a longer wavelength again (320 to 400nm) and is considered less dangerous again.

Prolonged exposure to UV-B and perhaps to UV-A are acknowledged to cause skin damage and possibly promote skin cancers as well as eye damage. But the vast majority of references point to UV-B light as the bogey.

The NEC FL8BL blacklight lamps used in this project emit mostly UV-A, with a peak wavelength of 365nm (which also explains why there is so much visible blue light from them). They are in fact the same as (or similar to) the blue lamps used in bug zappers.

Ideally, you should avoid long exposure, especially of the eyes, to any UV (or indeed any strong light). But the high wavelength of these tubes, their low power (all four combined are less than a single 36W fluoro tube), the fact that there is a sheet of UV-absorbing glass above them and the very intermittent nature of exposing PC boards using them means that they are reasonably safe.

Having said all that, keep children away and don’t let your teenage daughter use this as a mini face-tanning centre! If you are still concerned, a mains-rated interlock switch (eg, a microswitch operated by the lid) could be fitted in series with the active wires going to the ballasts.

 

The PC boards

 

Two PC boards are used, sharing functions between them.

The control/display PC board is connected to the timer board with 12-way and 4-way cables. I used these as it was easier to design and make single-sided PC boards to suit these than it was to make a double-sided board with a dual-in-line 16-pin plug. Because these are all on the low-voltage side of the circuit, ordinary hookup wire or even rainbow cable can be used here.

The timer PC board is screwed to a small panel of 1mm aluminium with stand-off’s. Cutouts and holes are required in the panel for the stand-off’s, LEDs, 7-segment displays and pushbutton switches. This panel is then screwed to the left side of the light box with a cutout to suit.

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Fig. 7: here's how to fold and cut the aluminium chassis, looking from the underside. The only critical positions are the notches for the tombstones which must of course line up with each other. The PC board, ballasts and starter holders can be placed in approximately the positions shown.

Looking now at Figs. 2 and 3, power is supplied to the circuit via a 1A fuse, PC-mounted transformer, (240V to two 6V windings, eg, Altronics 7012). Both 6V windings are connected in series, rectified and filtered, resulting in an unregulated DC supply of about 16V or so.

The unregulated supply is used to power the switching relay and also fed to a 5V voltage regulator (REG1, 7805). This provides the timer with a 5V regulated supply.

Most of the timer operation is carried out by the programmed PIC16F84 so the circuit is not as complicated as it would otherwise be if hardware alone did the task.

The PIC’s clock is set at 4MHz by crystal X1. Pins 17,18,1,2 (RA0 to RA3) send multiplexed BCD data to the display board via P4-P2. Pins 6 to 9 (RB0 to RB3) send multiplexed data to transistors Q1 to Q4 (display drivers) on the display board via P4-P2. Pin 11, RB5, is normally held low in standby.

When the timer is counting down it goes high, biasing on Q7 (BC337) which pulls in RLY1 (supplying power to the fluorescent tubes), at the same time biasing Q5 (BC547) on and Q6 (BC557) off.

These in turn extinguish standby LED3 and turn on running LED4.

When the timer has completed the countdown RB5 goes low, which turns off Q7 and turns on Q5 and Q6. Relay RL1 opens, the timer LED4 goes out and standby LED3 comes back on.

Pin 3, RA4, connects to the select switch via P3-P1; a pull-up resistor is required here. Pin 12, 13 (RB6, RB7) connect to the set and start switches (S3, S2) respectively.

Pin 10 (RB4) provides a positive pulse every second while the timer is active and this pulse is fed to two LEDs in series via a 220W resistor. These form a "colon" between the minutes and seconds LCD digits.

 

Making the chassis

 

Aluminium was chosen for the chassis as it is easy to work with and some UV light will reflect from this, distributing the UV fairly well through the artwork. The chassis is bent in a "U" shape with holes and slots cut out for the various components.

The layout is shown in Fig. 7, reproduced a little under half size. Ideally, the chassis should be bent to shape with a sheet metal folder but good results can be had with 25mm angle iron and a sturdy vice.

The aluminium sheet size is 320 x 265mm and the sheet can be 1 to 1.6 mm thick.

 

Making the Box

 

Once the chassis is made then the box can be made to fit. I made my box from 17mm plywood, 100mm high. A plywood lid was made to suit from the same material.

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Fig.6: the complete wiring diagram showing the underside of the 320 x 270mm U-shaped chassis. The tombstones poke up through slots in the chassis with the tubes on the upper side. All wiring to the fluoro tubes, starters and ballasts must be 250AVC rated.

A sheet just over 600 x 470mm (to allow for saw cuts) will achieve minimum wastage. The two sides and two ends need a slot cut in them, about 7mm down from the top, to accommodate the glass plate.

There has to be a slot about 6mm down from the top of the box to fit the glass plate. This is best done with a router using a ¼" (6.5mm) bit. Cut the slot about the same depth (6.5mm). As you are not removing much wood this can be done with one cut. The slot can also be cut with a circular saw if you are experienced enough – a router is better though and they can be obtained very cheaply these days.

As the smallest router bit I had was ¼", the glass plate had to be the same thickness, ¼" or 6.5mm. This was a fortunate accident, because that’s about the right thickness for stability but not too thick to have to worry about UV absorption in the glass. It is important that the glass does not have any scratches or imperfections as these will surely show up in your finished PC boards.

With the dimensions shown, the glass plate will be 6mm all round greater than the box internal, the chassis is 225mm wide by 320mm long, therefore it follows that the glass plate will be 237 x 332mm.

Of course this all depends on your carpentry skills. I used iron-on veneer on the cut edges of plywood and varnished the whole assembly with Estapol. This makes the job attractive as well as functional. Ply was used rather than straight wood as this tends to be truer so the pieces fit together better.

The lid is a single piece of plywood, the same size as the box and again finished with iron-on veneer. It is attached to the box with two medium-sized hinges.

Inside the lid a piece of 6mm high density foam plastic, covered with self-adhesive felt on one side, was stuck into place with double sided tape to fit into the space between the top of the box and the glass.

Its size, 320 x 230mm, allows it to clear the box edges as the lid is closed and press down hard on the blank PC board to hold it flat against the artwork.

 

Assembling the PC boards

 

Solder the 3 links on the display board first, followed by the resistors, IC socket, displays, sockets and capacitors.

The same order applies for the timer/power supply – the lowest profile components first and highest last. 300mm lengths of 12-wire cable and 4-wire cables using rainbow cable or single hookup wire lengths has to be made in order to connect the display/control board to the timer board.

Do not solder the LEDs into the display PC board yet.

 

Putting it all together

 

All components can now be fitted to the chassis as per Fig.4 and wired as per wiring schematic Fig.3. Be very careful in wiring the mains-carrying cable – that is to all the fluorescent tube holders, starters and ballasts.

Mains wiring may be taken directly to the block connector on the timer PC board, with the switched active connected to the rest of the circuit. Use single-core 10A lighting wire for wiring the lighting circuit up. That’s not because there are heavy currents involved, it’s for the safety afforded by the cable’s insulation.

Connections to the tombstones are achieved by pushing the stripped cable into the hole provided. The wires are locked into place by a spring loaded clamp and once they are in it is difficult to pull them back out again so try not to make mistakes. Make doubly and triply sure, however, that all strands of the wires have gone into the hole and none poke out to possibly short to the chassis.

Make sure you earth the chassis via the earth wire on the mains 3-core cable and plug.

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The connections to the starter holders are achieved by a clamping screw.

A rectangular cutout will have to be made for the control /display board on its mounting plate either in the front of the box or as I have done in the left side.

I have specified insulated stand-offs to mount the PC boards but metal ones could be used except for the one on the mains entry side of the timer/power supply board.

 

Display PC board

 

The display PC board is mounted to a small piece of ~1mm aluminium with holes drilled for the LEDs and switches along with a cutout for the display.

A piece of 1mm reddish plastic was glued into the cutout as a protective screen for the 7-segment displays and seconds LEDs. Use a small quantity of slow setting epoxy for this. The "Five Minute" type sets too quickly and is not as strong.

Drill PC board mounting holes in the panel by placing a photocopy of the display board overlay on the panel, lining up the 7-segment displays in the cut out and marking the center of the holes to be drilled with a prick punch or scriber.

I used 2mm mounting screws, nuts and washers. The stand-offs should be 8mm to allow the push-button switches to sit proud of the front panel.

If using 2mm screws you may have to make your own from 2mm brass tubing available from model aircraft stores. The 2mm screws don’t stand out on the front panel as much as 3mm.

Countersunk screws could be used and the front panel artwork fixed to the aluminium over the screw heads.

 

Testing.

 

It’s best to test the timer out before you wire it in on the chassis. Plug the two boards together and wire the main board temporarily to the timer board. Do not plug the IC’s in both boards as yet, that is the 4511 and the 16F84A. Make sure you have double checked everything especially the timer/power supply with its mains wiring.

In the interests of safety, cover the fuse and fuseholder with some insulation tape while testing. It’s the only section of the top of the PC board that’s likely to bite you – but if you contact it, it will do just that!

Switch on power, measure to see if you have approx +16V and a regulated +5V where marked on the power supply; also that +5V appears on pin 16 with respect to pin 8 on the 4511 socket and between pins 14 to 5 on the 16F84A socket. If all is well and you have no burning smells switch off and remove the mains plug from the power socket. Wait a short time for the electrolytic capacitors to discharge and insert the two ICs

Reconnect and switch power back on. You should get a readout of 00:30 on the display board. The relay should not be energized and the green standby LED should be illuminated. If you do not have this, switch off and recheck your work.

Hopefully all should be well and you can proceed to check the timer operation. Press the start button, the green led should go out and the red one should illuminate, at the same time the relay will energise and the display will begin to count down from 30 seconds to 0.

When the timer reaches 0 the relay will drop out, the red LED will extinguish and the green one will come back on. Pressing the start button again will bring back the 00:30 readout again.

Press the select button and the display should change to a different time setting. Do this 15 times. There are 16 timer settings stored in EEPROM in the programmed 16F84A.

You can change any or all of these if you so desire by the doing the following: select a setting to change by pressing the select button until the display is reading the setting you want to change.

Press the set button. The seconds will start flashing, incrementing one more every second, when the time in seconds is reading your requirement press set again. The single minute digit will start to flash incrementing as before, again when your desired time is reached press set again. The tens of seconds will start to flash incrementing as the single minute digit did.

Again, when your requirement is reached press set again. The timer will be set in EEPROM to your keyed-in time.

If you make a mistake then you will have to go through the entire procedure again. Usually you will only need to do this once or twice.

If all is well checking the timer then it can be wired into the chassis and the rest of the wiring completed.

 

Using your new light box to make a printed circuit board...

Exposing the image

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Getting a black black is actually more important that getting a clear white (believe it or not, you can expose through bond paper!). The pattern should be on the bottom side of the film, so it is intimate contact with the photo-sensitive emulsion.

To make boards from Kinsten stock the manufacturer’s recommend exposure time is 60 to 90 seconds using a high-contrast film. Set the timer for 1 minute 15 seconds using a test artwork. You may need to do a few test exposures and increase or decrease exposure times as required. Too long and you will end up with all the resist washed away, too short and it will be under developed with the "clear" areas not washing away.

Using this presensitized PC board I found the latitude is about 10 seconds either way but you may find it different.

You could use Riston negative-acting pre-coated board but it is more expensive and so is the special developer and stripper. Also you will require a negative of your artwork.

To give you an idea of cost, a fibreglass pre-coated Kinstenboard, 150mm x 300mm from Kalex costs $16.50 plus $2.75 for developer. The same size Riston board (from Jaycar) will cost you $49.95 plus $7.95 for developer and $8.95 for stripper (you will never get it off easily otherwise).

Kinsten resist can be removed with 00 gauge steel wool or acetone.

As you can see Kinsten is about one third the cost taking everything into consideration.

Developing the exposed board

As we mentioned eariler, the proprietary developer is easy to mix and use but we have also had success using a weak caustic soda brew. You’ll soon know if you’ve made it too strong or too weak – if it’s too strong the resist will all wash off (including the bits you want!) and if it’s too weak nothing will wash off.

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Developing a Kinsten pre-coated board. The clear areas in the transparency have been washed away leaving the resist to protect the wanted areas from the etchant. Yes, this is a different board to those shown above!

The same tray can be used to develop the board and to etch it – just make sure you wash the tray out between times.

Developing is achieved by full immersion, emulsion-side up, and gently rocking the tray so the developer "washes" over the resist. Rotate the tray as you go so the washing is even.

Brushing the board lightly with a soft brush (a makeup brush is ideal) can assist developing but be careful – it can result in flaws in the resist.

Before very long (a minute or so if your exposure is correct) you should see patches of developer starting to wash away from the board. It doesn’t take too much longer for development to be complete, with all unwanted areas (ie, between tracks, component holes etc) now cleared of developer.

Development time will increase with lower temperatures so down here in Tassie I heat up the developer with an old microwave oven for about a minute. Be careful – too hot and you will be left with no image either (it will all dissolve).

When finished, rinse it in cool fresh water.

Until it dries, the resist is normally fairly soft. The board can either be air-dried (say an hour or so), dried in direct sunlight (half an hour) or baked in a just-warm oven (an electric frypan is also good!) for maybe ten to fifteen minutes.

Etching the developed Board

If you don’t make many boards then the easiest way to etch the board is to place it upside down in a plastic container of ferric chloride in solution for 10 to 30 minutes, depending on the temperature, or rocked in a large tray.

If you make a number of boards, a better way is to purchase an etching tank, fish tank water heater & air pump. The tank is available at both Jaycar & Altronics or you can make your own from glass or Perspex. This way boards can be etched in less than 10 minutes depending on the strength of the solution.

An alternative etchant is ammonium persulphate but this needs to be heated to around 50°C plus before it will work and standard fish tank heaters will only heat the etching solution to 30°C (tropical fish don’t like it much hotter than that). Also, ammonium persulphate is theoretically a use once solution so etching using this can be expensive. However, we’ve been able to use stored ammonium persulphate etchant many times over a few weeks.

By the way, the cheapest and best way to buy ferric chloride is in hydrated granules. This is available from RS Electronics in 2kg containers (Cat 551-277). Virtually everything I get for this hobby has to be mail order down here, so there is no point in paying postage for water.

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For very occasional PC boards, tray etching is quite practical. Etching needs to be helped along by rocking or sloshed with a non-metal soft brush. The etchant shown here is actually ammonium persulphate - it's a lot cleaner to use than ferric chloride but must be heated first to 50-60°C to be usable. This board is about 90% etched - most of the inter-track copper is gone with just a few larger areas to go. Below: if you're making several PC boards, this commercial etching tank, heater and air pump is definitely the way to go.

Mix 500g per litre of water and to clear the solution a little add 5g of sodium chloride (common salt.) Add the ferric chloride to the water not the other way around.

It takes 2.5 litres of solution to fill the Altronics/Jaycar etch tank but this lasts for quite a time, I have etched over 50 large and small boards in the one batch I have mixed up and it still etches quite well under 10 minutes.

One problem is that it ferric chloride is messy – don’t wear your favourite clothes – and after a while sediment builds up at the bottom of the tank. I wait until all the sediment settles overnight, then drain the etching solution off into a large plastic container leaving the sediment behind and then clean out the tank.

 

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I then pour the solution back into the tank topping up with fresh solution if required. There’s not much evaporation down here where I live in Tassie any time of the year, so the level in the tank doesn’t go down much. Once the board is etched and the etch solution is washed off then it is ready for drilling and finishing.

Finishing the completed board.

Don’t use a hobby PC board drill press as these just haven’t the torque required. Buy yourself a cheap Chinese drill press with ½" chuck, replace the bearings with good Australian-made ones and use tungsten carbide bits.

These are quite readily available, from 0.45mm to 6.31mm. These bits won’t dull on fibreglass but they are very hard and brittle so they are easily broken. With this in mind, buy more that one of each size.

The resist can be left on while drilling to protect the board from oxidising. When you have completed drilling holes, scrub off the resist with 00 steel wool and dishwashing detergent.

Once you have done that dry the board thoroughly and cut it to size with a hacksaw and finish with a file. Then give it a couple of coats of liquid resin flux from a Solder flux pen (Jaycar cat TS-1512)

This will help in soldering and also give the board protection from oxidation.

A methylated spirit/resin flux used to be available but I haven’t seen the product anywhere for years. However, you can make your own with Rosin (buy it at a specialist music shop – it is used on violin bows). Crush it then dissolve it in metho until no more will dissolve.

 

Parts List - PC Board Light Box

1 PC board, 120 x 64mm, code 10111071

1 PC board, 120 x 64mm, code 10111072

1 aluminium sheet, 155 x 80mm x ~1-1.5mm (for front panel) with label

1 aluminium sheet, 300 x 320mm (thickness 1-1.6mm) (for chassis)

1 240V to 12V (2x6V) PC board mounting mains transformer (eg, Altronics M-7012A)

1 12V SPDT PC board-mounting relay with mains-rated contacts (eg, Altronics S-4170A)

1 4MHz crystal (X1)

1 covered M205 fuseholder, PC board mounting (eg Altronics S5985)

1 1A M205 fuse

3 pushbutton membrane switches, PC board mounting (eg, Altronics S-1135)

3 16-pin machine IC socket

1 18-pin machine IC socket

1 4-pin 90° PC board male socket (eg, Altronics P5514)

1 12-pin 90° PC board male socket (eg, Altronics P5522)

1 4-pin straight PC board male socket (eg, Altronics P5494)

1 12-pin straight PC board male socket (eg, Altronics P5502)

2 4-pin plugs

2 12-pin plugs

1 300mm length 4-wire cable (either rainbow cable or individual wires)

1 300mm length 12-wire cable (either rainbow cable or individual wires)

1 3-way mains-rated PC board mounting terminal block (eg, Altronics P2037A)

1 sheet 17mm plywood, ~600 x 470mm and 17mm iron-on edge veneer

1 sheet 3mm plywood, ~360 x 270mm (for base)

1 sheet 335 x 245 x 6mm clear glass (no flaws, scratches or tinting)

1 sheet 320 x 230 x ~7mm foam plastic (high density if possible)

1 sheet 320 x 230mm felt

1 piece red transparent plastic, 65 x 20 x ~1.5mm (for display lens)

2 hinges for lid

4 rubber feet

4 8W UV (actinic blue) fluorescent tubes (eg, NEC blacklight FL8BL or similar)

8 miniature fluoro tube holders, type ST 268 (known as "tombstones"),

4 fluorescent starter holders (HPM 390 or similar)

4 4-20W fluorescent starters (Osram ST151 or similar)

2 13W fluorescent ballasts (EC13 or similar)

1 3-core mains lead fitted with 3-pin plug.

1 mains cord clamp

1 earth lead lug (crimp-on preferred)

Lengths mains-rated hookup wire for fluoro tube, ballast and starter wiring

Semiconductors

1 PIC16F84-4, loaded with light_box_timer.hex (IC1)

1 4511 7-segment display driver (IC2)

1 7805 5V regulator (REG1) with U-shaped heatsink

5 BC557 or BC558 transistors (Q1-Q4, Q6)

1 BC547 or BC548 transistor (Q5)

1 BC337 or BC338 transistor (Q7)

2 3mm red LEDs (LED1, 2)

1 5mm green LED (LED3)

1 5mm red LED (LED4)

4 1N4004 1A silicon diodes (D1-D4)

4 0.5-inch 7-segment common cathode displays (DISP1-4)
(eg, Jaycar ZD1855 or Altronics Z0190)

Capacitors

1 2200mF 25V electrolytic

4 100nF monolithic

1 100nF 250VAC X2 TYPE

2 22pF ceramic

Resistors (0.5W, 1%)

3 10kΩ 1 4.7kΩ 1 220Ω 1 150Ω 8 47Ω


 

Electrical parts, including the miniature tube holders ("tomb-stones"), ballasts, etc are fairly common items available from (or ordered via) most electrical wholesalers.

The 8W "blacklight" fluorescent tubes are not so common but should also be available from major electrical wholesalers (even if on special order). Those used in the prototype were obtained from KALEX, 718 High St, Glen Waverley, Vic 3150. Tel (03) 9802 0788.

* Autotrax and Easytrax PC board layout software are available as free downloads from www.altium.com/Community/Support/Downloads/