LED UV lightbox, Inkjet stencils, SMT, PIC 16F887 and BCD code - diyAudio
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Old 27th August 2010, 12:16 AM   #1
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Default LED UV lightbox, Inkjet stencils, SMT, PIC 16F887 and BCD code

I’m posting this in the ‘construction tips’ section, I thought about posting it in the Wiki, but there are sections that I don’t want anybody to be able to edit, particularly the code. I see there are some similar threads, but there are some details here they don't include.

I used to have a UV lightbox, I made it about 15 years ago, I haven't been able to lay my hands on it for a couple of years now. Still, I had access to one at work, so I didn't worry too much

Recently I had to quit that job, and I've been ill, so sitting at home I've got a lot of ideas unfulfilled, partly for want of a lightbox..

One of these was to construct a new lightbox.

This time I decided to go with LEDs. The previous one used fluorescent tubes installed in some fittings taken from an old camper van, I used to run it off a 12V lead-acid battery. The whole thing was constructed out of scavenged bits; a box I picked up in the street that had slots for a sliding lid that neatly accommodated a sheet of glass that I cut to fit.

This time I bought 100 UV LEDs from a seller in Nanjing, China for $10 US shipped. I had some Veroboard which I cut into 3-conductor-wide strips, this gave me 10 strips, each of which accommodated 9 LEDs.

I decided to run the LEDs in strings of 3, without a ballast resistor. The forward voltage is quoted as 3.4~3.8 volts. When I ran up the 90 LEDs to 600mA (30 strings @ 20mA) I got ~11 volts on the lab power supply, so I figured 2 rectifier diodes in series with 12V regulated from a 7812 would be about right to drop the voltage to an appropriate level. It’s not recommended to run LEDs this way, usually there’s a ballast resistor in every string or a constant current supply, but I broke from these practices first when building torches, and it turns out LEDs are more robust used like this than theory and literature would suggest.

I built a 12V power supply with a 20VA RS toroidal transformer I had lying around. The other components for the bridge and the smoothing caps were just ‘lying around’ too. I used a 78S12 regulator in conjunction with the aforementioned dropper diodes in order that the UV array wouldn’t be overdriven.

w

Last edited by wakibaki; 27th August 2010 at 12:25 AM.
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Old 27th August 2010, 12:24 AM   #2
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You can see the LED array here.

Click the image to open in full size.

Here it is illuminated...

Click the image to open in full size.

And here it is with picture frame arranged above it with a piece of printer paper to show the illumination of the target.

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w
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Old 27th August 2010, 12:26 AM   #3
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Arranging the LEDs was a problem. First they had to be seated firmly against the Veroboard to try and make the beams parallel. Then the Veroboard had to be held flat. I've got a piece of 3 mm fibreboard with 2 x 1/4 x 1/2in balsa strips adhered to it with doublesided sticky pads. Then the Veroboard is held to the balsa strips with thumbtacks, or drawing pins if that name is more familiar to you.

This has not resulted in as even illumination as I had hoped. The beams of the LEDs are quite narrow. This has the advantage that the overall beam does not spread too much, but I have had to keep the target sheet 12 inches away from the LEDs to ensure that the spots spread into one another. I’m looking for an LCD monitor to dismantle, they have some high-performance diffusion screens in them, which might fix things. I think next time I might revert to fluorescent tubes. The illumination period is 10 minutes, where the box I had previously only required about 4 minutes. It was 12 watts, this is only ~6. This was cheap, however, given that I had the Veroboard, it works, and it’s low voltage.

w
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Old 27th August 2010, 12:33 AM   #4
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Default Inkjet transparency stencils

When at work I used to make stencils on a laser printer. I used transparencies up to A4, and architectural drafting film up to A2. I made some pretty big PCBs, butting 3 sheets together to get up to 32 inches. Once I figured how, I made the big boards by isolation routing.

A big board:-

Click the image to open in full size.

I was concerned that stencils made on an inkjet printer might not work. Transparency material for inkjet has a coating on one side to absorb the ink. This has tiny dots (imperfections) irregularly spaced presumably as a result of the coating drying. I thought these might show up on the PCB. I also didn't know if the contrast ratio between the inked areas and the clear would be sufficient to produce a good image in the photoresist. As it turns out, this is not a problem. Although the contrast could be better, it is still possible to produce adequate results down to 10 mil traces. This can be seen on the doublesided board shown later.

w
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Old 27th August 2010, 12:33 AM   #5
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Default Processing

I process the PCBs on a one-shot basis, as was common with film processing in the latter days of film photography.

I use Sodium Hydroxide for preference rather than proprietary resist developer, it's cheap. I do the processing in flatbottomed plastic tubs which came free from the Chinese takeaway. I have some bigger dishes too from the old photography days.

I mix a quarter boiling water with threequarters cold tap water to half-a-pint (a mugful) and add a shallow level teaspoon of granulated hydroxide and stir immediately. This is insufficiently strong to burn my fingers so I can handle the wet board in comparative safety once the exposed resist has dissolved. A little physical manipulation is sometimes necessary to remove some stubborn resist where perhaps the illumination has been poor. Sodium hydroxide is both caustic and poisonous so take care when handling it and dispose of it carefully immediately after use.

Experience has taught me how few crystalline balls of ferric chloride will adequately process the size of board I am working on. I tip them in, followed by a small quantity of recently boiled water plus the PCB. The solution does not stay hot enough to finish the board in one go, so I zap the solution with 20 seconds in the microwave once I can feel that it has cooled down. Used ferric chloride contains dissolved copper and is highly inimical to marine life and should not be disposed of into sewage systems. Take local advice on how to dispose of it.

w
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Old 27th August 2010, 12:38 AM   #6
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Default A finished board

I made the board for the PSU using the new LED array driven from a lab PSU and did the timing with a stopwatch. Itís a conventional thru-hole job, single-sided.

Click the image to open in full size.

Click the image to open in full size.

I may have to move the regulator off the board to get it on a heatsink.

w
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Old 27th August 2010, 12:42 AM   #7
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Default Double-sided, surface mount

I like surface mount components. The more surface mount you have, the less holes you have to drill. There are no problems with requiring thru-hole plating, although obviously, in the case of a double-sided board, vias are necessary. I generally use a larger head on the vias than I would for a commercially produced board. This is not ideal, but the real-estate taken up is generally not critical in DIY projects.

Click the image to open in full size.

Double-sided boards obviously require registration between the sides. This can be accomplished by peeling the protective plastic covering from one side and sticking one stencil to the unexposed board and drilling a few holes. You will have to remove some swarf from between the stencil and board before exposure. You can use reference marks included for the purpose, or simply drill at the board corners, or through a few vias. Then the protective film is peeled from the other side and the second stencil carefully matched to the holes. An alternative method matches the stencils against each other, taping them to prevent relative movement, and inserting a piece of scrap board with an edge coincident with one of the board edges on the stencil and then taping this in place. The unexposed board can then be inserted between the stencil sheets and butted up against the scrap piece and some tape added to obviate movement between exposures.

w
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Old 27th August 2010, 12:47 AM   #8
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Default Vias

Click the image to open in full size.

Click the image to open in full size.

Vias are all placed at the same time. Take about a metre of plated copper wire and grasp the ends in long-nose pliers, taking a couple of turns round each of the plier jaws. Hold the pliers firmly and pull them apart until the wire can be felt to give by a centimetre or so. This will put a 'set' on it, i.e. leave it straight and slightly work hardened.

Now cut sufficient 'needles' from the straightened wire to complete the board. Cut the needles with side-cutters held at a 45 degree or better angle so that a sharp pointed end is produced.

Place the board on a piece of sponge such as frequently found in kitchens or bathrooms. Place the needles to fill all the via holes, pushing them down securely into the sponge for about half their length. Once they are all placed, you can go round and solder them all. Detach the board from the sponge and trim the excess wire from the top surface. Now you can invert the board and solder the other side. Even if the board is subsequently reflowed in a toaster oven, the vias are unlikey to fall out due to surface tension, particularly if the wire is not a sloppy fit in the holes.

Click the image to open in full size.

w
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Old 27th August 2010, 12:52 AM   #9
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Click the image to open in full size.

The board illustrated is a timer for the lightbox. It employs a PIC 16F887 which was selected, amongst other things, for its internal oscillator.

Click the image to open in full size.

Click the image to open in full size.

The board provides a simple timer function with a relay to control the UV light. The circuit is shown here.

Click the image to open in full size.

When switched on the timer displays 000. There are 3 control button inputs, 2 for setting the period.. The first causes the display to count upwards. The count proceeds at second intervals when the button is held down until the count reaches 5, then the delay period is reduced and the count increases at a faster rate, this means that setting relatively long delays can be accomplished in a relatively short time.

The second button functions in a similar fashion to the first, but the count is down.

The third button turns on the UV light and starts the counter counting down at 1 second intervals. When the count reaches zero the UV is turned off and the display reverts to the number originally set. Subsequent exposures can be initiated by pressing the start button again.

A number of the components used are mounted unconventionally. The voltage regulator, electrolytic caps and diodes and relay are all conventional thru-hole types applied to one side of the board and ‘glued’ there with solder. In some cases I have used a custom foil pattern, and bent the component legs to mate. The 7-segment displays are common-anode types, each segment is ballasted with a 470R resistor. They are mounted in a 30-pin DIL socket which is surface-mounted to the PCB, this provides some stand-off and means that the components can be replaced or re-used if so desired. Since 30-pin sockets are not generally available, one can be made up from 2 smaller or cut down from a larger one (just cut off 2 pins leaving the plastic intact). Past experience with the MPU and displays (the LED forward voltage is >3V) have shown that the chip will easily support the dissipations encountered with 470R ballasts.

Where possible I like to use solder paste and reflow in a toaster over. A good source for solder paste is DealExtreme: Cool Gadgets at the Right Price - Site-Wide Free Shipping - DX, although there is a considerable delay in postage. Obviously solder paste can be obtained from sources closer at hand, but many suppliers like to ship fairly large quantities in an insulated package, to keep the temperature of the contents low. This can result in high prices and the quantities involved are greater than an amateur could reasonably use before it deteriorated. Solder paste is commonly used in commercial applications. If the paste fails to reflow suitably thousands of dollars worth of components can be affected in a production run. This can result in litigation in the worst cases. While this is understandable, the insistence on shipping large quantities in controlled circumstances is inconvenient to say the least, and the availability of small quantities from sites such as dealextreme is a way round this.

The MCU itself is a TQFP44. Soldering these by hand only requires the correct technique, which bears repeating here. Carefully apply solder to a corner pad of the PCB foil pattern. Slide the IC into place and secure the one pin. Adjust the positioning until you are completely happy with it. Now solder a single pin at the opposite corner. Starting with one of the unsecured edges, solder thickly and indiscriminately all the pins along that edge, pulling the solder bead from one end to another, making sure all pins on that edge are wetted. Now solder the remaining pins in the same manner. Remove the excess solder with a solder-sucker, working quickly to use as little heat as possible. If a string or sphere between 2 pins is difficult to remove, try re-applying some solder in that area and try again with the sucker.

w

Last edited by wakibaki; 27th August 2010 at 01:08 AM.
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Old 27th August 2010, 12:53 AM   #10
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Default Software

A couple of years ago I looked around for a BCD routine written under Mpasm, the free PIC assembler. I couldnít find anything. I wanted to display the contents of the 10-bit PIC A/D register on 7-segment displays, effectively to build a digital meter. The routine here does this and can be used to build a meter with some analog circuitry added to process e.g. AC current to voltage or WHY.

The board is programmed with a Pickit2 USB programmer via an ISP header which is simply a 6-pin 0.1 pitch SMT pin-header, connected to the appropriate pins on the PIC. The Pickit2 is available for a comparatively low price from regular suppliers, and even cheaper as a clone from ebay. It is very simple to knock up a board with a socket and header which will permit the programming of a wide variety of conventional thru-hole PICs using the Pickit2. Although it is possible to build programmers running off the PCís serial or parallel ports, the cost and effort mean that this is hardly worthwhile, and the Pickit2 is extremely versatile. There is now a Pickit3 available with (I believe) better debug facilities, but I have always found the PK2 to be adequate. Anyone who likes to be able to add a few digital features to a design should have one.

Since the display runs 000-999, a ten-bit buffer is required to contain the count. Arranging for this to overflow correctly is comparatively straightforward. The 3 input switches select amongst up- and down-counting routines while the program continuously cycles in one big loop. The loop processes the count in the buffer into 3 secondary Binary Corrected Decimal (hundreds, tens and units) buffers which are in turn processed into 7-segment display buffers. The process is comparatively straightforward, if tedious.

Each bit in the count buffer is examined and, if set, a corresponding number is added to the BCD registers, i.e. if BUF_HI, bit 1 is set, 5 is added to the hundreds count, 1 to the tens count and 2 to the units count, and so on, down to BUF_LO, bit 0, which is worth 1. When all the bits have been examined the BCD units buffer is reprocessed to move any accumulated tens to the BCD tens buffer by repeated subtraction of tens from the units buffer until a borrow occurs, when the units count is restored by adding ten. Similarly the BCD tens buffer is stripped of any accumulated hundreds. The size of the numbers involved is small enough that there is no risk of unintended overflows.

A simple delay routine at the end of the loop pads the time taken to execute to ~1 second. A second, shorter delay is switched in during the up and down setting routines after the buttons have been held down for a count of 5. This means that setting up a high count e.g. 600 does not become too tedious. The RC clock in the MPU is not guaranteed to keep accurate time, but the seconds delay can be trimmed to fair accuracy by timing the count over a minute or so and adding or subtracting a few cycles from the delay routine. On my board the accuracy is within 1 second per minute, which is plain lucky with such an uncomplicated delay routine.

The seconds delay means that, depending on where the routine is in the loop, there may be a brief wait for the system to respond to a button press, but I havenít bothered to complicate the routine by eliminating that.

The relay can easily be used with a minor modification to the circuit to switch mains voltage if mains-driven fluorescents are used. In that case only a 5V supply would be required.

The accompanying circuit diagram shows the proper correspondence between the PIC port bits and the common-anode display pins. The displays used are 0.5in character height which are readily available at a reasonable price on ebay.

w
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