UV pcb exposure box

Today, I want to write a few words about another of my older projects.

UV exposure box - whole view

A while ago, I started building this DIY UV exposure box. The electrical part consists of a self-designed count-down timer based on an AVR Tiny2313 CPU, 4x8W PHILIPS UV tubes, ignition/driver circuits from cheap fluorescent bulbs, a multiplexed numeric LED display, a rotary encoder and some wiring. A piece of ~4mm aluminum serves as a faceplate, picture frame glass as the exposure surface. We’ll have a look inside in a moment.

When turned on, the controller presents the configuration menu first. All options are preset – or should I say ‘preprogrammed’ because the preset cannot be changed, as of yet – to the values I use most frequently. Configuration includes the time (up to 9min 59sec), a variable tube preheat time up to 59 seconds and a zone selection. The zone selection does nothing at the moment, but the pcb features a mounting spot for a second solid state relais which I have not yet installed. Which means, all four tubes are activated.

Exposure box - menu

Minutes…

Exposure box - menu

Seconds…

Exposure box - menu

Zones…

Exposure box - menu

Preheat…

Exposure box - menu

…ready to go!

Exposure device - running

Enjoy

Press the selector one more time and the process starts. To stop again, either press the button again, switch off the mains or wait patiently until the time runs out.

Now, on to the inner values. After undoing 7 torx screws, the lid can be removed to access the elecronics compartment behind the front panel:

Exposure device - mains part

Mains input and ignitors

Exposure device - microcontroller

Microcontroller board

Exposure device - electronics compartment

Electronics compartment

Inside the compartment is the four-fold ballast circuit, which consists of four board found in the sockets of fluorescent energy saving lights. These are pretty cheap compared to commercial electronic or inductive ballasts and they can be used right as they come. The only crucial number is the power rating – my tubes are rated for 8W while the ballasts came from 9W bulbs, which works just fine. Care has to be taken when opening the sockets, though. The bulbs should be left lying around for some time prior to “dissection” because they contain a pretty juicy capacitor. The circuit inside is basically a simple switchmode current supply. When that’s done, an easy way is to cut them open along the circumfence about in the middle of the socket’s height with a fine saw blade. Don’t cut too deep or you will damage the pcb. Once the casing is open, mark the pairs of wires coming from each end of the tube before cutting them. These pairs need to go to the ends of the UV tube, don’t mix them up or you’ll short out the circuit. How the two wires connect to the two pins of the tube on each side is completely up to your choice, though.

As always, remember the hazards involved when dealing with mains equipment, especially such that was never designed to be opened or even run in the open. Also, don’t go breaking any fluorescent tubes as they might contain traces of mercury.

The four ballasts are wired in parallel to the mains, with just a fuse, the power switch and a solid state relais in series. Fuse-wiring got a little complicated because I forgot to place separate fuse sockets for controller and drivers onto the pcb, but nothing dangerous here. A connection between faceplate and protective earth is also present for safety reasons, seeing that there is lots of live wiring very near. You may have noticed the absence of any cooling fan or holes – these are not necessary as the device is run for a few minutes at a time and never unobserved. The ballast circuits are designed for operation in a very tight unventilated space anyways, so no trouble to that end. After ~5 minutes of exposure the glass surface becomes just noticeably warm.

To the right is the control circuit, consisting of said ATTiny2313, a small 6VA transformer-powered 5V supply, the solid state relais (SHARP S202S02) and three npn transistors as segment drivers for the LED display. I still have some pictures from back when I made the pcb (about a year ago now):

Drilling holes…

Finished pcb

…all soldered.

I have used this exposure box several times now and am pretty content with the results. There still remains some creepage of light between the layout print and the photosensitive layer, resulting in fuzzy edges of traces and larger groundplanes sprinkled with small holes. This is partly thanks to an absolutely ridiculous laser printer made by HP (Color LaserJet 2600nse). No matter what settings are used (even in the expert options), the printer will never do dense black withing planes and very often blur traces on either the leading or trailing edge. Text works fine, though.

You may download the schematics in Eagle 6 format at your leisure. I do not guarantee correctness of the layout, though. There was a small problem in an earlier version (Pin 1 of ribbon cable connector was not connected to ground) which I have fixed now.

I do not take any responsibility for whatever happens to you. It’s up to you to decide if you want to and are able to build something like this.

>> EAGLE 6.0.0 Schematic and Layout

>> Script files for older versions of EAGLE – untested!

>> Sourcecode and .hex for ATTiny2313

* NOTE: Make sure that pin 1 of the front panel connector is really connected to ground, the traces were etched away in my case. The result is erratic behaviour of the rotary encoder.

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CNC – Step 2

I remade the linear bearing sleds today, the previous ones were not precise enough – they were only the proof of concept anyway. The new ones glide on the rails as if they were not touching at all, VERY nice feeling if you lean on the bridge support with approx. 10 kg and slide it back and forth. No bumps, just smooooooooth metal! No clue how precise they are in terms of height differences and imprefections though, but there is a little potential to adjust if something should be too far off.

The portal looks very good so far! BUT….the cross-bar is skewed -.- Not by much, only one or two degrees around the length-axis, but it is enough for the right bearing sled to not sit on the rail properly. I can force it on, of course, but that is not the great idea here as the sleds should fit snugly on the rails without any force besides its own weight applied. In the beginning MDF was planned for this part, but I settled for beech wood instead. Best option here is to dump the current one, buy a precise MDF cut tomorrow and stuff the beech back into the scraps box for some future case build. After that, if the dimensions and angles of the portal as a whole are within tolerances, two more bearing sleds are needed to keep the portal on the rails while moving and when milling action occurs. Two more of the exact same type should suffice, but maybe some kind of spring-loaded mount would be better for long-term stability.

The side supports of the portal will be redone after the mill is completed. As will some other parts, probably. I will have access to a cnc mill then, so why not do the holes and cut-outs a little more precise 😉

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CNC – Step 1

Alright, so I finally decided what I want to do as kind of a long-term project in the coming time: A CNC mill which will get heavy use in the fabrication of PCBs, faceplates and enclosures. My case-building skills plain suck, so this nifty device will be helping me in the future. Also, this is my first real all-out mechanical build – meaning no straight way to the goal. In the end, it will probably have become a lot more expensive than I currently still hope for, but we’ll see about that ;-). (Account the extra expenses to learning and refilling the scrap parts box, distracts the wallet from groaning somewhere in a dark corner. Works most of the time.)

Sketchup of cnc mill

Sketchup of cnc mill

As I already wrote above, the main tasks for the machine will be PCB milling and drilling, machining wood, acrylic, plastics and aluminum. Maybe copper, and in the far future I want to take a shot at some steel sheeting, but all that is off the radar right now. I will try to aim for high precision and adequate stability. A portal mill seemed like a good idea at the time, especially because a moving table makes the machine just so much bigger while the workpiece must stay within relatively modest dimensions. I played with the thought for a while but then dropped it in favor of this design.

The whole thing will be controlled by linux-EMC or Mach3, maybe I can even fit a small EPIA-800 mainboard as a control computer in there if the drive mechanism allows enough leftover space. The controller will be custom made, mainly for education purposes on my side.

First, I did a draft in SketchUp to get a feel for the problems and dimensions, though it is already outdated by now. I will update it in the next days when I need exact measurements of the whole device again. Right now pen, paper and calipers are sufficient. The materials are almost all bought at the local hardware store, MDF sheets, V2A screws and aluminum parts in standard sizes. Fortunately they do have a decent free cutting service whose operators hit the measures on the tenth of a millimetre (or up until now, at least). Saves me some of the work.

Construction began with the machine table, which is a solid piece of 22mm thick MDF wood, 500 x 600 mm in dimension (long side along rails). The sheet will be reinforced with aluminum from below so it doesn’t bend or skew, but first the drive mechanics and electronics compartment below the table need to be done. Just as a side note, the milling area will be somewhere around 450 x 450 mm but that number is not final. I have noticed that some more changes to table and X-axis construction might be necessary and those will of course change the useable space on the table.

CNC machine table

Table construction

Machine table:

The table is carried by two 67 x 35 x 2.5 mm aluminum rectangular tubing. The tubing also supports the linear rails which are self-constructed and still need to prove their usefulness. The first rail is already installed (see pictures), the running sleds are in preparation (still need some standoffs) and the portal flanks are halfway done.

Linear rails

Linear rails

Linear bearings:

Made from 15.5 x 15.5 x 1.5 mm aluminum tubing with a slight groove in the exact center of each side, running along the length. The other part is a C-rail that fits exactly into this groove and supports the tubing nicely, giving it an exact 45° angle but believe me is a HELL to drill, even with a drill press. The carriage sleds of the portal consist of another 45°-rotated piece of the same tubing with ball bearings mounted to its sides in a way that they stand at 45° angles on the surfaces near the upper and lower edge of the rail. I have no picture of this part, will get some later today after I get around to reassembling the carriages.

The second rail is still in the making, will probably be finished today or tomorrow. I’ll try to get more pictures of the process but holding a camera while drilling seems not all that fun.

CNC stepping drive motors

Stepper motors

A small teaser:

The stepping motors I got for quite a bargain, pretty powerful with around 1.85Nm holding torque. These beauties weigh in at 1.4 kg per piece, measure about 100 x 56 x 56 mm,  sport a 6mm diameter shaft and survived the whole shipping torture without stuffing material or bubble wrap (YES seriously, who in his right mind ships 5 x 1.4 kg worth of steel and copper unwrapped in some otherwise empty cardboard box?!?) without even a slight dent in the shafts. The faceplate got a bit banged though, but nothing that can’t be polished up.

The drivers for these will be of my own making, probably AVR-controlled MOSFET fullbridge circuits. Half-stepping is planned, but I have not yet decided on microstepping. I guess that will be added if necessary for precision.

So long, stand by for more 🙂

EDIT: Just noticed that this wordpress blog seems to distort my pictures if they open in the lightbox. Will see if I can fix that, must be something wrong with the script.

EDIT: Fixed, seems like some unwanted css found its way into the template.

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