IMPORTANT NOTE:

There is a design flaw in this project, the actual UV leds LZ1-00UA00 are 3W not 5W like in that marketing bullsh|t from their datasheet. 5W is only permitted in short pulses.
I am a bit upset because mine burned out after a year of use since the current regulator was set for this 5W, and they are pretty expensive.

To solve the issue please use 1.8Ohm resistor ( 2x3.6Ohm in parallel )  in the output of the LM317, regulating the current to 700mA.

And use adequate cooling for the whole stuff, these leds are pretty sensitive to high temperature.
I made a new PCB board for them from a 0.3mm thick laminate and glued it to the heatsink, thus having minimum rosin thickness between the copper + led and the heatsink, better dissipating the heat.

I posted a video on Youtube about my way to use Positiv20 and how I make my PCBs. 
http://www.youtube.com/watch?v=p2kFazl-aEE

Many people were asking me about the UV exposure device from that video.
It is 100% custom designed and made by me. The ideea was to make a device as small as possible since I have constraints with storage space, so UV sources from heavy duty stuff (UV bulbs, UV fluorescent lamps) with high power demand are excluded.
I opted for a low profile and power efficient solution, so I have chosen the UV led approach. The only problem with it is the price, since high power UV leds are expensive.
In this blog I will present this project, post schematics, BOM and firmware for other enthusiast to build it also.

Device parameters:
   - power input: 12V DC
   - UV source:   2 x 5W 400nm UV LED
   - low power red ambient light
   - 200mm mast length, can project uniform UV light to 200x200mm area
   - controls:  - timer, dimmable UV and ambient light


NOTE:  All rights are reserved related to this design / firmware. You can reproduce it only for personal use. No commercial use is permitted.

This is the device in all his glory :-)

The main unit.
The PCB from the top of the picture is the power panel. It contains the LM317 current regulator, fuse,   switching transistors, etc.
The lower PCB is the timer and control unit
The chassis is made from L shape aluminum and it holds the whole assembly and the two 3mm thick glass sheets.
Initially I planned to make an enclosure also, but it works this way also, and any DIY enthusiast know that making an enclosure is the most difficult part.

View from bottom.
As you can see, the power panel was hand drawn and etched that way, pretty messy work, I'm not proud of it :-D
The timer board was the first piece of electronics made by lithographic method (exposed using an egg timer), you can see the difference.

The mast with the UV leds and cooler:
I made this from a 6mm brass pipe, on one end a 5pin radial connector glued with epoxy and on the other end the PCB with the UV leds and with the heatsink glued to it.

The lighting head.
Still a messy hand drawn PCB. It could be made by heat transfer but it was quicker to just draw it using a permanent marker.
In the middle is the soul of the device - the two 5W 400nm UV leds as close as possible to each other to have the best spot light to prevent shadow ghosting.
At the top you can see two little red leds - those are for the ambient lighting used for setting up the film and stuff. Useful if you expose something which is more sensitive to light than Positiv 20.

Here you can find the schematics, PCB, BOM, and the firmware:

https://drive.google.com/folderview?id=0B7tTimpGxVq4ejRBX2FuVW1ZNnc&usp=sharing

File list:

- expo.hex - firmware (Intel hex file)

- PowePanel.bom and TimerPanel.bom - list of components for the two electronic boards.

- SCH_logic, SCH_UV_power - schematics for power panel and timing and control electronics.

- PCB_logic,  PCB_power   - top and bottom layer and component placement (1:1 size)

I have used the following UV leds: LZ1-00UA00   (click on the link for more info)

Building and bring up:

    

    First construct the the chasis and build the power panel and the lighting mast with the UV leds mounted on it. Test it if all the voltages are correct (especially test J3 pin 1 for 5V  -see the schematics).

Try out the control for UV and ambient leds and for the cooling fan by shorting on J3 the following pins: 1 with 2 / 3 / 4.

For safety try out the current regulator with a 12V 50W lamp first - it should hold ~1.15A. this way we are sure we don't burn out those expensive UV leds.

    

    Now the timer board. Here we need an ISP programmer for the Atmega8.

Check for shorts and interruptions, connect with the power board, connect the programmer, set the fuse bits for external high speed crystal oscillator, without internal ck/8 divider; optionally the brown out detector can be activated to prevent eeprom corruption on power spike when switching off. Flash the provided expo.hex file and disconnect the programmer. It should start up in standby mode with 07:00 on display.

User manual:

Buttons:

      - left:   Start / Stop         (S)

      - right upper:  + button   (+)

      - right lower:  -  button   (-)

      - right middle:  menu button   (M)

Modes:

   - Standby (default):  

      button (S) will start the exposure, switches off the ambient light, waits 4 seconds and starts the UV.

      button (+)/(-) sets the timer -> set the secods, press (M), set the minutes, press(M) -> Standby

      button (M) enters in menu

   - Exposure:

      button (S) stops the exposure -> Standby

      button (+)/(-) adjusts UV power on-the-fly. It isn't memorized

   - Menu:

      button (+)/(-) select the following menu elements:

              - Upwr:  UV power in 10% steps

              - Apwr: ambient light power in 10% steps

              - Disp: display brightness in 10% steps

      button (M) enters in the selected menu point

      button (S) exitst from menu

Menu points are set by (+)/(-),  (M) will exit to the menu, (S) will exit to Standby, each of them saves the setup in eeprom.

Have fun.