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Preface

Fig.1 – The UV-LEDs bromograph

A bromograph (or UV Exposure Unit, or UV Screen Exposure System) is a fundamental instrument for making PCBs using the photo-etching method. Unfortunately its price is usually prohibitive for a normal DIY maker and the UV-neons are expensive anyway if you’re thinking of building one.

I thought that UV-LEDs could represent a valid alternative to them and I made some experiments to test their effectiveness. Encouraged by the first positive results, I considered that even using UV-LEDs with a wide view angle, such as 120°, I would need 54 LEDs to light uniformly a surface of a common 160×100 mm board. That means at least 1A of current consumption or doubling in case of a dual-layer bromograph, plus, a small amount consumed by a timer circuit which I planned to include in the project. So, I decided to avoid the use of a transformer that is quite cumbersome and expensive. I surfed on the internet looking for a solution and I stumbled on Wutel‘s website. There, I found some circuits that, by exploiting the capacitive reactance, supply LEDs directly from the 220V.

A dual-layer exposure unit with no transformer

Fig.2 – The selected timers

When the lid is opened the bromograph is in standby mode. By closing it, the exposure starts according to the time selected on the selector (see figure 2) or until you decide to interrupt the process by opening the lid. In any case the timer reset and the bromograph is ready for a new exposure. NB In figure 1 the LEDs are on only because I pressed down the switch with some tape.

As usual, a practical demonstration is worth more than a thousand words, so I recorded the video below in which you can watch the bromograph in action.

The upper and lower LEDs boards

I bought a batch of 100 120° UV-LEDs and a couple of 160×100 mm stripboards in order to build the upper and lower boards. Thanks to the layout I’m going to explain below, I managed to build a bromograph capable to expose up to 160×100 mm single layer or 120×100 mm dual layer boards.

Fig.3 – The lower and the upper boards’ schematic

THE UPPER BOARD

Fig.4 – Detail of the LEDs soldered on the boards

The upper board is entirely covered by 54 UV-LEDs arranged in 9 rows of 6 columns.

I soldered the LEDs in series according to the schematic in figure 2: once having inserted the LEDs into the right holes of the board, solder all the A terminals and flush cut them. Then, bend the K terminals towards the next LEDs and cut them to the right size that allows you to join the A one. Finally, solder the two ends together (see the result in figure 4).

THE LOWER BOARD

Fig.5 – The components on the lower board

The lower board includes 42 UV-LEDs (9 rows x 7 columns) and the circuitry that supplies both boards.

In order to understand properly the connections between the components, I drew the schematic according to the real placement in figure 5.

Solder the LEDs using the method explained on the upper board section. Similarly, you can connect the other components by soldering their own terminals together when possible. When it is not, use wires or make tracks of tin, as I actually did.

The timer board

The timer board is supplied directly from the 220V as well. The exposure time is managed by a small microcontroller, the PIC12F509, according to the rotary switch’s position. On the other hand, the triac Z0405, driven by the opto-isolator MOC3021, supplies the upper and lower LEDs boards.

Fig.6 – The timer board’s schematic

BUILDING THE PCB

The PCB is dual layer but it should be easy to make because the tracks are quite wide and there are only three vias (as usual solder them first).

Fig.9 – The PCB bottom-mask

Fig.8 – The PCB top-mask

Fig.7 – The PCB layout

The rotary switch and the LED are the only components you must solder on the bottom side of the board; hence, place all the others on the top side one. Once built, by using a 3-wires cable, connect the lower board to the upper one and the timer board to the lower one.

Pay attention to respect the numbers on each terminal (for instance connect X2-1 to X4-1 etc.).

Testing the circuits – BE CAREFUL!

Now, check and recheck 1000 times your job because you are going to test the whole circuit by supplying the board directly with 220V! Ready?

Good. Once supplied, if the red LED turns on it means that you are on the right path. Close the LID switch then (or short cut the J1 terminal) to power up the 96 UV-LEDs. If you were successful, install the boards on a proper box like the one I built.

Building the box

The box I built is essentially made in wood but you can use the material you prefer. In any case you can find the size of each part on the slides below. The parts in light blue (slide 4) are made in glass and I suggest not to use plastic materials instead because they are usually lighter (you need some weight to press down the PCB mask on the board) and they are easier to scratch.

Download the project

Pressing the button below you can download a compressed file with all the necessary material to build this project:

the PIC12F509, Z0405 and the MOC3021 data sheet;
the pictures of the final project, the schematic, the code, the HEX file, the PCB mask and the PCB layout as shown in this post;
the schematic and the board files in Eagle format;
the box’s file drawn in FreeCAD format.

If you need some help please do not hesitate to contact me or leave your comments below. Enjoy it!

BP0004 (2.9 MiB, 134 downloads)

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Grazie di cuore, Andrea Dal Maso