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UV-LED exposure

UV-LED exposure unit with timer ATtiny2313

This application describes a UV-LED array to expose fotosensitive PCBs and an ATtiny2313 timer to control the exposure time.

The UV-LED array has the following properties: The Timer has the following properties:

0. Content

1 Hardware

1.1 Exposure-Hardware

Scheme ExposureThe hardware of the UV-LED exposure consists of 14 constant current sources with an npn transistor BD439, driving seven UV-LEDs each. With a disconnected timer unit the Zener diode and the 2.7 k resistor provides a 5 V source voltage at the bases of the transistors and switches the UV-LED diodes on.

UV light Dangure! The light is very intensive. Direct exposure of your eyes can cause serious damages. Via the input clamps named "Base" the driving voltage can be tied to zero volt, and the UV-LEDs are switched off.

Assembling the components and the UV-LEDs on a 10-by-16 cm breadboard is shown in the Layout. The black dots are UV-LEDs, the yellow rectangles are transistors, the lightbrown rectangles resistors, the blue rectangles clamps and the red rectangles are spare place for screws. The layout shows the component side, all components are mounted on top of the board. On the bottom side, the components are soldered and connected using enameled copper wire.

The board requires at least 24 V and max. 30 V source voltage at a current of 300 mA.

1.2 Timer-Hardware

Timer-Scheme The ATtiny2313 is clocked with an external xtal of 2.4576 Mcs/s.
The LCD display is connected via a 14-pole connector to Port B (eight data bits), to the portbits PD0 and PD1 (control bits RS and E) and to the potentiometer (contrast adjustment).
The three keys (white, black and red) are connected to the portbits PD2, PD3 and PD4, the resistors 4k7 and the tantal capacitors 4µF debounce the keys.
On portbit PD6 a two-coloured LED is connected to display the operating status. The board is supplied with 5 V, via the voltage regulator 7805, from the supply voltage of the UV-LED array.
The in-system-programming interface is available via a standard KANDA connector.
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2 Operation

2.1 Operation of the exposure unit

Exposure Box The UV-LED array should be mounted in a minimum distance of 15 cm to the PCB. A larger distance is of advantage, because it smoothes the exposure at the borders of the PCB.

Exposure Cover Advantageous is mounting the array to the inside cover of a wooden or plastic box, that is high enough. The box then protects the eyes from the intensive UV light. You can purchase such boxes or build your own one.

2.2 Operation of the Timer

After connecting the timer unit to the operating voltage, the LCD displays a message with the function of the device, the software version and the copyright of the software for a few seconds. After that period the device is ready and the LED shows green light.

Pushing the white key prolongs the exposure time by 10 seconds, the black key diminishes exposure time by 10 seconds. Pushing the black key while holding the red key down stores the exposure time in the internal EEPROM. Pushing the white key while holding the red key down starts the exposure. Pressing the white key again while holding the red key down stops the exposure cycle. During exposure, the LED blinks red.
The necessary exposure time for your PCBs can be determined by tests. In my box with 14.5 cm height the optimal exposure time of with laser-printed thin sheet and a 1 mm plastic panel on top is roughly four minutes.

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3 Software

3.1 Software Source

The software for the timer is available in HTML format or as Assembler source code file. The assembled Hex code file and a prepared EEPROM hex file is also available.

3.2 Fuse selection

Warning! New ATtiny2313 by default have their internal RC generator as clock source. After burning the flash and before using the timer, the ATtiny2313 has to be changed to use the external xtal as clock source. This is done by setting the appropriate fuse. Be sure you change the fuse setting with an external xtal connected. Otherwise the fuse setting process fails and error messages result. The easiest way is changing the fuse setting with the ATtiny2313 mounted in the target circuitry.

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