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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:
- 98 UV-LEDs with 5 mm and 4000 mCd each.
- Operation with 20 mA constant current and supply from a 30 V source.
- 14 rows with 7 LEDs each.
The 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
- Adjustable between 10 seconds and 100 minutes exposure time.
- Two line LCD for the selected exposure time and for the downcounter.
- Operation with three keys, allows adjusting the time in 10 sec intervals.
- Selected time can be stored in the internal EEPROM, timer always starts with
that preselected time.
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.
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
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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.
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
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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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.
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
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