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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 array can have two sizes: with 98 and with 169 UV diodes. The larger
version allows the exposure of Euro-sized PCBs (160-by-100 mm), the smaller
has problems with those.
The UV-LED array has the following properties:
The timer device with an ATtiny2313 has the following properties:
- 98 or 169 UV-LEDs with 5 mm and 4000 mCd each.
- Operation with 20 mA constant current and supply from a 30 V source
or with a 230/110 V power supply described here.
- 14 rows with 7 LEDs each (smaller version) or 19 rows with 7 LEDs each
plus 6 rows with 6 LEDs each (larger version).
The hardware of the UV-LED exposure consists of 14 resp. 25 constant
current sources with an npn transistor BD439, driving seven resp. six
UV-LEDs with 19.5 mA each (average). Neither the varying hFE of
the transistors nor the number of LEDs per row (six or seven) nor a
varying operating voltage has an influence on the constant current
sources, as long as the operating voltage is large enough:
- 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.
Vmin = VLED + VEmitter + VCEsat
= 3.2 * N + 4.3 + 0.2
= 26.9 V (N = 7)
= 23.7 V (N = 6)
The operating voltage does not need to be stabilized and can be as high
as 45 V without forced transistor cooling.
The base voltage is either at 5 V to switch the UV LEDs fully on
or below 0.7 V for UV LEDs fully off. The current required is
between 2 and 3 mA, so can be supplied by any portpin output of
an AVR. The base current would be up to 24 mA, if all hFEs of all
transistors would be at their minimum guaranteed value of 20 and if
25 rows were driven. At average the hFE in my case was 137 (123 minimum,
The Zener diode with 4.7 or 5.1 V at the base of the transistors
protects the LEDs from extensive currents that would be associated with
an overvoltage. 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, by default, switches the UV-LED diodes on.
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 for the smaller version.
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 enlarged version adds 71 additional UV LEDs on a 80-by-160 mm
PCB. The picture shows the wiring of the columns with seven resp. six
The picture demonstrates the coverage of a (here as black background
plate) Euro PCB. The angle of the UV LEDs is at 30°, the distance
of the LEDs from the PCB is at 120 mm, which means a circle of
64 mm diameter.
The picture shows that the coverage on the three PCB edges is not
at average. It is therefore recommended to cover the three side walls
of the casing with aluminium sheet to enforce UV light mirroring on
The exposure devive needs 26 V operating voltage. Voltages
above that level are consumed by the Collector-Emitter of the
transistors and increase their heat power. Up to 45 V can
be applied without forced cooling of the transistors.
The smaller version consumes roughly 300 mA, the larger version
This shows such a power supply. A fuse of 500 mA limits
the wall current, a 30VA toroidal transformer with two secondary
coils of 30 V supplies the 30 V, two power diodes and
two parallel 2.2 mF electrolytical capaitors form the DC.
This shows the voltage on the capacitors on start-up of the
power supply. The classification of the capacitors for 50 V
is required as minimum, as the picture shows.
At a load of 500 mA the voltage decreases down to 36 V.
At 470 mA I measured 37 Volts, which is well in accordance
with the simulation. This is by far enough voltage for the LED
operation. The voltage swings of 0.69 V (at 50 Hz) are
easily suppressed by the constant current regulating transistors.
The picture shows the mounting of the toroidal transformer and the
small PCB with diodes and capacitors in a wooden box.
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. You can also use a 6-pin connection instead.
Some caution is required if you supply the base voltage without the
LED operating voltage in place. In this case the CE does not deliver
Ibase max = (5 V - 0.7 V / 440 Ω = 9.8 mA
flow into each transistor base. With 14 rows this is theoretically
137 mA, with 25 even larger. This is more than a portpin of an
AVR can deliver and the 5 V operating voltage can break down,
by that interrupting LCD operation. The transistors are not at risk.
1.4 Alternative 5V supply
In the schematic of the timer a 7805 regulator is shown. This can
only be used if the LEDs are powered by an external power supply
with not more than 30 V. If the 30 V power supply is
used, which can have up to 50 V (without load), such a regulator
would not be usable (due to over-voltage). An alternative supply for
the 5 V of the timer can be build as follows.
This has a little bit more components than an integrated regulator
but works with the 30-to-50 V power supply.
On the input a green LED (with two resistors) and a Zener diode
(also with two resistors) reduce the no-load voltage of the power
supply down to transistor-friendly 42 Volts. The Zener diode
of 9.1 V is blocked by a 10µF capacitor to suppress
noise. The base of the transistor BD439 is at 9.1 Volts, its
emitter at 8.45 V. This goes into a small 78L05 integrated
regulator. Du to the small input voltage difference, the 5 V
supply can deliver up to 100 mA current. The transistor
should be cooled with a small 20 K/W heat sink.
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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
with a laser-printed thin sheet and a 1 mm plastic panel on
top is roughly four minutes.
That is how the box looks like from above.
That is how the extended LED field looks like.
To the right the LEDs are switched on. The previous LEDs and
the extended LEDs obviously have different intensities. But I
have not seen any difference in exposures of large PCBs, so
the difference is academic.
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The software for the timer is available in
HTML format or as
code file. The assembled Hex code file
and a prepared EEPROM hex file is also
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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