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Even though SILICON
CHIP publishes most
PC board patterns and/
or has them available for
download, making your own
PC boards has for many been
put in the “too hard” basket.
Here’s one reader’s way of
producing commercial-quality
PC boards at home. He starts
off by building an exposure
light box with timer.
by
Robert Scott
I
have been using Autotrax*
1.61 to design
PC boards for my own
creations for a few years
now, ever since it became
available at the right price
(free!). Before that I used Easytrax* and way in the past I used
Bishop Graphics tapes and pads.
That at least got me a PC board artwork.
Now the challenge was to convert that to a
PC board.
I tried using “PressnPeel”, a photo-sensitive film
which transfers a toner direct to the PC board surface
using a hot iron. This then acts as the resist for etching.
However, despite the glowing reports I’ve seen on this product
on the ’net, I found it had its limitations.
First, the blank PC board must be extremely clean for the toner imsiliconchip.com.au
November 2007 69
D1–D4: 1N4004
A
REG1 7805
OUT
GND
100nF
MMC
100nF
MMC
IN
T1
K
+16V K
A
K
A
NEUTRAL
6V
240V
6V
2200 F
25V
1A FUSE
K
A
ACTIVE
+5V
CON4
14
+5V
6
1
2
3
2
1
18
17
6
7
8
4
7
8
RA3
RA2
RA1
RA0
RB0
MCLR
4
5
4.7k
5
CON3
B
IC1
RB1 PIC16F84–04
RB2
11
RB5
9
RB3
10
RB4
16
OSC1
3
RA4
13
15
RB7
OSC2
12
RB6
Vss
9
10
11
12
Q6
BC557
Vdd
E
C
C
E
Q5
BC547
RLY1
K
100nF
250VAC
X2 TYPE
D5
1N4004
B
A
LOAD
(FLUORO
TUBE
CIRCUITS)
10k
10k
10k
C
B
Q7
BC337
E
X1 4MHz
22pF
22pF
4
3
2
1
220
100nF
MMC
CON2
11
6
3
16
3
14
10
1
13
7
12
6
10
9
8
7
IC2
4511B
11
4
7
10
2
5
9
1
6
4
5
9
4
2
2
1
15
10
a
f
g
e
b
f
6
e
2
B
E
C
2
1
8
Q1
BC557
B
LED1
c
d
1
K
8
8
LED2
C
K
B
E
C
Q3
BC557
E
B
C
Q4
BC557
K
7805
BC337, BC547,
BC557
2
1
150
A
TIMER
RUNNING
LED4
SET
TIME
S3
START
TIMER
S2
SELECT
S1
LIGHT BOX CONTROL TIMER
GND
B
E
C
IN
GND
K
SC
2
b
A
4
3
2007
g
e
4
c
K
A K
E
f
6
d
A
a
7
b
g
e
4
c
K
Q2
BC557
f
6
d
1
K
g
10
a
7
b
DISP4
9
10
a
7
4
c
d
SECONDS
DISP3
9
8
150
CON1
DISP2
9
8
12
STBY
LED3
MINUTES
DISP1
7 x 47
OUT
LEDS
K
A
1N4004
A
K
Fig.1: the light box controller is built on two PC boards and this circuit diagram is split in two vertically, each part
containing the contents of one of the boards. They are joined by two short cables, one 4-way and one 12-way, which
plug into connectors 1/3 and 2/4 respectively.
70 Silicon Chip
siliconchip.com.au
age on the film to stick to it. Second,
if the PC board artwork is quite a bit
larger than the iron then it is hard to
get the blank board up to the correct
temperature all over for the toner to
stick again.
Quite often you would pull away
the film only to be left with a result
where, Dalo pen in hand, you would
have to repair the pattern as best you
could.
It wasn’t a very satisfactory situation
and to make matters worse, PressnPeel
at a retail level adds quite a lot to the
finished board cost.
The good news is that I have heard
sodium hydroxide works just as well.
The bad news is that I have not been
able to get sodium hydroxide anywhere down here in Tasmania yet so
I cannot verify if the above is true.
Editor’s note: at SILICON CHIP we have
been producing one-off PC boards using Kinsten pre-coated blanks for some
years (in fact, we published a feature
on it in March 2001) and had heard
exactly the same thing.
We can confirm that properly diluted sodium hydroxide will develop
Kinsten boards perfectly. Too strong a
solution and the whole image washes
straight off. Too weak and nothing
happens. Experimentation is a wonderful teacher. Incidentally, we found
that sodium hydroxide is not difficult
to obtain from specialist chemical
supply houses here in Sydney.
OK, with the availability of the
blank board and suitable UV lamps the
next step was finding a way to transfer
the computer-generated PC board pattern to a transparency through which
the Kinsten coated blank boards could
be exposed. The idea is to have as high
a contrast as possible – black blacks
and clear “whites”.
The problem with most printers,
especially printing onto transparency
film (eg, overhead projector film) is
that the blacks are anything but. Hold
one up to the light and you’ll see what
I mean.
If you are very accurate, to some
degree this can be alleviated by us-
Pre-coated boards
I had been looking for a source of
relatively cheap, photo-resist coated
blank board and found it in “Kinsten”
positive acting photo-resist coated PC
board.
Kinsten coated PC boards are available in both SRBP and fiberglass,
single or double-sided and in a variety of sizes, from several sources – I
obtained mine from KALEX (718 High
Street Rd Glen Waverley Vic 3150. Ph
03 98020788). They can also supply
via mail order.
Also available from Kalex are the
8W UV lamps used in this project at
$9.75 each plus GST. The developing
solution for this resist is available too.
While it appears to be just plain old
sodium hydroxide (NaOH; caustic
soda), it is actually sodium metasilicate, mixed at 50g per litre of water. A
50g pack will cost $2.50 plus GST.
ing two sheets. I get very good results
from two toner-coated transparencies
from a laser printer stuck together with
thin double sided tape. I haven’t tried
inkjet transparencies or even know if
this is possible with inkjet. I find a
good HP or Canon laser printer such
as the LaserJet 4 or Cannon LBP 1260
does the job admirably.
I have one of each of these; even the
LaserJet II or III will do. These can be
obtained quite cheaply second-hand
and refurbishing the cartridge is quite
easy, even if rather messy.
Editor’s note: inkjet prints can be
just as good as, if not better than,
laser prints. However, the problem of
non-black blacks still exists. Incidentally, great results can be achieved by
printing onto plain bond paper – with
an appropriate increase in exposure
time.
Exposing PC boards
The Kinsten coated boards are
exposed by shining UV light through
the artwork transparency. The clear
part of the transparency “softens” the
emulsion on the PC board, which is
then “developed” away with the sodium hydroxide solution mentioned
earlier.
Two problems exist. One is to keep
the PC board pattern transparency in
intimate contact with the board so
that there is no light “scatter”, causing
break-up of tracks. Even the thickness
of the film itself can cause problems, so
the image on the film should always be
BALLAST 1
(EC13 OR SIMILAR)
12-WAY
CABLE
8W UV FLUORO
4–20W
STARTER
CON2
TIMER
MODULE
siliconchip.com.au
4–20W
8W UV FLUORO
4–20W
MAINS
INPUT
STARTER
BALLAST 2
(EC13 OR SIMILAR)
4-WAY
CABLE
Fig.2: Light Box mains wiring. The
two PC board modules control two
sets of two 8W fluoro blacklight
tubes, as shown here. Incidentally,
with suitable mains insulation,
these modules could also be used
as a general-purpose timer.
8W UV FLUORO
NEUTRAL
ACTIVE
CON3
CON1
TIMER
CONTROL
MODULE
CON4
LOAD
STARTER
A
N
E
8W UV FLUORO
EARTH CONNECTED
TO METAL CHASSIS
4–20W
STARTER
November 2007 71
Outline of the project
CON4
4004
4004
4004
4004
22pF
ALTRONICS
P2037A
TERMINAL
BLOCK
SEC
BC547
10k
Q5
CON3
Q7
BC337
E
SEC
NC
D5
10k
X2 TYPE
C
BC557
BC557
Q1
1
4511B
47
47
IC2
47
47
47
CON2
47
47
100nF
220
LED3
Y BT S
150
NRG
N
O
LED4
BC557
BC557
Q3
Q2
ACTIVE
240V
NEUTRAL
LOAD
(TO
FLUORO
TUBES)
100nF 250VAC
NO
BC557
1
ALTRONICS 7012
ALTRONICS
S 4170A
B
10k
Q6
T1
RLY1
C
CON5
IC1
4.7k
100nF
COVERED
M205
FUSE (1A)
(ALTRONICS
S5985)
REG1
7805
22pF
X1
16F84
4.00MHz
+
100nF
Q4
88 88
LED1
+
+
LED2
DISP3
DISP2
DISP1
TS3
ES
TRS2
ATS
DISP4
T CS1
ELES
DER
CON1
150
7 0 B EF 8 2
L T C R E MI T
Figs 3 & 4, the component overlays for the Exposure Controller (top) and the display
board/timer controller (bottom). CON5 should be a 3-way terminal block, as shown.
The electronics side of the
project consists of two PC boards, each
120 x 64mm. One is for the timer lamp
control and power supply, the other
the timer control and display panel.
One of these is mounted on standoffs on the underside of a folded
aluminium chassis, which also contains the fluorescent tube ballasts and
starters. The other is mounted on the
side of (and through) the lightbox
so that its LED displays and setting
pushbuttons are all accessible and
viewable from outside.
On the top side of the chassis are
mounted the eight “tombstones”
which hold four 8W NEC fluorescent
“blacklight” (UV) tubes. These are not
like the deep purple (almost black)
blacklight tubes you see in clubs and
discos. Instead, these are described as
“actinic blue” and appear white when
off but are very strong in UV as well
as visible blue light when on.
This chassis is secured by screws
in a wooden box, outside dimensions
360 x 120 x 100mm, which has a
72 Silicon Chip
D1–D4
2200 F
1
on the PC board side, ie, “emulsion to emulsion.”
The second problem is to keep
the amount of UV exposure constant in both time and strength,
so that results are consistent.
Various methods of exposure
have been tried over the years
– including using the very high
UV content of sunlight. But this
highlights problem two – the
sun’s strength varies according
to time of day, cloud cover, latitude, pollution levels, etc!
The answer is to use a dedicated light box. With a timer,
the exposure could be set. With
pressure applied to the transparency, the two parts could be
held together properly.
I thought I would see if a light
box project was feasible. First
thing? Check the net!
There appeared to be a lot of
info but only one with anything
like what I was looking for. It
consisted of a PIC16F84 programmed as a timer with a basic
circuit displaying on 7 segment
displays. While it held promise,
I believed that with redesign of
the firmware for the PIC and
particularly the hardware would
make it much better.
6mm glass pane located in a channel
in the sides of the box, which places
it about 25mm above the fluorescent
tubes. There is a hinged lid on the
box which has a piece of 6mm foam
covered with felt glued to its underside. When the lid is locked closed,
the foam and felt force the PC board
(and the transparency underneath it)
hard against the glass pane.
This ensures that the blank board
and the transparency have intimate
contact with one another so that the
image on the transparency accurately
transfers to the blank PC board.
The circuit
Fig.1 shows the wiring of the exposure lamps, ballasts and starters, under
the control of the timer PC board.
Power is switched to the fluoro
tubes via a mains-rated relay, under
the control of the PIC and switching
transistors.
The four UV tubes are arranged
in two identical parallel circuits,
shown in Fig.2. Each one consists
of two lamps, two starters and a ballast all in series. The starters are the
4-20W (more sensitive type) for the
lower level currents involved with
8W tubes.
It’s a little unusual to have two
tubes share one ballast, so a word of
explanation might be necessary.
When power is applied, both
starters will arc and close due to the
internal bimetallic strip. The tube
heaters will heat up and the inductor
(ballast) will build up a 50Hz varying
magnetic field. When one of the starters cool down and open the magnetic
field round the inductor will collapse
causing a somewhat large EMF to be
developed across the inductor. This
will appear across the open starter
and its associated tube.
The gas inside the tube will ionise
and the tube will strike. Once any
fluorescent tube strikes, the voltage
dropped across it due to current flowing through it is much reduced. If the
siliconchip.com.au
There are some differences between these photos and the final version – specifically the mains connector, the fuse type, suppressor capacitor and the relay.
other starter then opens induced EMF
across the inductor again will strike
the second tube.
All this happens rather fast and
both tubes should be glowing within
a second or so.
Sometimes both starters open
nearly simultaneously and the startup
strikes occur together. This type of
circuit is possible with low wattage
tubes as the distance between tube
heaters is small compared to say, a
36W standard lighting fluorescent
and the voltage drop is small.
The PC boards
Two PC boards are used, sharing
functions between them.
The control/display PC board is
connected to the timer board with
12-way and 4-way cables. I used these
as it was easier to design and make
single-sided PC boards to suit these
than it was to make a double-sided
board with a dual-in-line 16-pin plug.
Because these are all on the lowsiliconchip.com.au
voltage side of the circuit, ordinary
hookup wire or even rainbow cable
can be used here.
The timer PC board is screwed to a
small panel of 1mm aluminium with
stand-off’s. Cutouts and holes are
required in the panel for the standoff’s, LEDs, 7-segment displays and
pushbutton switches. This panel is
then screwed to the left side of the
light box with a cutout to suit.
Looking now at Figs. 2 and 3, power
is supplied to the circuit via a 1A fuse,
PC-mounted transformer, (240V to
two 6V windings, eg, Altronics 7012).
Both 6V windings are connected in
series, rectified and filtered, resulting
in an unregulated DC supply of about
16V or so.
The unregulated supply is used to
power the switching relay and also
fed to a 5V voltage regulator (REG1,
7805). This provides the timer with
a 5V regulated supply.
Most of the timer operation is carried out by the programmed PIC16F84
so the circuit is not as complicated
as it would otherwise be if hardware
alone did the task.
The PIC’s clock is set at 4MHz by
crystal X1. Pins 17,18,1,2 (RA0 to
RA3) send multiplexed BCD data to
the display board via P4-P2. Pins 6
to 9 (RB0 to RB3) send multiplexed
data to transistors Q1 to Q4 (display
drivers) on the display board via P4P2. Pin 11, RB5, is normally held low
in standby.
When the timer is counting down
it goes high, biasing on Q7 (BC337)
which pulls in RLY1 (supplying
power to the fluorescent tubes), at the
same time biasing Q5 (BC547) on and
Q6 (BC557) off.
These in turn extinguish standby
LED3 and turn on running LED4.
When the timer has completed the
countdown RB5 goes low, which turns
off Q7 and turns on Q5 and Q6. Relay
RL1 opens, the timer LED4 goes out
and standby LED3 comes back on.
Pin 3, RA4, connects to the select
switch via P3-P1; a pull-up resistor is
required here. Pin 12, 13 (RB6, RB7)
connect to the set and start switches
(S3, S2) respectively.
Pin 10 (RB4) provides a positive
pulse every second while the timer
is active and this pulse is fed to two
LEDs in series via a 220Ω resistor.
These form a “colon” between the
minutes and seconds LCD digits.
Making the chassis
Aluminium was chosen for the
chassis as it is easy to work with and
some UV light will reflect from this,
distributing the UV fairly well through
the artwork. The chassis is bent in a
“U” shape with holes and slots cut out
for the various components.
The layout is shown in Fig. 7,
reproduced a little under half size.
Ideally, the chassis should be bent to
shape with a sheet metal folder but
good results can be had with 25mm
angle iron and a sturdy vice.
The aluminium sheet size is 320
x 265mm and the sheet can be 1 to
1.6 mm thick.
Making the Box
Once the chassis is made then the
box can be made to fit. I made my box
from 17mm plywood, 100mm high.
A plywood lid was made to suit from
the same material.
A sheet just over 600 x 470mm (to
allow for saw cuts) will achieve miniNovember 2007 73
Parts List – PC Board Light Box
1 PC board, 120 x 64mm, code 10111071
1 PC board, 120 x 64mm, code 10111072
1 aluminium sheet, 155 x 80mm x ~1-1.5mm (for front panel) with label
1 aluminium sheet, 300 x 320mm (thickness 1-1.6mm) (for chassis)
1 240V to 12V (2x6V) PC board mounting mains transformer (eg, Altronics M-7012A)
1 12V SPDT PC board-mounting relay with mains-rated contacts (eg, Altronics S-4170A)
1 4MHz crystal (X1)
1 covered M205 fuseholder, PC board mounting (eg Altronics S5985)
1 1A M205 fuse
3 pushbutton membrane switches, PC board mounting (eg, Altronics S-1135)
3 16-pin machine IC socket
1 18-pin machine IC socket
1 4-pin 90° PC board male socket (eg, Altronics P5514)
1 12-pin 90° PC board male socket (eg, Altronics P5522)
1 4-pin straight PC board male socket (eg, Altronics P5494)
1 12-pin straight PC board male socket (eg, Altronics P5502)
2 4-pin plugs
2 12-pin plugs
1 300mm length 4-wire cable (either rainbow cable or individual wires)
1 300mm length 12-wire cable (either rainbow cable or individual wires)
1 3-way mains-rated PC board mounting terminal block (eg, Altronics P2037A)
1 sheet 17mm plywood, ~600 x 470mm and 17mm iron-on edge veneer
1 sheet 3mm plywood, ~360 x 270mm (for base)
1 sheet 335 x 245 x 6mm clear glass (no flaws, scratches or tinting)
1 sheet 320 x 230 x ~7mm foam plastic (high density if possible)
1 sheet 320 x 230mm felt
1 piece red transparent plastic, 65 x 20 x ~1.5mm (for display lens)
2 hinges for lid
4 rubber feet
4 8W UV (actinic blue) fluorescent tubes (eg, NEC blacklight FL8BL or similar)
8 miniature fluoro tube holders, type ST 268 (known as “tombstones”),
4 fluorescent starter holders (HPM 390 or similar)
4 4-20W fluorescent starters (Osram ST151 or similar)
2 13W fluorescent ballasts (EC13 or similar)
1 3-core mains lead fitted with 3-pin plug.
1 mains cord clamp
1 earth lead lug (crimp-on preferred)
Electrical parts, including the
Lengths mains-rated hookup wire for fluoro tube, ballast and starter wiring
miniature tube holders (“tombstones”), ballasts, etc are fairly
Semiconductors
common items available from
1 PIC16F84-4, loaded with light_box_timer.hex (IC1)
(or ordered via) most electrical
1 4511 7-segment display driver (IC2)
wholesalers.
1 7805 5V regulator (REG1) with U-shaped heatsink
5 BC557 or BC558 transistors (Q1-Q4, Q6)
The 8W “blacklight” fluorescent
1 BC547 or BC548 transistor (Q5)
tubes are not so common but
1 BC337 or BC338 transistor (Q7)
should also be available from
2 3mm red LEDs (LED1, 2)
major electrical wholesalers
1 5mm green LED (LED3)
(even if on special order).
1 5mm red LED (LED4)
Those used in the prototype
4 1N4004 1A silicon diodes (D1-D4)
were obtained from KALEX,
4 0.5-inch 7-segment common cathode displays (DISP1-4)
718 High St, Glen Waverley,
(eg, Jaycar ZD1855 or Altronics Z0190)
Vic 3150. Tel (03) 9802 0788.
Capacitors
1 2200mF 25V electrolytic
* Autotrax and Easytrax PC
4 100nF monolithic
board layout software are avail1 100nF 250VAC X2 TYPE
able as free downloads from
2 22pF ceramic
www.altium.com/Community/
Support/Downloads/
Resistors (0.5W, 1%)
3 10kW
1 4.7kW
1 220W
1 150W
8 47W
74 Silicon Chip
siliconchip.com.au
mum wastage. The two sides and two
ends need a slot cut in them, about
7mm down from the top, to accommodate the glass plate.
There has to be a slot about 6mm
down from the top of the box to fit the
glass plate. This is best done with a
router using a ¼” (6.5mm) bit. Cut the
slot about the same depth (6.5mm). As
you are not removing much wood this
can be done with one cut. The slot can
also be cut with a circular saw if you
are experienced enough – a router is
better though and they can be obtained
very cheaply these days.
As the smallest router bit I had was
¼”, the glass plate had to be the same
thickness, ¼” or 6.5mm. This was
a fortunate accident, because that’s
about the right thickness for stability but not too thick to have to worry
about UV absorption in the glass. It
is important that the glass does not
have any scratches or imperfections
as these will surely show up in your
finished PC boards.
Below: the completed Light Box
with its plywood base removed. The
second PC board is on the left side.
siliconchip.com.au
Is Ultraviolet light dangerous?
From time to time warnings appear about the dangers of UV light. Even
as we go
to press, UV tanning salons have been implicated in at least one recent death
through
melanoma (skin cancer).
From the outset, let’s state that staring at any light, especially intense light,
is not
good for the eyes. Very bright light, especially if strong in ultraviolet wavelen
gths in
particular, is known to cause eye discomfort and damage.
Ultraviolet light is generally regarded as having a wavelength from about 200
to 400nm
(nanometres). This is further divided into three sub-bands, UV-C, UV-B and
UV-A.
UV-C (200-280nm) has the shortest wavelength and is often used as a germ
killer
or steriliser. It is regarded as dangerous stuff! Anything which emits UV-C
usually has
interlocks to prevent accidental exposure to the eyes or skin.
UV-B (280-320nm) has a longer wavelength and is considered less dangero
us but
exposure can redden and possibly burn the skin and may cause damage to
the retina.
UV-A has a longer wavelength again (320 to 400nm) and is considered less
dangerous again.
Prolonged exposure to UV-B and perhaps to UV-A are acknowledged to cause
skin
damage and possibly promote skin cancers as well as eye damage. But the
vast majority
of references point to UV-B light as the bogey.
The NEC FL8BL blacklight lamps used in this project emit mostly UV-A, with
a peak
wavelength of 365nm (which also explains why there is so much visible blue
light from
them). They are in fact the same as (or similar to) the blue lamps used in bug
zappers.
Ideally, you should avoid long exposure, especially of the eyes, to any UV (or
indeed any
strong light). But the high wavelength of these tubes, their low power (all four
combined
are less than a single 36W fluoro tube), the fact that there is a sheet of UV-abso
rbing
glass above them and the very intermittent nature of exposing PC boards
using them
means that they are reasonably safe.
Having said all that, keep children away and don’t let your teenage daughte
r use this
as a mini face-tanning centre! If you are still concerned, a mains-rated interlock
switch
(eg, a microswitch operated by the lid) could be fitted in series with the
active wires
going to the ballasts.
November 2007 75
Fig. 7: here’s how
to fold and cut the
aluminium chassis,
looking from the
underside.
The only critical
positions are the
notches for the
tombstones which
must of course line up
with each other. The
PC board, ballasts
and starter holders
can be placed in
approximately the
positions shown.
320mm
20mm
BEND UP 90
o
37mm
TIMER
PC BOARD
*
STARTER
STARTER
#
#
#
*
50mm
BALLAST
*
*
#
225mm
50mm
# PRECISE
POSITION
NOT
IMPORTANT
* 8 SLOTS FOR
TOMBSTONES
19 x 16mm
IN POSITIONS
SHOWN
*
*
BALLAST
#
50mm
*
*
STARTER
STARTER
#
37mm
BEND UP 90
20mm
With the dimensions shown, the
glass plate will be 6mm all round
greater than the box internal, the
chassis is 225mm wide by 320mm
long, therefore it follows that the glass
plate will be 237 x 332mm.
Of course this all depends on your
carpentry skills. I used iron-on veneer
on the cut edges of plywood and varnished the whole assembly with Estapol. This makes the job attractive as
well as functional. Ply was used rather
than straight wood as this tends to be
truer so the pieces fit together better.
The lid is a single piece of plywood,
the same size as the box and again
finished with iron-on veneer. It is
attached to the box with two mediumsized hinges.
Inside the lid a piece of 6mm high
density foam plastic, covered with
self-adhesive felt on one side, was
stuck into place with double sided
tape to fit into the space between the
top of the box and the glass.
Its size, 320 x 230mm, allows it
to clear the box edges as the lid is
76 Silicon Chip
closed and press down hard on the
blank PC board to hold it flat against
the artwork.
Assembling the PC boards
Solder the 3 links on the display
board first, followed by the resistors, IC socket, displays, sockets and
capacitors.
The same order applies for the
timer/power supply – the lowest profile components first and highest last.
300mm lengths of 12-wire cable and
4-wire cables using rainbow cable or
single hookup wire lengths has to be
made in order to connect the display/
control board to the timer board.
Do not solder the LEDs into the
display PC board yet.
Putting it all together
All components can now be fitted
to the chassis as per Fig.4 and wired
as per wiring schematic Fig.3. Be very
careful in wiring the mains-carrying
cable – that is to all the fluorescent
tube holders, starters and ballasts.
#
o
Mains wiring may be taken directly
to the block connector on the timer
PC board, with the switched active
connected to the rest of the circuit.
Use single-core 10A lighting wire for
wiring the lighting circuit up. That’s
not because there are heavy currents
involved, it’s for the safety afforded
by the cable’s insulation.
Connections to the tombstones are
achieved by pushing the stripped
cable into the hole provided. The
wires are locked into place by a spring
loaded clamp and once they are in it
is difficult to pull them back out again
so try not to make mistakes. Make
doubly and triply sure, however, that
all strands of the wires have gone
into the hole and none poke out to
possibly short to the chassis.
Make sure you earth the chassis via
the earth wire on the mains 3-core
cable and plug.
The connections to the starter
holders are achieved by a clamping
screw.
A rectangular cutout will have
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2007 77
11/07
4–20W STARTER
BALLAST 1
S
DAOL
“U”-SHAPED
ALUMINIUM
CHASSIS
CABLE
TIES
EC13
+
A
N
A
N
TIMER BOARD
V21+
REMIT ERUSOPXE BCP
4–20W STARTER
4–20W STARTER
EC13
4–20W STARTER
BOTTOM OF
FLUORO TUBE
END SOCKETS
(TOMBSTONES)
ACTIVE
(BROWN)
MAINS
CABLE
1
70BEF72
BALLAST 2
EARTH WIRE
CONNECTED
TO CHASSIS
CON4
1
CON3
1
NEUTRAL
(BLUE)
+5V
BOTTOM OF
FLUORO TUBE
END SOCKETS
(TOMBSTONES)
TIMER CONTROL BOARD
1
1
88 88
+
CON2
+
YBTS
NO
TES
TRATS
TCELES
DER
1
CON1
NRG
78 Silicon Chip
70BEF82
LTC REMIT
Fig.6: the complete wiring
diagram showing the
underside of the 320 x
270mm U-shaped chassis.
The tombstones poke up
through slots in the chassis
with the tubes on the upper
side. All wiring to the fluoro
tubes, starters and ballasts
must be 250VAC rated.
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PC BOARD UV LIGHTBOX AND TIMER
SILICON
CHIP
STBY
to be made for the control /display
board on its mounting plate either in
the front of the box or as I have done
in the left side.
I have specified insulated stand-offs
to mount the PC boards but metal ones
could be used except for the one on the
mains entry side of the timer/power
supply board.
Display PC board
The display PC board is mounted
to a small piece of ~1mm aluminium
with holes drilled for the LEDs and
switches along with a cutout for the
display.
A piece of 1mm reddish plastic was
glued into the cutout as a protective
screen for the 7-segment displays and
seconds LEDs. Use a small quantity of
slow setting epoxy for this. The “Five
Minute” type sets too quickly and is
not as strong.
Drill PC board mounting holes in
the panel by placing a photocopy of
the display board overlay on the panel,
lining up the 7-segment displays in
the cut out and marking the center
of the holes to be drilled with a prick
punch or scriber.
I used 2mm mounting screws, nuts
and washers. The stand-offs should be
8mm to allow the push-button switches
to sit proud of the front panel.
If using 2mm screws you may have to
make your own from 2mm brass tubing
available from model aircraft stores.
The 2mm screws don’t stand out on the
front panel as much as 3mm.
Countersunk screws could be used
and the front panel artwork fixed to
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Fig.7: same-size
artwork for
the Lightbox
front panel.
Photocopy this
and use as a
template for
drilling the
holes in the
aluminium
sheet.
ON
SET
TIME
START
TIMER
the aluminium over the screw heads.
Testing.
It’s best to test the timer out before
you wire it in on the chassis. Plug the
two boards together and wire the main
board temporarily to the timer board.
Do not plug the IC’s in both boards as
yet, that is the 4511 and the 16F84A.
Make sure you have double checked
everything especially the timer/power
supply with its mains wiring.
In the interests of safety, cover the
fuse and fuseholder with some insulation tape while testing. It’s the only
section of the top of the PC board that’s
likely to bite you – but if you contact
it, it will do just that!
Switch on power, measure to see
if you have approx +16V and a regulated +5V where marked on the power
supply; also that +5V appears on pin
16 with respect to pin 8 on the 4511
socket and between pins 14 to 5 on
the 16F84A socket. If all is well and
you have no burning smells switch off
and remove the mains plug from the
power socket. Wait a short time for the
electrolytic capacitors to discharge and
insert the two ICs
Reconnect and switch power back
on. You should get a readout of 00:30
on the display board. The relay should
not be energized and the green standby
LED should be illuminated. If you do
not have this, switch off and recheck
your work.
Hopefully all should be well and
you can proceed to check the timer
operation. Press the start button, the
green led should go out and the red
PRESET
SELECT
one should illuminate, at the same
time the relay will energise and the
display will begin to count down from
30 seconds to 0.
When the timer reaches 0 the relay
will drop out, the red LED will extinguish and the green one will come back
on. Pressing the start button again will
bring back the 00:30 readout again.
Press the select button and the display should change to a different time
setting. Do this 15 times. There are 16
timer settings stored in EEPROM in
the programmed 16F84A.
You can change any or all of these
if you so desire by the doing the following: select a setting to change by
pressing the select button until the
display is reading the setting you want
to change.
Press the set button. The seconds
will start flashing, incrementing one
more every second, when the time in
seconds is reading your requirement
press set again. The single minute
digit will start to flash incrementing as
before, again when your desired time
is reached press set again. The tens of
seconds will start to flash incrementing
as the single minute digit did.
Again, when your requirement is
reached press set again. The timer
will be set in EEPROM to your keyedin time.
If you make a mistake then you will
have to go through the entire procedure
again. Usually you will only need to
do this once or twice.
If all is well checking the timer then
it can be wired into the chassis and the
rest of the wiring completed.
November 2007 79
Using your new light box to make a printed circuit board . . .
Exposing the image
To make boards from Kinsten stock the manufacturer’s recommend exposure
time is 60 to 90 seconds using a high-contrast film. Set the timer for 1 minute
15 seconds using a test artwork. You may need to do a
few test exposures and increase or decrease exposure
times as required. Too long and you will end up with all
the resist washed away, too short and it will be under
developed with the “clear” areas not washing away.
Using this presensitized PC board I found the latitude is
about 10 seconds either way but you may find it different.
You may need up to five or six minutes or so to expose a
board through bond paper. But it can be done!
You could use Riston negative-acting pre-coated
board but it is more expensive and so is the special
developer and stripper. Also you will require a negative
of your artwork.
To give you an idea of cost, a fibreglass pre-coated
Kinsten‑board, 150mm x 300mm from Kalex costs
$16.50 plus $2.75 for developer. The same size Riston
board (from Jaycar) will cost you $49.95 plus $7.95 for
developer and $8.95 for stripper (you will never get it Getting a black black is actually more important that getting
off easily otherwise). Kinsten resist can be removed a clear white (believe it or not, you can expose through bond
paper!). The pattern should be on the bottom side of the film, so
with 00 gauge steel wool or acetone.
it is intimate contact with the photo-sensitive emulsion.
As you can see Kinsten is about one third the cost
taking everything into consideration.
Developing the exposed board
As we mentioned eariler, the proprietary developer is easy to mix and use but
we have also had success using a weak caustic soda brew. You’ll soon know if
you’ve made it too strong or too weak – if it’s too strong the resist will all wash
off (including the bits you want!) and if it’s too weak nothing will wash off.
The same tray can be used to develop the board and to etch it – just make
sure you wash the tray out between times.
Developing is achieved by full immersion, emulsion-side up, and gently rocking the tray so the developer “washes” over the resist. Rotate the tray as you go
so the washing is even.
Brushing the board lightly with a soft brush (a makeup brush is ideal) can
assist developing but be careful – it can result
in flaws in the resist.
Before very long (a minute or so if your exposure is correct) you should see patches of
developer starting to wash away from the board.
It doesn’t take too much longer for development to be complete, with all unwanted areas
(ie, between tracks, component holes etc) now
cleared of developer.
Development time will increase with lower
temperatures so down here in Tassie I heat up the
developer with an old microwave oven for about
a minute. Be careful – too hot and you will be left
with no image either (it will all dissolve).
When finished, rinse it in cool fresh water.
Until it dries, the resist is normally fairly soft.
The board can either be air-dried (say an hour
or so), dried in direct sunlight (half an hour) or
baked in a just-warm oven (an electric frypan
is also good!) for maybe ten to fifteen minutes.
80 Silicon Chip
Developing a Kinsten pre-coated
board. The clear areas in the
transparency have been washed
away leaving the resist to protect
the wanted areas from the etchant.
Yes, this is a different board to those
shown above!
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Etching the developed Board
If you don’t make many boards then the easiest way
to etch the board is to place it upside down in a plastic
container of ferric chloride in solution for 10 to 30 minutes,
depending on the temperature, or rocked in a large tray.
If you make a number of boards, a better way is to
purchase an etching tank, fish tank water heater & air
pump. The tank is available at both Jaycar & Altronics or
you can make your own from glass or Perspex. This way
boards can be etched in less than 10 minutes depending
on the strength of the solution.
An alternative etchant is ammonium persulphate but
this needs to be heated to at least 50°C (or more – the
hotter the better) before it will work and standard fish
tank heaters will only heat the etching solution to 30°C
(tropical fish don’t like it much hotter than that).
Also, ammonium persulphate is theoretically a use
once solution so etching using this can be expensive.
However, we’ve been able to use stored ammonium persulphate etchant many
times over a few weeks (heating it each time before use).
By the way, the cheapest and best way to buy ferric chloride is in hydrated
granules. This is available from RS Electronics in 2kg containers (Cat 551-277).
Virtually everything I get for this hobby has to be mail order down here, so there
is no point in paying postage for water.
Mix 500g per litre of water and to clear the solution a little add 5g of sodium chloride (common salt.) Add the ferric chloride to the water not the other way around.
It takes 2.5 litres of solution to fill the Altronics/Jaycar etch tank but this lasts
for quite a time; I have etched over 50 large and small boards in the one batch I
have mixed up and it still etches quite well under 10 minutes.
One problem is that it ferric chloride is messy – don’t wear your favourite clothes
– and after a while sediment builds up at the bottom of the tank. I wait until all the
sediment settles overnight, then drain the etching solution off into a large plastic
container leaving the sediment behind and then clean out the tank.
I then pour the solution back into the tank topping up with fresh solution
if required. There’s not much evaporation down here where I live in Tassie
any time of the year, so the level in the tank doesn’t go down much. Once
the board is etched and the etch solution is washed off then it is ready for
drilling and finishing.
For very occasional PC boards, tray
etching is quite practical. Etching
needs to be helped along by rocking
or sloshed with a non-metal soft
brush. The etchant shown here is
actually ammonium persulphate –
it’s a lot cleaner to use than ferric
chloride but must be heated first to at
least 50-60°C to be usable. This board
is about 90% etched – most of the
inter-track copper is gone with just a
few larger areas to go.
Below: if you’re making several PC
boards, this commercial etching
tank, heater and air pump is
definitely the way to go.
Finishing the completed board.
Don’t use a hobby PC board drill press as these just haven’t the torque
required. Buy yourself a cheap Chinese drill press with ½” chuck, replace the
bearings with good Australian-made ones and use tungsten carbide bits.
These are quite readily available, from 0.45mm to 6.31mm. These bits
won’t dull on fibreglass but they are very hard and brittle so they are easily
broken. With this in mind, buy more that one of each size.
The resist can be left on while drilling to protect the board from oxidising.
When you have completed drilling holes, scrub off the resist with 00 steel
wool and dishwashing detergent.
Once you have done that dry the board thoroughly and cut it to size with
a hacksaw and finish with a file. Then give it a couple of coats of liquid resin
flux from a Solder flux pen (Jaycar cat TS-1512)
This will help in soldering and also give the board protection from oxidation.
A methylated spirit/resin flux used to be available but I haven’t seen the
product anywhere for years. However, you can make your own with Rosin
(buy it at a specialist music shop – it is used on violin bows). Crush it then
dissolve it in metho until no more will dissolve.
SC
siliconchip.com.au
November 2007 81
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