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A Beam-Break
Flash Trigger
By JIM ROWE
Here’s an easy-to-build accessory for the Time Delay Photoflash
Trigger described in our February 2009 issue. It triggers the delay
unit and your photoflash in response to an object interrupting an
invisible beam of infrared (IR) light. Alternatively, it can be used
on its own to directly trigger a photoflash.
A
FEW MONTHS ago (in February
2009), we described a “Time Delay Photoflash Trigger”. This unit was
triggered by a sudden sound picked up
by an electret microphone. It then immediately opened the camera’s shutter
and then fired the photoflash shortly
after, depending on the delay period
programmed into the unit.
Using sound pick-up in this manner is a popular and effective method
of triggering a flash for “stop motion”
and other kinds of special effects photography. However, in addition to the
electret mic input, we also gave the
delay unit a second “contact closure”
input, so that it could be triggered using other techniques. Which was just
62 Silicon Chip
as well, because as soon as the delay
unit was published we started getting
requests for a light beam trigger.
This simple “Beam Break Trigger
Unit” is the result of those requests.
It’s mainly intended as an alternative
triggering front-end for the Time Delay
Photoflash Trigger and is connected to
the latter’s “contacts” input. However,
it can also be used to trigger a photoflash unit directly if you don’t need the
programmable time delay capabilities.
Note, however, that using the unit
to directly trigger the flash has one
important limitation. Unlike the Time
Delay Photoflash Trigger, it doesn’t
also trigger the shutter. This means that
you have to open the shutter manually
before the infrared beam is interrupted
(eg, at night or in a darkened studio).
The new project is in two parts: (1)
an IR Source unit which produces the
IR beam and (2) a Detector unit which
monitors the IR beam and closes its
output trigger contacts briefly if the
beam is interrupted. These two units
are linked with an interconnecting
cable which supplies the Source unit
with power.
By the way, if you’re already wondering how you accurately line up
the Source and Detector units when
the IR light beam is invisible to the
human eye, wonder no more. That
problem has been solved by providing
the detector unit with a visible green
siliconchip.com.au
S1
CON2
CON1
LINE-UP
GUIDE
LED4
A
22k
100nF
K
220k
IC1: LM358
1k
820
6
A
LED1
5
K
8
3
2
IC1a
10nF
1
+1.0V
A
1k
470k
10k
B
C
Q1
BC338
D
Q2
2N7000
G
S
E
TRIGGER
OUT
100
PD1
K A
LED3
22k
4
A K
LED2
7
IC1b
9V
BATTERY
(6 x AA
CELLS)
470 F
16V
220 F
CON3
10k
2.7k
K
OBJECT
BREAKING
BEAM
IR LEDS
SC
2009
'BEAM BREAK' TRIGGER UNIT
PD1
(ZD-1948)
ACTIVE
AREA
BC338
LED4
2N7000
B
K
A
A
K
K
A
E
C
D
G
S
Fig.1: the infrared beam is generated by LEDs 1-3 and picked up by photodetector diode PD1. Op amp IC1b functions
as a current-to-voltage converter while IC1a is wired as a non-inverting amplifier. The latter drives transistor Q1 &
Mosfet Q2 to briefly switch the trigger output when the IR beam is interrupted.
LED which lights when the IR beam is
being received. This makes the liningup process easy.
Both parts of the project run from
a 9V battery fitted inside the Detector
unit’s box. The total current drain is
about 15mA which means that the
battery should be either a set of six
AA (1.5V) alkaline cells or a single
high-energy 9V lithium battery. A
standard 9V zinc-carbon or alkaline
battery is not up to the job, as its life
would be too short.
Circuit details
Take a look now at Fig.1 for the
circuit details. There’s really not a
great deal in either part of the circuit.
In fact, the IR Source unit is nothing
more than three IR LEDs connected in
series and with an 820Ω series resistor.
This resistor limits the current from
the 9V supply (and thus the current
through the IR LEDs) to about 7.5mA.
Power is derived from the battery
in the Detector unit via a cable fitted
with a 3.5mm jack plug (CON1). This
mates with CON2 on the detector unit.
In the Detector unit, the IR beam
from the Source unit normally falls on
PD1, an IR photodetector diode. This
photodetector is connected between
ground and the inverting input (pin
6) of op amp IC1b (an LM358).
siliconchip.com.au
Op amp IC1b is connected as a
current-to-voltage converter. Its pin 7
output sits somewhere between +1.7V
and +4.0V when the IR beam is present
but rests close to +1.0V when no IR
light is falling on PD1. This “dark”
output voltage of +1.0V is basically
set by the voltage divider formed by
the 22kΩ and 2.7kΩ resistors, with the
220µF capacitor providing filtering.
This is used to directly bias pin 5 of
IC1b and to bias pin 2 of IC1a via a
1kΩ resistor.
The output at pin 7 of IC1b is fed to
the non-inverting input (pin 3) of IC1a,
which is configured as a non-inverting
amplifier with a voltage gain of 471.
Because of this very high gain, IC1a
acts very much like a comparator. Its
pin 1 output sits at over +8V when the
IR beam is present but falls to 0V when
there is no IR light falling on PD1 (ie,
the IR beam is interrupted).
IC1a’s output in turn drives the
base of transistor Q1 via a 10kΩ resistor. As a result, Q1 is turned on or off
depending on whether the IR beam is
present or not. When the IR beam is
present, Q1 is on and when the beam
is interrupted, Q1 turns off.
LED4 and its series 1kΩ resistor form
the collector load of Q1. This means
that LED4 lights when Q1 is on and
turns off when Q1 is off. This allows
LED4 to be used as a guide when
lining-up the Source’s IR beam with
PD1, as described previously.
Switching the trigger output
Because Q1 is switched on when
the IR beam falls on PD1, its collector
voltage is normally held down to about
0.4V. However, if the beam is interrupted, Q1 turns off and its collector
voltage rises to nearly +9V.
This sudden voltage change is used
to switch on Q2, a 2N7000 MOSFET
which is used as an output switch
across triggering output CON3. As
shown, a 10nF coupling capacitor and
Q2’s 10kΩ gate resistor form a simple
differentiating circuit. This results
in Q2 being switched on only briefly
when Q1’s collector voltage rises when
the beam is interrupted. The 100Ω
resistor in series with the coupling
capacitor is there to suppress any
possible oscillation during switch-on
or switch-off.
That’s about it, apart from power
switch S1 and the 470µF and 100nF
capacitors which decouple the supply
rail voltage to keep it constant. The
current drain of the detector circuit
varies between about 7.5mA when the
IR beam is present and 1.5mA when
it is interrupted, so the total battery
drain for both sections varies between
June 2009 63
Parts List
IR Source Unit
1 PC board, code 13106092, 57
x 26mm
1 UB5 jiffy box, 82 x 53 x 31mm
4 6mm long untapped spacers
4 M3 x 12mm screws, countersink head
4 M3 hex nuts
1 Nylon cable tie, 75mm long
1 2m length of light-duty figure-8
cable
1 3.5mm mono jack plug, cable
type (CON1)
3 5mm IR LEDs (LEDs1-3)
1 820Ω resistor
Detector Unit
1 PC board, code 13106091,
122 x 58mm
1 UB3 jiffy box, 129 x 68 x 44mm
1 SPDT mini toggle switch (S1)
1 PC-mount 3.5mm stereo jack
(CON2)
1 PC-mount 2.5mm concentric
plug (CON3)
4 M3 x 15mm tapped spacers
8 M3 x 6mm machine screws,
pan head
2 1mm PC board terminal pins
1 9V battery clip lead
1 8-pin DIL IC socket
1 30mm length of 12-15mm
diameter black PVC conduit
or brass tubing
1 piece of IR-transparent red
film, approx. 16mm square
1 9V battery snap connector OR
1 x 4-way AA cell holder plus
1 x 2-way AA cell holder – see
text
Semiconductors
1 LM358 dual op amp (IC1)
1 BC338 NPN transistor (Q1)
1 2N7000 N-channel MOSFET
(Q2)
1 IR photodetector (PD1) (Jaycar
ZD-1948 or similar)
1 5mm green LED (LED4)
Capacitors
1 470µF 16V RB electrolytic
1 220µF 16V RB electrolytic
1 100nF metallised polyester
1 10nF metallised polyester
Resistors (0.25W 1%)
1 470kΩ
1 2.7kΩ
1 220kΩ
2 1kΩ
2 22kΩ
1 100Ω
2 10kΩ
64 Silicon Chip
The IR Source board carries the three infrared LEDs (LEDs1-3) plus an 820Ω
current-limiting resistor. It’s mounted inside a UB5 case on 6mm untapped
spacers and derives its power from the Detector unit.
15mA (beam present) and 9mA (beam
interrupted).
Construction
As shown by the photos, the two
units which make up the Beam Break
Trigger are each housed in a small jiffy
box. The IR Source circuit is built on
a small PC board coded 13106092 (57
x 26mm), while the Detector parts are
installed on a larger PC board coded
13106091 (122 x 58mm).
Start the assembly by building the IR
Source board – see Fig.2. This should
take you just a few minutes since there
are only four components to install –
the three infrared LEDs and the 820Ω
current-limiting resistor.
Be sure to orientate the three IR
LEDs correctly as shown in Fig.2.
In addition, these three LEDs must
be fitted with their leads bent down
by 90°, so they face out of the end of
the box when the board is mounted
inside. In particular, note that the
centre LED (LED2) is fitted with its
body relatively low down near the
board, while the two outer LEDs are
fitted higher and with their leads bent
inwards towards LED2. This is done
so that they form a triangular group,
to provide a relatively compact beam
source (see photo).
Once these parts are in, install the
power cable by soldering its leads to
the +9V and 0V and pads. The cable
is then anchored using a small Nylon
cable tie which passes through the two
3mm holes on either side.
Having completed the board, it can
be mounted inside its UB5 jiffy box
on four 6mm long untapped spacers
and secured using four M3 x 12mm
countersunk head screws and nuts. As
shown in the photos, the IR LEDs face
outwards through a 10mm hole in one
end of the box, while the power cable
exits via a small notch filed in the top
at the opposite end. Fig.3 shows where
to drill the holes in both boxes.
Finally, complete the IR Source unit
by attaching the front panel label to
Table 1: Resistor Colour Codes
No.
1
1
2
2
1
2
1
1
Value
470kΩ
220kΩ
22kΩ
10kΩ
2.7kΩ
1kΩ
820Ω
100Ω
4-Band Code (1%)
yellow violet yellow brown
red red yellow brown
red red orange brown
brown black orange brown
red violet red brown
brown black red brown
grey red brown brown
brown black brown brown
5-Band Code (1%)
yellow violet black orange brown
red red black orange brown
red red black red brown
brown black black red brown
red violet black brown brown
brown black black brown brown
grey red black black brown
brown black black black brown
siliconchip.com.au
CABLE TO
CON1
(MATES
WITH
CON2
BELOW)
CABLE
TIE
29060131
9002 ©
+9V
V9+
820
V0V
0
INFRARED LEDS
K LED3
LED2 LED1A
An infrared transparent filter is fitted to the inside of the case at the
receiving (PD1) end of the UB3 box, while a 30mm x 12mm-diameter “lighthood” (eg, brass or plastic tubing) is attached to the outside of the case.
MOVING
OBJECT
PD1
ZD-1948
K A
220k
100nF
IC1
LM358
220 F
2.7k
22k
470k
10k
22k
1k
siliconchip.com.au
1k
LED4
470 F
A
There are more components on the
Detector board but its construction is
still straightforward – see Fig.2. Install
the resistors first, taking care to use the
correct value at each location. Table
1 shows the resistor colour codes but
it’s also a good idea to check each
one using a digital multimeter before
soldering it in place.
Follow these parts with the metallised polyester capacitors, then fit the
two electrolytic capacitors. The latter
are polarised, so be sure to orientate
them as shown. The two PC board
terminal pins used to make the battery
connections can then be fitted. Note
that both pins are fitted on the copper
side of the board, to make it easier to
10nF
K
Detector board assembly
solder the battery clip leads to them.
Switch S1 and connectors CON2 &
CON3 are next on the list, followed
by an 8-pin socket for IC1. Be sure to
orientate the socket with its notched
end towards the adjacent 100nF capacitor, to guide you when plugging
in IC1 itself later on.
Transistor Q1, photodetector PD1,
MOSFET Q2 and LED4 can now all go
in, again taking care to orientate them
correctly. Note that PD1 is mounted
vertically with its curved side facing
outwards and with the centre of its
body about 5mm above the PC board.
LED4 should also be mounted vertically, with the bottom of its body about
12mm above the board (this ensures
that it will protrude slightly from its
matching hole in the box lid after assembly).
The Detector board can now be
completed by plugging IC1 into its
9002 ©
19060131
the lid. A full-size artwork is shown
in Fig.3 and is also available for download from the SILICON CHIP website.
Q1
BC338
RE G GIRT KAER B MAE B
Follow this photo and the parts layout
diagram (Fig.2) at right to build the Detector PC board.
100
10k
Q2
2N7000
S1
9V
BATTERY
POWER
CON3
+
–
CON2
R
S
TRIGGER OUT
TO FLASH, ETC
T
(TO EMITTERS)
Fig.2: install the parts on the two PC
boards as shown on this layout diagram.
LED4 (green) on the Detector board is
mounted vertically but be sure to bend
the leads of IR LEDs1-3 through 90°
before installing them on the IR Source
board – see text & photo.
June 2009 65
LID OF UB3 BOX
IR SOURCE
POWER
TRIGGER
OUT
CL
POWER
RECEIVING END
OF UB3 BOX
BEAM
FOUND
TOWARDS
BEAM
B
BEAM BREAK FLASH
TRIGGER UNIT
SILICON
CHIP
10
47
IR BEAM
OUT
9V DC
INPUT
24.5
BEAM BREAK FLASH
TRIGGER UNIT
IR BEAM SOURCE
24.5
SILICON
CHIP
A
A
CL
4.75
B
ALL DIMENSIONS IN MILLIMETRES
HOLES A:
HOLES B:
HOLE C:
HOLES D:
HOLES E:
30.5
47
CL
C
8
A
E
3mm DIAMETER
5mm DIAMETER
6.5mm DIAMETER
10mm DIAMETER
3mm DIA, COUNTERSUNK
E
A
CL
IR LED END
OF UB5 BOX
14
50.5
CL
8
9.5
D
9.5
11.75
D
D
10
E
5
5
10
TRIGGER OUTPUT END OF UB3 BOX
E
BOTTOM OF UB5 BOX
POWER CABLE END OF UB5 BOX
Fig.3: these drilling diagrams for the UB3 & UB5 boxes can be either be copied and used directly as templates or you can
mark the holes out manually using the measurements indicated. Also shown are the two front-panel artworks. They can
either be copied and used direct or downloaded from the SILICON CHIP website and printed out.
66 Silicon Chip
siliconchip.com.au
socket (take care with the orientation).
The detector board is then ready to be
mounted behind the lid of the UB3
box.
The first step is to drill and ream
out the various holes in the base and
lid, as shown in Fig.3. That done, fit
the front panel label and cut out the
holes using a sharp hobby knife, then
secure the board to the lid using four
M3 x 15mm tapped spacers and eight
M3 x 6mm machine screws.
Note that you’ll need to remove the
upper nut from the ferrule of switch
S1 before doing this, so the ferrule can
pass up through its matching hole in
the lid. Once the board is in place,
the nut can be replaced and threaded
down against the top of the lid. The
lower nut and lockwasher can then be
threaded up against the underside of
the lid, using a small spanner.
The next step is to fit a small square
of red “IR transparent” film inside the
box behind the single 5mm hole at the
PD1 end. It can be held in place using
a couple of narrow strips of transparent
tape, one on either side.
A short “light hood” is now be attached to the photodetector (PD1) end
of the box. This must cover the 5mm
hole and be as close as possible to
concentric with it.
The hood itself can be fashioned
from a 30mm length of 12mm diameter
brass tubing (see photos) or from a
similar length of opaque (preferably
black) PVC conduit. Whichever you
use, it’s simply glued to the end of
the box using 5-minute epoxy cement.
Now for the final assembly. First,
connect the battery snap lead to the
terminal pins on the underside of the
board, then place the battery in the
bottom of the box and fasten it in place
using either a small aluminium “U”
bracket or a strip of gaffer tape. Finally,
lower the lid and PC board assembly
into the box before fitting the screws
to hold everything together.
Trying it out
No adjustments are required, so you
can try it out simply by plugging the
power cable from the IR Source into
CON2 on the Detector unit and turning
on power switch S1.
If the Detector’s light hood is now
aligned with the output from the IR
Source (or any other source of IR radiation), LED4 should immediately begin
glowing. If it does, block the end of the
hood with your thumb or a small piece
siliconchip.com.au
Above: the Detector
board is secured to
the lid of the UB3
case using four M3 x
15mm tapped spacers
and eight M3 x 6mm
machine screws
Left: a “light hood”
is fitted to the end of
the Detector unit to
prevent interference
from stray IR light
sources.
of opaque material and check that the
LED immediately switches off.
The same thing should happen if
you turn the IR Source away from the
Detector or if you simply block the
beam with your hand or some other
small opaque object. If this happens,
then your Beam Break Trigger Unit
is probably working correctly and is
ready for use.
If you’re going to be using it in
conjunction with the Time Delay Photoflash Trigger unit, all that remains is
to make up a suitable cable to connect
the two together. This simply involves
connecting the Detector’s trigger output to the “external trigger contacts”
input (CON4) of the delay unit.
By the way, the Beam Break Trigger
Unit should give reliable triggering
with the IR Source unit placed up to
a metre or so from the Detector box
in normal room lighting. This “beam
length” range can be extended considerably in dark (eg, night-time) conditions but in bright sunlight it will be
shortened due to the relatively high
SC
level of IR in the ambient light.
Direct Flash Triggering: Making The Cable
A
s mentioned in the article, the Beam Break Trigger can also be used to trigger an
electronic flash directly, rather than via the Time Delay Photoflash Trigger. To do
this, trigger output CON3 is simply connected to the photoflash via a suitable cable.
However, when you’re making up this cable, make sure that the positive side lead
from the flash input is connected to the centre contact of the plug that goes to CON3. If
the polarity is reversed, MOSFET Q3 in the Beam Break Trigger Unit could be damaged.
The procedure is to first use your DMM to check the polarity of the voltage at the
end of the cable that’s plugged into the flash unit (ie, with the flash unit powered up
and ready for triggering). Once that’s done, you’ll then know which way around to
connect the cable to the plug that goes to CON3 on the Detector unit.
While you’re checking the polarity of the cable leads, make a note of the actual
voltage itself. If it is below 60V, that won’t be a problem. Conversely, if it’s higher
than 60V, you’ll need to replace the 2N7000 MOSFET with one having a higher
voltage rating – such as a IRF540N.
June 2009 67
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