These instructions may be used
in combination with the instructions for building a delay
unit in order to provide a photogate with a selectable delay.
For instructions on building the delay unit, go
The following parts are included
with the SPG kits. (If you purchased the SPG in combination
with a delay unit, then a single set of wires was provided
with the combination.)
Infrared phototransistor (PT)
Infrared emitter (LED)
with SPG2 kit
with either SPG1 or SPG2
555 timer IC
400-V SCR (EC103D)
1 470-Ω resistor (yellow-violet-brown)
1 10-kΩ resistor (brown-black-orange)
10-kΩ potentiometer (white knob)
0.01-µF capacitor (103)
Wires 3-ft of 2-conductor cable
3-ft of 3-conductor cable
9-V battery cable*
fresh 9-V battery is required but not included with
the kit. You'll also need a wire cutting and stripping
tool such as the one shown to the right. If you
do any soldering, you'll need a soldering iron, solder,
and a heat sink.
Click for larger view
Click on the thumbnails
in order to view full-size images of the breadboard with
the components that have been added in each step.
Using the Breadboard
click to view
The breadboard offers
an easy way to build electrical circuits without soldering.
The 2"x3" breadboard provided with your
kit contains an array of holes where wires and components
are to be inserted. The holes in the center portion
of the breadboard are identifiable by row (vertical
in the photos) and column (horizontal). There
are two sets of 30 rows numbered by 5's, and each
set of rows has 5 columns labeled a-e and f-j. The
5 holes on each row are electrically connected to
each other (but not across the center channel), so
any components inserted into the same row would be
connected just as if they had been soldered.
However, the components can be removed and replaced
with other components at any time, without the hassle
of unsoldering and resoldering parts.
On either side of the breadboard
are two columns marked by blue and red lines. The
25 holes in each column are electrically connected,
but the columns aren't electrically connected to each
other. The outermost column marked with the
red line at the top will be used for all +9 V connections,
while the outermost column marked with the blue line
at the bottom will used for all ground (negative)
Assembling the Photogate
that the photographs show a delay unit already built
on the right side of the board. The photogate
may be used with or without the delay unit. However,
the 9-V battery cable is required for the operation
of either kit. This is the cable coming in from
the left with the red and black leads above and below
the 555 timer IC. The column of 25 holes to which
the red wire is connected will be termed the positive
column, while the column to which the black wire is
connected will be termed the negative column.
While wiring the circuit, be sure to have the battery
disconnected from the battery cable.
click to view
1: Adding the 555 Timer
The 555 timer is an 8-pin
IC that also has a notch and circle identifying Pin
1. Orient the IC so that the notch faces the
left side of the breadboard. Now find Row 4 and look
across to where it meets Column e. Place Pin 1 there.
Pin 8 should easily fit in Row 4, Column f. Press
the IC firmly down in place; again, it should be seated
across the center division of the breadboard.
click to view
2: Adding the Potentiometer
The potentiometer allows
you to adjust the sensitivity of your photogate. It
has three legs, two in the front and one in the rear.
Place the two front legs over Rows 8 and 10 on Column
a, and the rear leg over the nearest hole on the nearby
negative column. The front legs should be facing the
center of the breadboard, while the rear leg faces
the outside of the breadboard. Press the legs in firmly
as far as they will go, but avoid bending them.
click to view
= anode (+)
G = gate
C = cathode (-)
Step 3: Adding the
rectifier is the output of the photogate circuit.
Putting in this SCR is easy since all three leads
go in consecutive rows along Column e. Hold the SCR
as in the diagram to the right in order to identify
the leads. Put the cathode into Row 11 on Column e.
The gate will then go into Row 12, and the anode into
Row 13 of that column.
click to view
4: Adding the Capacitor
Locate the capacitor
labeled 103. This has a value of 0.01 µF. Insert one
lead of this capacitor into Row 7, Column i, and the
other lead into Row 9 of the same column.
You may wish to trim
the leads of the capacitor so that it sits closer
to the breadboard. This will reduce the chance
that the leads of two components accidentally touch
each other and create a short. When you add
the resistors in the next step, you may wish to trim
their leads also.
click to view
5: Adding the Resistors
Locate the brown-black-orange
resistor (10 kΩ). Insert one end into Row
6, Column c, and the other end into Row 12 of the
same column. Next, find the yellow-purple-brown
(470 Ω) resistor. Insert one end into Row
2, Column g. The other end should reach over to the
nearest hole in the negative column.
click to view
6: Adding the Wires
Now you'll connect all
your electronic components together. Each wire only
needs to be 2 inches in length or less. You can estimate
how much you'll need to bridge across two holes before
cutting, although it's always better to have longer
wires than ones that are too short.
Strip about 1/4" of insulation
off each end. The list below will tell you which rows
and columns your wire ends should fit into. The longest
wires are listed first so if you happen to cut a piece
that's too short, you'll be able to use it later.
wires (~1.5 inches)
9, Column f
5, Column d
6, Column g
11, Column c
7, Column d
Row 4, Column g
Row 4, Column c
8, Column d
6, Column i
4, Column h
*Row 13, Column c
Row 18, Column h
*This wire is only
needed if you're connecting the photogate to a
Photogate connections shown with output cable;
click to view
Note that for some versions of this kit, the colors
of the wires in the 3-conductor cable are red,
black, and green. In that case, simply replace
the word white with green in the
7a: Connecting the LED and phototransistor (or interrupter)
The photogate has a light-emitting
and a light-sensing component. The former is
a light-emitting diode (LED), which emits an infrared
beam. The sensing component is an infrared phototransistor
(PT). When the beam is broken by an object, the blockage
causes the voltage to rise across the PT, which gates
the SCR at the output of the circuit.
For the SPG1 kit,
the PT and the LED are the individual components
shown to the right. The LED is the component
with a blue case, and the PT has a clear case.
(In an earlier version of this kit, the PT and
LED both had clear cases. If you have this version,
note that the LED has longer legs than the PT.)
For both components, one leg is shorter than the
other. The shorter leg is positive on the PT,
while on the LED, the longer leg is positive.
The wiring instructions given later in this section
will ensure that the correct polarity is maintained.
For the SPG2 kit,
the PT and LED are housed in the two posts of
the single component, termed an interrupter, shown
to the right.
To begin wiring, use
the gray 3-conductor cable. The 4 legs of the PT and
LED (or interrupter) will be soldered to these three
Strip 1" of insulation
from each of the conductors on one end of the cable.
The PT and LED will be attached to this 1" end.
Now strip ½" of insulation from each conductor
on the other end of the cable. This ½" end will
connect with the breadboard. Strip an additional 1"
of the gray outer shielding from the ½" end so
the individual conductors can reach to their destinations.
If using individual
PT and LED components, make the following connections:
Wrap the black wire
around the shorter leg of the LED.
Wrap the white (or green) wire around the longer
leg of the PT.
Next prepare a jumper
wire that will go from the longer leg of the LED to
the shorter leg of the PT. The length of this
jumper will depend on how far apart you want to separate
the PT and LED for your photography. Strip the
wire back about an inch on each end wrap it onto the
legs of the components.
Now wrap the red wire
of the 3-conductor cable to either one of the legs
onto which you wrapped the jumper wire.
For best connections,
solder the red, black, and white (or green) wires
to the legs onto which they are wrapped. Before soldering
each leg, clip a heat sink (a metallic alligator clip
will work for this) to the leg just below the plastic
case. This will prevent the component from heating
excessively during soldering. Before starting to solder,
make sure you're working in a well-ventilated area
in order to avoid inhaling the solder fumes. A fan
to blow the fumes away from you will help. Prepare
the tip of the soldering iron by holding the solder
to it so that solder can melt and flow over the tip.
This will improve heat conductivity. Touch the
solder on the leg to which you're soldering the wire.
Hold the flat of the soldering iron tip on the leg
but not directly on the solder. As soon as the leg
is hot enough, the solder will flow. Move the solder
around so as to melt solder into the wire and onto
the leg along the length of the leg.
using an interrupter, make the following connections:
on the top view of the interrupter to the right
refer to the LED and phototransistor (PT) respectively.
The numbers refer to the legs on the underside (not
Solder the black wire
to leg 2.
Solder the white (or green) wire to leg 3.
Solder a short jumper wire between legs 1 and 4.
Solder the red wire to the jumper between legs 1and
If you have an interrupter
with short legs, you won't be able to wrap wires around
the legs. Just hold the wire to the leg and tack solder
the two parts together. Since you won't be able to
use a heat sink, minimize the amount of time that
the soldering iron is in contact with the leg.
Whether using individual
PT/LED or an interrupter, do the following:
Connect the free ends
of the 3-conductor cable to these holes:
Black to Row 2, Column
white (or green) to Row 6, Column j
Red to the positive column
Step 7b. Connecting
the flash unit
You may already have
an output cable for your flash if you prepared one
for the delay circuit. If not, use the following instructions.
The 3 feet of 2-conductor
is used to connect the output of the photogate trigger
to the PC cord of a flash unit. From one end of the
2-conductor cable, strip 1" of the gray insulation,
being careful not to cut the insulation on the red
and black wires. Then strip 1/2" of insulation
from each of the red and black wires. These
will connect to the breadboard. Next, strip
2" of the gray insulation from the other end
of the cable. Strip each of the individual wires
back 1". These will connect to the PC cord.
One way to make this connection is to cut the socket
off the end of the PC cable, strip the insulation
on the individual PC wires back by 1", splice
the red wire of the gray cable to the positive wire
of the PC cable, and splice the black wire of the
gray cable to the negative wire of the PC cord.
(The positive wire of the PC cord is usually the wire
that goes to the center pin of the PC socket.
For more information on connecting to a PC cord, see
this page: http://hiviz.com/tools/triggers/makeown.htm#connect.)
In order to connect
the flash unit to the output of the photogate, connect
the red wire to Row 13, Column b, and the black wire
to the negative column.
If you're connecting
the flash to a delay unit,
For the delayed output
(Output 2) of the delay circuit, connect the red wire
to Row 18, Column b and the black wire to the negative
column. (This is the connection shown in the photograph.)
For the undelayed output
(Output 1) of the delay circuit, connect the red wire
to Row 16, Column b and the black wire to the negative
8: Operating the Trigger
If you're using the individual
PT and LED components, lay them down on a table a
few inches apart pointing at each other. You
may want to tape down the cables so that the components
can't shift positions. If you're using an interrupter,
the components are already fixed in position.
With a 9-V battery connected
to the battery clip and your flash unit connected
to one of the outputs as described in step 7b, you
can now test your circuit. Run your finger between
the PT and LED in order to break the photogate beam.
If your flash cable is connected directly to the photogate
or to Output 1 of the delay circuit, you should notice
an immediate discharge of your flash unit. If
your flash cable is connected to Output 2 of the delay
unit, you may notice a short delay before discharge,
depending on the setting of your delay circuit.
If your flash unit doesn't discharge, you may need
to adjust the sensitivity of the photogate.
the sensitivity: Turn the 10-kΩ
potentiometer in one direction or the other until
the flash discharges spontaneously. Then back up the
dial just before the point of spontaneous discharge.
If you change the distance
between the PT and LED (if using the individual components)
or if the orientation of either component changes
slightly, you may need to readjust the sensitivity.
The maximum separation is about 6 inches. The larger
the separation, the more care you need to take in
aligning the components.