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 here.
following parts are included with the TPG kit. (If you purchased
the TPG in combination with a delay unit, then a single
set of wires was provided with the combination.)
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 below 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. 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 Potentiometer
potentiometer (gray knob) 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 7 and 9 on Column j, and the
rear leg over the positive 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.
2: Adding the SCR and Transistor
rectifier (labeled EC-103D) is the output
of the photogate circuit. Hold the SCR
as in the diagram to the right in order to
identify the leads. Putting in this
SCR is easy since all three leads go in consecutive
rows along Column c. Put the cathode into
Row 8 on Column c. The gate will then go into
Row 9, and the anode into Row 10 of that column.
SCR pin diagram
= anode (+) G = gate C = cathode (-)
transistor looks identical to the SCR but is
labeled PN2222A (or 2N2222A). Its three leads
go in consecutive rows along Column g.
To identify the leads of the transistor, hold
it as in the diagram to the right. Put
the emitter into Row 8 on Column g. The
gate will then go into Row 9, and the collector
into Row 10 of that column.
Transistor pin diagram
= emitter (-) B = base C = collector (+)
click to view
3: Adding the Resistors
Locate the brown-black-orange
resistor (10 kΩ). Insert one end into Row
9, Column d, and the other end into Row 8, Column
f. Next, find the yellow-violet-brown
(470 Ω) resistor. Insert one end into Row
3, Column g. The other end should reach over to the
nearest hole in the positive column.
click to view
4: Adding the Wires
Two short wires are needed
for this step. Strip about 1/4" of insulation
off each end. One wire should join Row 8, Column b
to the nearby negative column. The other should
join Row 10, Column h to the nearby positive column.
shown with 3-conductor
cable; click to view
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
5: Connecting the LED and phototransistor
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 TPG 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.
To begin wiring, use
the gray 3-conductor cable. The 4 legs of the PT and
LED will be soldered to these three conductors.
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.
Wrap the red wire around
the longer leg of the LED, and the white (or green)
wire around the shorter leg of the PT. Next
prepare a jumper wire that will go from the shorter
leg of the LED to the longer 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 black wire of the 3-conductor cable to either
one of the legs onto which you wrapped the jumper
Connect the free ends
of the 3-conductor cable to these holes on your breadboard:
Black to the negative
White (or green) to Row 9, Column h
Red to Row 3, Column f
You may want to wait
to solder the leads until you've tested the circuit
in step 8. When you do get around to soldering,
here are some tips.
Note about soldering:
When you connect a wire to a leg of the PT or LED,
first wrap the wire tightly around the leg several
times. Then 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.
click to view
6: Connecting the Photogate Trigger to a Delay Unit
If you're using a delay
unit, add a wire from Row 10, Column a, the output
of the photogate trigger, to Row 18, Column h, the
input of the delay unit. This will allow the photogate
trigger to trip the delay unit.
click to view
7a: Connecting the Photogate Trigger to a Flash Unit
If you're using a delay unit with
the photogate, skip to step 7b. If you're connecting
the output of the photogate directly to a flash unit,
use the instructions below.
Prepare an output cable to your flash
unit as follows.
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.)
Once the cable is ready, insert the
red wire of the cable into Row 10, Column a and the
black wire to the negative column.
click to view
Step 7b: Connecting the Delay Unit to a Flash Unit
If you've constructed
a delay unit, then you've already prepared an output
cable for your flash. The photograph to the
left shows the output cable connected to Output 2
of the delay unit.
8: Operating the Circuit
Connect a 9-V battery
to the battery clip. Lay the PT and LED down
on a table a few inches apart, pointing at each other.
You may wish to tape them in place so the components
can't shift positions. Run your finger
between the PT and LED quickly in order to break the
photogate beam. (If you pass your finger
through too slowly, the photogate may not respond.)
If your flash cable is
connected directly to the photogate or to Output 1
of the delay unit, 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
you don't get a discharge, one possible reason is
that the sensitivity isn't adjusted correctly.
Adjusting the sensitivity:
Turn the 100-kΩ potentiometer in one direction
or the other until the flash discharges spontaneously.
Then back up the dial just before the point of spontaneous
If you change the distance
between the PT and LED 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.