instructions are written for the SPG1, SPG1-BB, SPG1-DU, SPGI-DU-BB, MT, and MT-BB
kits. All of these kits use a separate infrared LED emitter
and infrared phototransistor (PT) to provide variable separation between the LED and PT. The LED is
the component with a blue case, and the PT has
a clear case, as shown at left. 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. The LED is
the component with a blue case, and the PT has
a clear case, as shown to the left. The figure to the right, courtesy of a helpful DIYer, provides a visual display of the connections that you'll be making to the infrared LED and phototransistor.
Here are the parts you'll need:
3-conductor cable, 3-ft length
Yellow hookup wire, 1.5-ft length
Infrared phototransistor (clear case)
Infrared LED (blue case)
3/32" heat shrink tubing (HST), 4" length
For tools, you'll need the following:
15-30 W soldering iron, solder, wet sponge,
heat sink (a metal
clip will do)
Lighter or matches to shrink HST
Stripping the wires
Photo 1. At one end of the 3-conductor cable, strip the outer casing back
by 9 inches. This will reveal the three inner
conductors, colored red, black, and green. Strip each
of these conductors back by ¾ inches. This
will expose conductors that will be wrapped around
the appropriate component legs later.
Photo 2. At the other
end of the cable, strip the outer casing back by 2". Then strip the individual conductors back
¼". This short length will make these
multiple-stranded conductors easier to push into the
breadboard later. Photo 2 shows both ends of the cable stripped.
Photo 3. Strip both ends of the 18" yellow hookup wire back by 3/4".
Photo 4. Twist the red wire and one end of the yellow wire
together tightly, as shown in Photo 4.
Fitting the heat shrink
tubing and making connections
Photo 5. Cut the heat shrink tubing (HST) into four 1"
pieces. Place one piece over each wire (black, green,
jumper, red+yellow) as shown in Photo 5, and slide
it back onto the wire. Be on the lookout for pieces
falling off if wires are held upside down.
Photo 6. Make connections by wrapping the
wires around the legs of the PT and LED. When wrapping,
try to get at least two complete turns; more are better.
Before twisting any wires together, make sure the
HST for that wire is still present and hasn't fallen
Make the following connections by twisting the wires
around the component legs.
Twist the green wire tightly around the longer
leg of the PT (clear case) and the black wire of
the 3-conductor cable around the shorter leg of
the LED (blue case). (See Photo 6.)
Twist the combined red+yellow wire around the
longer leg of the LED and the other end of the yellow
wire around the shorter leg of the PT.
When done, your connections should look like those in Photo 7.
Soldering the connections
Photo 8.Trim any stray wire strands on the connections so
the heat shrink tubing will slip over them.
It's a good idea to place a metal clip to serve as
a heat sink between the head of the PT or LED and
the leg where you will be soldering. (See Photo 8.) This will help avoid damage from overheating.
If you don't use a heat sink, complete the soldering
quickly to minimize heat buildup.
Photo 9. The completed soldering job is shown in Photo 9.
Adding the heat-shrink tubing
Photo 10. After soldering, slide the heat shrink tubing over
each of the solder joints so that the legs of each
component are insulated from each other. (See Photo 10.) Keep the pieces about 1/8” away
from the component heads to protect them from overheating
when the tubing is heated.
Photo 11. Using a lighter or a match, move the flame smoothly
back and forth along the entire length of the tubing,
with the tip of the flame just beneath it. (See Photo 11.) If you hold the flame too long in
one spot or too closely to the tubing, you will notice
smoke. If this happens, lower your flame and continue
moving it back and forth.
The tubing will visibly shrink and will be acceptably
tight-fitting after only 10-15 seconds of heating.
connections the photogate
This completes the construction of the cable. You can test it with your breadboard circuit as follows. Make the following
connections between the breadboard side of your photogate
cable and your completed trigger circuit.
Green wire to 4J
Black wire to 1J
Red wire to + column
Connect a 9-V battery to your circuit and turn the
sensitivity knob (white pot) to about the middle position. (Don't expect to see the IR LED light up, because infrared light isn't visible to the human eye. If you want to check whether the LED is actually emitting light, you can photograph it in a dark room with an exposure of a few seconds at about f/5.6, ISO 400. A glow will be recorded on the image if the LED is lit.)
Next align the PT and IR LED on the photogate cable
so that the round ends are facing each other a few inches apart as shown to the left. When they're aligned, the red LED of
the photogate circuit should light. When you break
the photogate beam, the red LED should go off. If
you see this, your connections are good, disconnect
the battery and return to the circuit assembly instructions.
If the test doesn’t work, try realigning the PT and
IR LED, and ensure that all wires are wrapped tight around
the component legs. Recheck previous steps if necessary
to ensure the right wires are connected to the component