The variable-width cable uses a separate infrared LED emitter
and infrared phototransistor (PT). The LED is
the component with a blue case, and the PT has
a clear case, as shown to the 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 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
3.5mm stereo plug
Stripping the wires
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. See Photo 1 showing
the cable after stripping.
Strip both ends of the yellow hookup wire back by 3/4
inches. See Photo 2.
Twist the red wire and one end of the yellow wire
together tightly, as shown in Photo 3.
Fitting the heat shrink
tubing and making connections
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 4, and slide
it back onto the wire. Be on the lookout for pieces
falling off if wires are held upside down.
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 5.)
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 6.
Soldering the connections
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 7.) This will help avoid damage from overheating.
If you don't use a heat sink, complete the soldering
quickly to minimize heat buildup.
The completed soldering job is shown in Photo 8.
Adding the heat-shrink tubing
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 9.) Keep the pieces about 1/8” away
from the component heads to protect them from overheating
when the tubing is heated.
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 10.) 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.
Preparing to connect the 3.5mm plug
The photogate cable will connect to the project box enclosure with a 3.5mm stereo male connector. One is shown in Photo 11. The process of adding a connector to the cut end of the 3-conductor cable is described next.
Remove the jacket of the connector and push it onto the cut end of the cable as shown in Photo 12.
Strip the gray insulation back about 1/4" and the individual wires about 1/8" as shown in Photo 13.
Connections to the plug
The terminals of the connector are numbered in Photo 14. The black wire will connect to 1, the green wire to 2, and the red wire to 3. If you have trouble getting all the strands through a hole, you can clip off the strands that won't fit.
Soldering the connections
Here are some important things to keep in mind about soldering. First, don't crimp the tabs of the shaft around the cable before soldering. If you do, the heat of the soldering can melt the insulation and create a short. Secondly, tin the tip of the soldering iron with solder to improve conductivity. Then hold the tip on the metal near the wire to be soldered. Touch the wire to the metal, not to the tip of the iron. If you don't get the metal as hot as the melted solder, then the solder will bead up rather than flowing, and your connection may not actually conduct.
Photos 15 and 16 show two views of the completed soldering job. Snip any stray, unsoldered wires and make sure that the three conductors do not touch each other at any point.
Completing the cable
Crimp the tabs around the cable as shown in Photo 17.
Screw the jacket on to complete the connector as shown in Photo 18.
The completed cable is shown in Photo 19.
Mounting the cable
Photo 20 shows one method of mounting the emitter and detector. Three sections of 1/2" PVC pipe and two PVC elbows are used. (These parts aren't provided.) A 13/64" hole is drilled through each of the two upright sections of PVC. Then a 1/4" hole is drilled through just the outer part of each section. This allows the emitter and detector each to be slipped through the corresponding hole on the outside without passing all the way through the pipe. Hot glue is used to hold the emitter and detector firmly in the mount. The wires are taped to the horizontal section of PVC. One could also drill a hole through the center of the horizontal section for a mounting bolt for the assembly.