information and inspiration
for students, teachers and hobbyists
About Tools Products Activities Galleries Projects Links Contact Facebook LinkedIn  flickr


Assembly and Operating Instructions for Kits


Assembly Instructions for the Variable-width Photogate Cable Kit


Assembly instructions for other kits


This cable is used with the Multi-Trigger breadboard.


Variable-width photogate cable (for Multi-Trigger Breadboard)

About the variable-width cable


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.


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)
  • 7-in length of 3/32-in heat shrink tubing (HST)
  • 2-in lengths each of red, green, and black hook up wire

The tools that you'll need include a wire stripper, a 15-30 W soldering iron, solder, and a lighter or matches. Electrical tape will also be helpful.

stripping the 3-conductor cable

Photo 1

stripping the yellow wire

Photo 2

twisting the red and yellow wires

Photo 3

Stripping the wires


At one end of the 3-conductor cable, strip the outer, gray 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 inch. This will expose the conductors that will be wrapped around the appropriate component legs later.

At the other end of the cable, strip the outer casing back by 2 inches. Then strip each of the red, black, and green conductors back by 3/4 inch. This will expose the conductors that will connect to the breadboard.

See Photo 1 showing the cable after stripping.




Strip both ends of the yellow hook up 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.

adding the heat-shrink tubing

Photo 4

twisting the black and green wires onto the components

Photo 5

twisting the remaining wires onto the components

Photo 6

Fitting the heat shrink tubing and making connections


Cut the heat shrink tubing (HST) into seven 1" pieces. Set 3 of the pieces aside for later use. Place one piece over each wire (black, green, jumper, red+yellow) as shown in Photo 4, and slide it back onto the wire.








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 off.


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. See Photo 6.

soldering the legs

Photo 7

completed soldering

Photo 8

Soldering the connections


Trim any stray wire strands on the connections.

It's a good idea to place a metal clip (a paper clip will work) 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.


slipping on the HST

Photo 9

shrinking the HST

Photo 10

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.

Photo 11

Taping the wires


In order to manage the wires better, the two groups of wires leading to emitter and detector may be taped together with electrical tape or other means.

Photo 12

Photo 13

Photo 14

Connections for the breadboard


Now we move to the opposite end of the cable to add extensions that will make connections to the breadboard easy. You previously stripped the gray insulation back by 2 inches and the individual conductors by 3/4 inch. Now strip 2-in sections each of the red, green and black hook up wire to a distance of 3/4 inch. Wrap the stripped ends of the photogate cable conductors around these 2-in extensions. See Photo 12.




Solder the connections as shown in Photo 13.




Slip 1-in sections of heat-shrink tubing over the bare connections and heat to shrink. Then strip the ends back by 1/4 inch. See Photo 14.

Variable-width photogate cable

Photo 15

The completed cable


The completed cable is shown to the left. For instructions on how to connect it to the breadboard, see the instructions for your breadboard kit. The Multi-Trigger (v11) instructions are here.

Completed SPG1 cable Photo 16

Mounting the cable


Photo 16 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.




About Tools Products Activities Galleries Projects Links Contact Facebook LinkedIn flickr
copyright © 1995-2020