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Assembly and Operating Instructions for Kits


Instructions for Preparing the SPG1 (variable-width) Photogate Cable


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About the variable-width cable


These 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.IR LED/PT connections 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:

  • Wire stripper
  • 15-30 W soldering iron, solder, wet sponge, heat sink (a metal clip will do)
  • Lighter or matches to shrink HST

stripping the 3-conductor cable

stripping the yellow wire

twisting the red and yellow wires

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.

adding the heat-shrink tubing

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 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 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 legs

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.


slipping on the HST

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.

Testing 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 legs.

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