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Assembly and Operating Instructions for Kits


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Instructions for Preparing the TPG Photogate Cable


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These instructions are written for the TPG-BB and TPG-DU-BB kits. These kits use 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 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.

3-conductor cable, 3-ft length
Yellow jumper wire, 1-ft length

Infrared phototransistor (clear case)
Infrared LED (blue case)

Heat shrink tubing (HST), 4" length

Tools and supplies:
Wire stripper
For soldering: 15-30 W soldering iron, solder, wet sponge, heat sink (a metal clip will do)

Lighter or matches to shrink HST


One end of the photogate cable will be stripped, and its individual conductors connected to the appropriate legs of the infrared PT, LED, and jumper wire. The other end of the photogate cable will be stripped differently, for easier connection to the breadboard. The individual wires will be wrapped around the legs of the components, and the connections tested with your photogate trigger to ensure proper connections before soldering. The solder joints will then be covered with pieces of heat shrink tubing for electrical insulation.


The figure below, courtesy of a helpful DIYer, provides a visual display of the connections that you'll be making to the infrared LED and phototransistor. Click on the image for a larger view.

Preparing the photogate cable


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At one end of the cable, strip the outer casing back by 6 inches (15 cm). This will reveal the three inner conductors, colored red, black, and green. Strip each of these conductors back by ¾ inches (2 cm). This will expose free wire that will be wrapped around the appropriate component legs later. At the other end of the cable, strip the outer casing back by 2" (5 cm). Then strip the individual conductors back ¼" (¾ cm). This short length will make these multiple-stranded conductors easier to push into the breadboard. See the thumbnail to far left showing both ends of the cable after stripping. Click on the thumbnail for a larger image.


Also strip both ends of the jumper wire back by ¾ inches. See the thumbnail to the near left.


Twist the black wire and one end of the jumper wire together tightly, as shown on the right.


Fitting the heat shrink tubing and making connections


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Cut the heat shrink tubing (HST) into four 1" pieces. Place one piece over each wire (black, green, yellow, black+yellow) as shown to the left, and slide it back onto the wire. Be on the lookout for pieces falling off if wires are held upside down.


Now it's time to 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 shorter leg of the PT (clear case) and the red wire of the 3-conductor cable around the longer leg of the LED (blue case). (see upper right photo)
  • Twist the combined black+yellow wire around the shorter leg of the LED and the other end of the yellow wire around the longer leg of the PT.

When done, your connections should look like those to the right.


Testing connections before soldering


Next, you'll test your photogate connections using the trigger it will be used with. Make the following connections between the breadboard side of your photogate cable and your completed photogate trigger.

  • Green wire to 4F
  • Red wire to 2F
  • Black wire to 2D

Also connect your flash unit to the output of the photogate if you haven't done so already. The red conductor of the output cable is connected to A4 and the black conductor to the ground (-) column.


Connect a 9-V battery to your circuit and turn the sensitivity knob (brown pot) to about the middle position. Next align the PT andLED on the photogate cable so that the round ends are facing each other about an inch apart.

If the test doesn’t work, try realigning the PT and 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.


Soldering the connections


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Now that all your connections are good, it’s time to solder. If you're new to soldering, see the tips below.


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 the photo to the left.) 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 is shown to the right.


    Soldering Tips

    • Solder in a well-lit, well-ventilated, open area. Avoid contact with all metal surfaces on the iron.

    • Keep the tip of the soldering iron clean by wiping it against a wet sponge or towel before and after each use. A clean tip should look shiny and silvery; any yellow or black material on the tip will get into the solder and may weaken your solder joint.

    • Once the tip of your soldering iron is clean, touch a bit of solder to the tip just before use. This is called tinning, and helps the solder run more evenly.

    • Heat the connection to be soldered by holding the soldering iron to it, until solder applied at the junction between the two melts and flows freely. This ensures the connection and the solder are both hot enough to yield a good solder joint. This should take no more than 10-15 seconds. After the connection is heated, try to get solder along the entire length of the connection by briskly moving the solder and iron along.

    • Avoid touching only the solder to the connection, and then the soldering iron to the solder to melt it onto the connection. The connection will be cooler than the melted solder and won’t form a good solder joint.

    • Let new solder joints cool for several seconds before examining them. There should be solder all the way around the connection, forming a rigid joint. When done, unplug your soldering iron and let it cool.

Finishing up


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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 to the upper left.) 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 to lower left.) 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.


That’s it! Now you can align the PT and LED as shown to the right and reconnect to your photogate circuit.




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