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Assembly and Operating Instructions for Kits


Instructions for Preparing the SPG2 Photogate Cable


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These instructions are written for the SPG2-BB and SPG2-DU-BB kits. For these kits, the infrared emitter and detector are housed in the two posts of the interrupter, shown below.



3-conductor cable, 3’ length

1" piece of hookup wire (Cut this from the hookup wire supplied with your kit.)

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





One end of the photogate cable will be stripped and its individual conductors connected to the appropriate legs of the interrupter. The other end of the photogate cable will be stripped differently, for easier connection to the breadboard.


Preparing the photogate cable


click for larger view  

At one end of the gray 3-conductor cable, strip the outer casing back by 2" (5 cm). Then strip the individual conductors back ¼" (¾ cm). This will yield short lengths of wire that will be easy to tack solder to the short interrupter legs. See the photo to the left.


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. (no image for this)


Strip all the insulation from the 1" piece of hookup wire so that you have a bare wire.



Soldering the connections

  click for large view

The symbols and on the top view of the interrupter below refer to the LED and phototransistor (PT) respectively. The numbers refer to the legs on the underside (not shown).


The interrupter has short legs, so you won't be able to wrap the photogate cable wires around the legs. Instead, you will need to hold the wire to the leg and tack solder the two parts together. A clamp to hold the interrupter may be useful for maintaining alignment while you are soldering. Since you won't be able to use a heat sink, minimize the amount of time that the soldering iron is in contact with the leg. We recommend melting some solder on the tip of the iron first. Then, while holding a wire to a leg, touch the liquid solder to the pair and let just enough solder flow to make a bond. Avoid touching the body of the interrupter to keep from melting the plastic. See the series of photos to the right.


After soldering each connection, remove the soldering iron and hold the wire to the leg for a few seconds to allow the solder to solidify.


When ready to solder, make the following connections.

  • Solder the black wire to leg 2.
  • Solder the green wire to leg 3.
  • Solder the 1" piece of hookup wire between legs 1 and 4.
  • Loop the red wire anywhere along the length of the 1" hookup wire and solder them together.

Check to make sure that you don't have any solder bridging between legs. If you do, you'll need to melt the solder to clear the bridge. A completed soldering job is to lower 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


If you're concerned about the wires breaking off of the interrupter legs from use, try covering the base of the interrupter with hot glue.


All there is left to do is to make the following connections between the breadboard side of your photogate cable and your completed photogate trigger. Then you're done!


Green wire to 4J

Black wire to 1J
Red wire to + column


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