The cable that you prepare from this kit can be used
with the SPG2 photogate circuit.
For soldering: 15-30 W soldering
iron, solder, wet sponge, heat sink (a
metal clip will do)
The SPG2 Cable
About the SPG2 cable
For this cable, the IR emitter and detector are housed in the U-shaped piece shown in Photo 1. In addition to this piece, you'll need the following parts:
3-ft length of 3-conductor cable
the PC board shown in Photo 2
a 2-inch piece of red hookup wire
3.5mm stereo plug
Stripping the wires
At one end of the 3-conductor cable, strip the
outer casing back by 2". Then strip the
individual conductors back 1/8". See Photo 3.
Also strip the ends of the 2-inch piece of red hookup wire back by 1/8" (not shown).
Placement of the interrupter on the PC board
Photo 4 shows how the interrupter will be placed on the PC board. Photo 5 is an overhead view with the numbers 1-4 overlaid on the photo. The numbers correspond to those on the graphic of an interrupter to the right. The symbols and refer to the LED and phototransistor (PT) respectively.
Insert the legs of the interrupter into the board in exactly the placement shown in Photo 5. This is important to insure that the connections that you make to the board go to the correct legs. Photo 6 shows the placement of the legs of the interrupter from below.
Soldering the interrupter
to the PC board
In preparation for soldering, temporarily tape the interrupter on the board to hold it in place. Turn the board over and solder the legs to the copper contacts. Since you won't be able to use a heat sink on the short legs of the interrupter, solder quickly to avoid overheating the interrupter. Or you can solder one leg and then wait a while to solder the next in order to give the part time to cool. When you solder, fill the hole with solder and make sure the solder flows onto the copper to bond with it. Be careful not to create a solder bridge with a neighboring copper contact. If you do create a bridge, run the tip of the soldering iron between the contacts as needed to remove the bridge or use a desoldering tool. The completed soldering job is shown in Photo 8.
Adding the red wire
Place the red wire on the board as shown in Photo 9 and solder to the underside.
Photo 10 shows the completed solder joints.
Connecting the 3-conductor cable
Solder the three wires of the 3-conductor cable to the PC board in the locations shown in Photo 11.
The completed solder job is shown in Photo 12.
Preparing to connect the 3.5mm plug
If you've already prepared the SPG1 cable, the instructions that follow for adding the 3.5mm plug are the same as for that cable. They are repeated here for completeness.
The photogate cable will connect to the project box enclosure with a 3.5mm stereo male connector. One is shown in Photo 13. 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 14.
Strip the gray insulation back about 1/4" and the individual wires about 1/8" as shown in Photo 15.
Connections to the plug
The terminals of the connector are numbered in Photo 16. 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 17 and 18 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 19.
Screw the jacket on to complete the connector as shown in Photo 20.
The completed cable is shown in Photo 21.
Mounting the interrupter
The PC board provides a convenient way to securely mount the interrupter assembly. The 4 holes on the corners of the board can be used for this purpose. Photo 22 shows just one way to do this. (The parts for this method of mounting are not provided in the MT2 kit.) The PC board is mounted to the outside of a small plastic project box using 4 3/4-inch standoffs and 4-40 bolts. A hole drilled in the lid and the side of the box provides a pass through for the cable. The cable is tied in an overhand knot inside the box to provide strain relief. A bolt through the back of the box can be used for mounting to a support. Photo 23 shows another view.