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Assembly and Operating Instructions for Kits


Instructions for Preparing the LAT Phototransistor Cable (Build 2)


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These instructions are written for the light-activated breadboard circuits.



2-conductor cable, 3-ft length
Phototransistor (shown to left)

3/32" dia. heat-shrink tubing, 2" length

3/16" dia. heat-shrink tubing, 3" 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 phototransistor cable will be stripped, and its individual conductors connected to the legs of the phototransistor. The other end of the phototransistor cable will be stripped for connection to the breadboard. The individual wires will be wrapped around the legs of the phototransistor, and the connections tested with your light-activated trigger to ensure proper connections before soldering. The solder joints will then be covered with pieces of heat shrink tubing for electrical insulation.


Preparing the cable


click for larger view

At one end of the cable, strip the outer casing back by 2 inches (15 cm). This will reveal the two inner conductors, colored red and black. At one end, 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. See the photo to the left showing both end of the cable after stripping.


Fitting the heat shrink tubing and making connections


click for larger view
click for larger view

Cut the smaller diameter heat-shrink tubing (HST) into two 1" pieces. Place one piece over each conductor (on the end that you stripped back) as shown to the left. Slide each piece of tubing over a conductor. See the photo to the left.


Now it's time to make connections by wrapping the wires around the legs of the phototransistor. Twist the red wire onto the longer leg and the black wire onto the shorter leg. See the photo to the right.


Testing connections before soldering


Before soldering, you can test your connections using the trigger it will be used with. Make the following connections of the phototransistor cable to the breadboard.


Black to 8J
Red to 5J


If you don't already have your flash unit connected to 9A (red) and ground (black), make those connections now. (See Step 7a of the LAT instructions.)


Connect a 9-V battery to your circuit and turn the sensitivity knob (brown pot) to about the middle position. Place the phototransistor as far from the flash as possible and shaded from it. Shine a flashlight, laser pointer, or other bright light source at the phototransistor to activate the trigger.  If your flash unit doesn't discharge, try adjusting the sensitivity. Turn the 100-kΩ potentiometer clockwise to increase sensitivity.


Soldering the connections


click for larger view  

Assuming that your tests worked, it’s time to solder. If you're new to soldering, see the tips below.


Trim any stray wire strands on the connections.

It's a good idea to place a metal clip to serve as a heat sink between the case of the phototransistor and the leg where you will be soldering. 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 left.


    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.

Completing the cable


click for larger view  

click for larger view

  click for larger view

After soldering, slide the heat-shrink tubing over each of the solder joints so that the legs of are insulated from each other. (See upper photo to the left.) 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. 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.


Now slide the larger diameter heat-shrink tubing onto the cable to complete cover the smaller diameter tubing just below the phototransistor. Shrink the tubing with a lighter. See the photo to lower left.


From the end of the cable opposite the phototransistor, strip the gray insulation a few inches. Then strip the red and black wires about 1/4 inch for connection to the breadboard.


The completed cable is shown to the right. Now you can connect the cable to the breadboard.

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