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Assembly and Operating Instructions for Kits


Instructions for Preparing the LAT Phototransistor Cable (Build 1)


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These instructions are written for the LAT-BB and LAT-DU-BB kits. These kits use a phototransistor that includes a lens on top of the case in order to narrow the field of view to about 10°. This component is shown in the photo below. While there are three legs, only two of them are connected to the trigger circuit. This will be described later.


2-conductor cable, 3-ft length
NPN phototransistor (visible)

Heat shrink tubing (HST), 2" 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 appropriate legs of the phototransistor. The other end of the phototransistor cable will be stripped differently, for easier 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 both ends 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. At the other end of the cable, strip each of the conductors back by 1/4". 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.


Fitting the heat shrink tubing and making connections


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click for larger view

Cut the heat shrink tubing (HST) into two 1" pieces. Place one piece over each conductor (on the end that you stripped back 3/4") as shown to the left. Slide each piece of tubing over a conductor. 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 phototransistor. You first need to identify the legs. The phototransistor has three legs, but only two are used. Hold the photransistor so that you are looking at the base of it with the legs pointed toward you and the tab on the case pointing up. The leg nearest the tab is the emitter and is connected to the black conductor. The leg to the left of the tab connects to the red wire. The last leg can be bent outward and will not be used. Twist the red and black wires onto the legs now. The photo to the right shows the red wire twisted onto the phototransistor.


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


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  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 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 case of the phototransistor 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


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 photo to the left.) Keep the pieces about 1/8” away from the phototransistor base to protect it 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.


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

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