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Assembly and Operating Instructions for Kits


Assembly Instructions for a Light-Activated Trigger on a Breadboard (LAT-BB, Build 1)


Note: These instructions are for v10 kits.


Assembly instructions for other kits


While these instructions are for the LAT-BB, the photos all include a delay unit. If you are building the light-activated trigger with a delay (LAT-DU-BB), see these instructions for building the delay unit as well as the light-activated trigger.


Parts List


The following parts are included with the LAT-BB kit.


NPN phototransistor, visible

555 timer IC

400-V SCR (EC103D)


1 1-kΩ resistor (brown-black-red)
100-kΩ potentiometer (brown knob)
0.01-µF capacitor (103)

6-ft of 2-conductor cable
3 3" pieces of hookup wire

2" of heat shrink tubing
9-V battery cable*

*A fresh 9-V battery is required but not included with the kit. 


You'll also need a wire cutting and stripping tool such as the one shown to the right. For soldering, you'll need a 15-30 W soldering iron and resin-core solder.

Click for larger view


Click on the thumbnails below in order to view full-size images of the breadboard with the components that have been added in each step.


Using the Breadboard


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The breadboard offers an easy way to build electrical circuits without soldering. The 2"x3" breadboard provided with your kit contains an array of holes where wires and components are to be inserted. The holes in the center portion of the breadboard are identifiable by row (vertical in the photos) and column (horizontal).  There are two sets of 30 rows numbered by 5's, and each set of rows has 5 columns labeled A-E and F-J. The 5 holes on each row are electrically connected to each other (but not across the center channel), so any components inserted into the same row would be connected just as if they had been soldered.  However, the components can be removed and replaced with other components at any time, without the hassle of unsoldering and resoldering parts.


On either side of the breadboard are two columns marked by blue and red lines. The 25 holes in each column are electrically connected, but the columns aren't electrically connected to each other.  The outermost column marked with the red line at the top will be used for all +9 V connections, while the outermost column marked with the blue line at the bottom will used for all ground (negative) connections.

Assembling the Light-Activated Trigger


Note that the photographs show a delay unit already built on the right side of the board.  The light-activated trigger may be used with or without the delay unit. However, the 9-V battery cable is required for the operation of either kit.  This is the cable coming in from the left with the red and black leads above and below the 555 timer. The column of 25 holes to which the red wire is connected will be termed the positive column, while the column to which the black wire is connected will be termed the negative column.   While wiring the circuit, be sure to have the battery disconnected from the battery cable.


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Step 1: Adding the 555 Timer


The 555 timer is an 8-pin IC that also has a notch and circle identifying Pin 1. (See diagram to the right.) Orient the IC so that the notch faces the left side of the breadboard. Now find Row 3 and look across to where it meets Column E. Place Pin 1 there. Pin 8 should easily fit into 3F. Press the IC firmly down in place; again, it should be seated across the center division of the breadboard.


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Step 2: Adding the Potentiometer


The potentiometer allows you to adjust the sensitivity of your light-activated circuit. It has three legs, two in the front and one in the rear. Place the two front legs over 9J and 11J, and the rear leg over the nearest hole on the nearby positive column. The front legs should be facing the center of the breadboard, while the rear leg faces the outside of the breadboard. Press the legs in firmly as far as they will go, but avoid bending them.


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Step 3: Adding the Wires


Now connect all your electronic components together. Each wire only needs to be 2 inches in length or less. You can estimate how much you'll need to bridge across two holes before cutting, although it's always better to have longer wires than ones that are too short.


Strip about 1/4" of insulation off each end. The list below will tell you which rows and columns your wire ends should fit into. 


3A to (-) 3J to (+) 6D to 3G 5-9I
7A to (-) 4D to 5G 7E to 8F *9E to 18F


*This wire is only needed if you're connecting the trigger to a delay unit.


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SCR pin diagram


A = anode (+)
G = gate
K = cathode (-)

Step 4: Adding the SCR


The silicon-controlled rectifier is the output of the light-activated circuit and can be connected to the input of the delay circuit.  Putting in this SCR is easy since all three leads go in consecutive rows along Column B. Place the cathode into 7B, the gate into 8B, and the anode into 9B. The diagram to the right shows the pin designations for reference.


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Step 5: Adding the Resistor


Locate the brown-black-red resistor (1 kΩ), and insert it from 5C-8C.


You may wish to trim the leads of the resistor so that it sits closer to the breadboard.  This will reduce the chance that the leads of two components accidentally touch each other and create a short.  When you add the capacitor in the next step, you may wish to trim its leads also.


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Step 6: Adding the Capacitor


Locate the 0.01-µf capacitor (103). Place one leg of this capacitor into 6H and the other into 8H.



Step 7a: Connecting the Output Cable to Your Flash


Three feet of the 2-conductor cable may be used to connect the output of the circuit to a flash unit.* If you have assembled a delay circuit, you will have already prepared the output cable from those instructions. If not, see the information in the yellow box.


A trigger cable is needed to connect your flash unit to the breadboard. The trigger cable kit is purchased separately, since there are different connectors depending on your flash unit. If you need a trigger cable kit, see this page.


Follow the instructions for assembling the flash trigger cable from your kit. (For quick reference, select a link: PC or FA kit / VPC kit.)


Once you've prepared the trigger cable, connect the wires to the breadboard according to this table.


Note that trigger circuits can also be connected to a camera shutter or wireless transmitter. See this page for more information.



*Connection to a camera shutter or wireless transmitter is also possible. See this page.


Visible light phototransistor

Step 7b: Preparing and Connecting the Phototransistor


The visible light phototransistor (right) is the light-sensing component of the circuit. There are two mounting options: directly onto the breadboard, or soldered to a 3-ft length of the included 2-conductor cable if you would like more flexibility.


Cable mount: Click here for assembly instructions, and return here when you are done with assembly. The free ends of the phototransistor cable will be connected to your breadboard at the following locations:

Black to 8J
Red to 5J

Click for larger image

Breadboard mount: The phototransistor has three legs, but only two will be inserted into the breadboard.  Looking down at the top of the PT (legs facing away), locate the tab on the case.  See photo to left. With the tab pointing up, the leg to the left of this tab goes into 8J, and the leg to the right of the tab goes into 5J.  The last leg can be bent outward, away from the breadboard.


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Completed LAT showing connections to trigger, output, and delay unit
(click to view)

Step 8: Testing and Operating the Circuit


Note if you're using a delay unit with the light-activated trigger: For the delay unit outputs, a jumper wire must be added from the output of the trigger 9E to the input of the delay unit 18F. When using the trigger without the delay unit, disconnect this latter wire, as some flash units can burn out the 556 timer if connected to the timer input.


Connect a 9-V battery to the battery clip and a flash unit to the breadboard. You can now test your circuit.  Turn the 100-kΩ potentiometer to about its midway 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 cable is connected directly to the LAT circuit output or to Output 1 of the delay circuit, you should notice an immediate discharge of your flash unit.  If your flash cable is connected to Output 2, you may notice a short delay before discharge, depending on the setting of your delay circuit.  If your flash unit doesn't discharge, try adjusting the sensitivity. Turn the 100-kΩ potentiometer clockwise to increase sensitivity.


Note about repeated discharges: A single triggering event can lead to repeated discharges of the flash. This can occur if the phototransistor is positioned so that it picks up the light from the flash. This can create a feedback loop in which the circuit self triggers. This can happen even if the phototransistor is shaded from the flash. If the phototransistor is too close to the flash, RF noise from the flash unit can trigger the LAT. One solution is to keep the flash as far from the trigger circuit as possible. Another is to increase the reset delay if you're using a delay unit.




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