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Assembly and Operating Instructions for Kits


Assembly Instructions for a Light-Activated Trigger on a Breadboard (legacy LAT-BB)


Note: These instructions are for kits before v10.


Assembly instructions


These instructions may be used in combination with the instructions for building a delay unit in order to provide a light-activated trigger with a selectable delay.  For instructions on building the delay unit, go here.


Parts List


The following parts are included with the LAT kit. (If you purchased the LAT in combination with a delay unit, then a single set of wires was provided with the combination.)


NPN phototransistor, visible


555 timer IC
400-V SCR (EC103D)
1 10-kΩ resistor (brown-black-orange)
100-kΩ potentiometer (white knob)
0.01-µF capacitor (103)

6-ft of 2-conductor cable
Hookup wire
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.
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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click to view

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.

Constructing 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.
bb_087.jpg (101426 bytes) click to view

Step 1: Adding the 555 Timer


The 555 timer is an 8-pin IC that also has a notch and circle identifying Pin 1.  Orient the IC so that the notch faces the left side of the breadboard. Now find Row 4 and look across to where it meets Column e. Place Pin 1 there. Pin 8 should easily fit in Row 4, Column f. Press the IC firmly down in place; again, it should be seated across the center division of the breadboard.

bb_088.jpg (96288 bytes) click to view

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 Rows 11 and 13 on Column j, 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 Phototransistor


The phototransistor (PT) is the visible light-sensing component of the circuit. The component has three legs as shown to the right, 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.  With the tab pointing up, the leg to the left of this tab goes into Row 1, Column i.  The leg to the right of the tab goes into Row 3, Column h.  The last leg can be bent outward, away from the breadboard.


If you wish to mount either phototransistor away from the breadboard for more flexibility, it can be soldered to a 3-ft length of the included 2-conductor cable.  This is described in a later step.

Visible light phototransistor

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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 e. Put the cathode into Row 11 on Column e. The gate will then go into Row 12, and the anode into Row 13 of that column.

SCR pin diagram

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

bb_091.jpg (98599 bytes) click to view

Step 5: Adding the Capacitor


One lead of this capacitor should fit into Row 7, Column h, and the other lead into Row 9 of the same column.


You may wish to trim the leads of the capacitor 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 resistor in the next step, you may wish to trim its leads also.

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Step 6: Adding the Resistor


Insert one end into Row 6, Column c, and the other end into Row 12 of the same column. 

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Completed LAT, not joined to delay unit
(click to view)


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LAT joined to delay unit via green wire (click to view) 

Step 7: 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. 


End 1  End 2 End 1  End 2
Row 1, Column g  negative Row 5, Column d Row 6, Column g
Row 11, Column c negative Row 7, Column d Row 4, Column g
Row 4, Column c negative Row 6, Column i Row 11, Column i
Row 9, Column f negative Row 3, Column j Row 6, Column j
Row 4, Column h positive *Row 13, Column c Row 18, Column h
    *This wire is only needed if you're connecting the trigger to a delay unit.
bb_094.jpg (106923 bytes)
Testing the LAT 


Step 8: Connecting the Cables


Cut the 6 ft of 2-conductor cable into two pieces.  The phototransistor can be soldered onto one of these cable lengths, while the other length of cable will be used to connect a flash unit to the output of this circuit.


Strip 1" of insulation from each of the conductors on one end of the phototransistor cable. The PT will be attached to this 1" end. Now strip ½" of insulation from each conductor on the other end of the cable. This ½" end will connect with the breadboard. Strip an additional 1" of the gray outer shielding from the ½" end so the individual conductors can reach to their destinations.


Note about soldering:  When you connect a wire to a leg of the PT, first wrap the wire tightly around the leg several times. Then clip a heat sink (a metallic alligator clip will work for this) to the leg just below the plastic case. This will prevent the component from heating excessively during soldering. Before starting to solder, make sure you're working in a well-ventilated area in order to avoid inhaling the solder fumes. A fan to blow the fumes away from you will help. Prepare the tip of the soldering iron by holding the solder to it so that solder can melt and flow over the tip. This will improve heat conductivity.  Touch the solder on the leg to which you're soldering the wire.  Hold the flat of the soldering iron tip on leg but not directly on the solder. As soon as the leg is hot enough, the solder will flow. Move the solder around so as to melt solder into the wire and onto the leg along the length of the leg.


You may want to wait to solder them until you've tested the circuit in Step 9.


Phototransistor Connection


Locate your phototransistor.  This component should already have been inserted onto the breadboard in Step 3.  The PT has a polarity that must be maintained when wiring. 

Looking down at the case of the PT (legs facing away), locate the notch on the case.  Make the following connections. 

Wrap the black wire around the leg to the left of the notch.
Wrap the red wire around the leg to the right of the notch.
The last leg is unused and can be bent outward.

Now connect the free ends of the 2-conductor cable to these holes:

Black to Row 1, Column i
Red to Row 3, Column h

Flash Connection


The remaining 3 feet of 2-conductor cable may be used to connect the output of either the LAT or the delay unit to the PC cord of a flash unit. You’ll need to provide your own PC cord. You may have already prepared your flash cable if you wired the delay unit. If not, follow the instructions in the next paragraph.


From one end of the 2-conductor cable, strip 1" of the gray insulation, being careful not to cut the insulation on the red and black wires.  Then strip 1/2" of insulation from each of the red and black wires.  These will connect to the breadboard.  Next, strip 2" of the gray insulation from the other end of the cable.  Strip each of the individual wires back 1".  These will connect to the PC cord.  One way to make this connection is to cut the socket off the end of the PC cable, strip the insulation on the individual PC wires back by 1", splice the red wire of the gray cable to the positive wire of the PC cable, and splice the black wire of the gray cable to the negative wire of the PC cord.  (The positive wire of the PC cord is usually the wire that goes to the center pin of the PC socket.  For more information on connecting to a PC cord, see this page:


In order to connect the flash unit directly to the output of the light-activated circuit, connect the red wire to Row 13, Column b, and the black wire to the negative column.


In order to connect the flash unit to the delayed output (Output 2) of the delay circuit, connect the red wire to Row 18, Column b and the black wire to the negative column.


In order to connect the flash unit to the undelayed output (Output 1) of the delay circuit, connect the red wire to Row 16, Column b and the black wire to the negative column.

bb_096.jpg (107875 bytes)
LAT joined to delay unit, with Output 2 connected to flash (click to view) 

Step 9: Operating the Circuit


With a 9-V battery connected to the battery clip and your flash unit connected to the breadboard, you can now test your circuit.  Shine a flashlight 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, you may need to adjust the sensitivity of the LAT.


Adjusting the sensitivity: Turn the 100-kΩ potentiometer clockwise to increase the sensitivity of the circuit but avoid turning it all the way in either direction.




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