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Assembly and Operating Instructions for Kits


Assembly Instructions for a Photogate Trigger on a Breadboard (SPG1- or SPG2-BB)


Note: These instructions are for v10 kits.


Assembly instructions for other kits


While these instructions are for the SPG1-BB and SPG2-BB kits, the photos all include a delay unit. If you are building a photogate with a a delay (SPG1-DU-BB or SPG2-DU-BB), see these instructions for building the delay unit as well as the photogate.


Parts List


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


with SPG1 kit
Infrared phototransistor (clear case)
Infrared emitter (blue case)

18" jumper wire (yellow)
4" heat shrink tubing


with SPG2 kit

with either SPG1 or SPG2
555 timer IC
400-V SCR (EC103D)

2 470-Ω resistors (yellow-violet-brown)
1 100-Ω resistor (brown-black-red)
10-kΩ potentiometer (white knob)
0.01-µF capacitor (103)

3-ft of 3-conductor cable
3 4" pieces of 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. 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 Photogate


Note that the photographs show a delay unit already built on the right side of the board.  The photogate 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 IC.  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. There is a semicircular notch that can be used to identify the pins.  See the diagram to the right. Orient the IC so that the notch faces the left side of the breadboard. Now find Row 2 and look across to where it meets Column E. Place Pin 1 there. Pin 8 should easily fit into 2F. 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 10-kΩ potentiometer (white knob) allows you to adjust the sensitivity of your photogate. It has three legs, two in the front and one in the rear. Place the two front legs over 9F and 11F, and the rear leg into 10J. 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.


If the legs won't stay seated in the breadboard, a tip is to bend all three legs slightly outward away from the potentiometer. Then insert the legs. The slight outward bend will help the potentiometer stay seated.


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Step 3: Adding the Wires


These are the wires that will connect all your electronic components together. Since the wires run beneath the components (or around, in the case of the 555 timer and potentiometer, to allow for easier component removal), it is important to cut the wires so they lay flat against the breadboard. You can estimate how long a wire needs to be by running a piece between the two breadboard holes you want to connect, then cutting the wire 1/4" longer than that at either end. Then strip 1/4" of insulation from each end. Note that the wires supplied with your kit won't necessarily be the same color as those in the photograph.


The list below will tell you which rows and columns your wire ends should fit into.


Note this error in the photo: The wire from 4H to 6E is not needed, although it has no effect on the circuit if included.


7F to (-) 3D to 4G 4I to 9J
8A to (-) 5D to 2G 7G to 10F
2A to (-) 6E to 4H (not needed) *10E-18F
2J to (+)  


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

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SCR pin diagram


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

Step 4: Adding the SCR and red LED


The silicon-controlled rectifier is the output of the photogate circuit and is 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. Put the cathode into 8B, the gate into 9B, and the anode into 10B.


The red LED can be used to check for correct operation of the circuit even without a flash unit connected. It has legs of different lengths to help indicate the proper polarity. Insert the short leg into 8J, and the long leg into the (+) column. You may wish to trim these leads so the LED sits closer to the breadboard; however, be sure to trim the short leg shorter than the long leg so you can connect the LED with the right polarity later.



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Step 5: Adding the Resistors


Locate the brown-black-brown resistor (100 Ω), and place it between 4-9D.


There are two yellow-violet-brown (470 Ω) resistors.  Insert one end of the first into 1F; the other end should reach over to the nearest hole in the (-) column. The second resistor connects between 3-8H.


You may wish to trim the leads of the resistors so they sit 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 capacitor labeled 103, which has a value of 0.01 µF. Place it between 5-7J.



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Photogate connections shown with output cable and jumper to delay unit; click to view


Step 7a: Preparing and Connecting the Photogate Cable


Select your kit model below for instructions on how to assemble the photogate cable. Return here after you've assembled the cable.




Now, connect the free ends of the 3-conductor cable to these holes on your breadboard:

Black to 1J
Green to 4J
Red to the (+) column

Step 7b. Connecting a Flash Unit


If you built a delay unit previously, then you've already prepared a trigger cable to connect the flash to the breadboard. 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.


To connect the output cable from the flash to your circuit, three different connections are possible if you're using a delay unit and one connection if you're not. Note that for the delay unit outputs, a jumper wire must be added from the output of the trigger 10E to the input of the delay unit 18F. When using the trigger without the delay unit, be sure to disconnect this latter wire, as some flash units can burn out the 556 timer if connected to the timer input.


red wire
black wire
jumper to delay unit
Photogate direct
ground (-) column
Do not connect 10E to 18F.
Immediate output of delay unit
ground (-) column
Connect 10E to 18F.
Delayed output of delay unit
ground (-) column
Connect 10E to 18F.


Step 8: Testing and Operating the Circuit


If you're using the individual PT and IR LED components, lay them down on a table a few inches apart pointing at each other.  You may want to tape down the cables so that the components can't shift positions.  If you're using an interrupter, the components are already fixed in position.


With a 9-V battery connected to the battery clip and your flash unit connected to one of the outputs as described in step 7b, you can now test your circuit.  The red LED of the photogate should be lit if the PT and IR LED are aligned correctly. Run your finger between the PT and IR LED in order to break the photogate beam.  The red LED should go out momentarily. If your flash cable is connected directly to the photogate or to Output 1, 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 photogate.


Adjusting the sensitivity: First make sure the photogate is working. Then turn the white knob counterclockwise until the photogate indicator LED goes off. Then back off the knob a little bit until the LED comes back on.


If you change the distance between the PT and IR LED (if using the individual components) or if the orientation of either component changes slightly, you may need to readjust the sensitivity.  The maximum separation is about 8 inches. The larger the separation, the more care you need to take in aligning the components. If you wish to have greater separation, a red laser pointer can be used instead of the IR LED.




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