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Assembly and Operating Instructions for Kits


Assembly Instructions for a Schmitt Trigger Photogate on a Breadboard (legacy SPG1- or SPG2-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 photogate with a selectable delay.  For instructions on building the delay unit, go here.


Parts List


The following parts are included with the SPG 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 (PT)
Infrared emitter (LED)

with SPG2 kit

with either SPG1 or SPG2
555 timer IC
400-V SCR (EC103D)
1 470-Ω resistor (yellow-violet-brown)
1 10-kΩ resistor (brown-black-orange)
10-kΩ potentiometer (white knob)
0.01-µF capacitor (103)

3-ft of 2-conductor cable
3-ft of 3-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.  If you do any soldering, you'll need a soldering iron, solder, and a heat sink.
Click for larger view

Click on the thumbnails 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.



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.
bb_044.jpg (2851134 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.

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Step 2: Adding the Potentiometer


The potentiometer 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 Rows 8 and 10 on Column a, and the rear leg over the nearest hole on the nearby negative 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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SCR pin diagram

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

Step 3: Adding the SCR


The silicon-controlled rectifier is the output of the photogate circuit.  Putting in this SCR is easy since all three leads go in consecutive rows along Column e. Hold the SCR as in the diagram to the right in order to identify the leads. 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.

bb_047.jpg (2899344 bytes) click to view

Step 4: Adding the Capacitor


Locate the capacitor labeled 103. This has a value of 0.01 µF. Insert one lead of this capacitor into Row 7, Column i, 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 resistors in the next step, you may wish to trim their leads also.

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Step 5: Adding the Resistors


Locate the brown-black-orange resistor (10 kΩ).  Insert one end into Row 6, Column c, and the other end into Row 12 of the same column.  Next, find the yellow-purple-brown (470 Ω) resistor.  Insert one end into Row 2, Column g. The other end should reach over to the nearest hole in the negative column.

bb_054.jpg (2838410 bytes) click to view

Step 6: Adding the Wires


Now you'll 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. The longest wires are listed first so if you happen to cut a piece that's too short, you'll be able to use it later.


Longest wires (~2 inches)

Medium length wires (~1.5 inches)

End 1  End 2 End 1  End 2
Row 9, Column f  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 8, Column d Row 6, Column i
Row 4, Column h positive *Row 13, Column c Row 18, Column h
*This wire is only needed if you're connecting the photogate to a delay unit.
bb_055.jpg (2989814 bytes)
Photogate connections shown with output cable; click to view
Note that for some versions of this kit, the colors of the wires in the 3-conductor cable are red, black, and green.  In that case, simply replace the word white with green in the instructions.

Step 7a: Connecting the LED and phototransistor (or interrupter)


The photogate has a light-emitting and a light-sensing component.  The former is a light-emitting diode (LED), which emits an infrared beam.  The sensing component is an infrared phototransistor (PT). When the beam is broken by an object, the blockage causes the voltage to rise across the PT, which gates the SCR at the output of the circuit.

For the SPG1 kit, the PT and the LED are the individual components shown to the right. The LED is the component with a blue case, and the PT has a clear case. (In an earlier version of this kit, the PT and LED both had clear cases. If you have this version, note that the LED has longer legs than the PT.) For both components, one leg is shorter than the other. The shorter leg is positive on the PT, while on the LED, the longer leg is positive. The wiring instructions given later in this section will ensure that the correct polarity is maintained.
For the SPG2 kit, the PT and LED are housed in the two posts of the single component, termed an interrupter, shown to the right.  

To begin wiring, use the gray 3-conductor cable. The 4 legs of the PT and LED (or interrupter) will be soldered to these three conductors.

Strip 1" of insulation from each of the conductors on one end of the cable. The PT and LED 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.


If using individual PT and LED components, make the following connections:

Wrap the black wire around the shorter leg of the LED.
Wrap the white (or green) wire around the longer leg of the PT.

Next prepare a jumper wire that will go from the longer leg of the LED to the shorter leg of the PT.  The length of this jumper will depend on how far apart you want to separate the PT and LED for your photography.  Strip the wire back about an inch on each end wrap it onto the legs of the components.


Now wrap the red wire of the 3-conductor cable to either one of the legs onto which you wrapped the jumper wire.


For best connections, solder the red, black, and white (or green) wires to the legs onto which they are wrapped. Before soldering each leg, 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 the 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.


If using an interrupter, make the following connections:

The symbols and  on the top view of the interrupter to the right refer to the LED and phototransistor (PT) respectively. The numbers refer to the legs on the underside (not shown).

Solder the black wire to leg 2.
Solder the white (or green) wire to leg 3.
Solder a short jumper wire between legs 1 and 4.
Solder the red wire to the jumper between legs 1and 4.

If you have an interrupter with short legs, you won't be able to wrap wires around the legs. Just hold the wire to the leg and tack solder the two parts together. Since you won't be able to use a heat sink, minimize the amount of time that the soldering iron is in contact with the leg.


Whether using individual PT/LED or an interrupter, do the following:


Connect the free ends of the 3-conductor cable to these holes:

Black to Row 2, Column j
white (or green) to Row 6, Column j
Red to the positive column

Step 7b. Connecting the flash unit


You may already have an output cable for your flash if you prepared one for the delay circuit. If not, use the following instructions.

The 3 feet of 2-conductor is used to connect the output of the photogate trigger to the PC cord of a flash unit. 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 to the output of the photogate, connect the red wire to Row 13, Column b, and the black wire to the negative column.

If you're connecting the flash to a delay unit,


For 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. (This is the connection shown in the photograph.)


For 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.


Step 8: Operating the Trigger


If you're using the individual PT and 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.  Run your finger between the PT and LED in order to break the photogate beam.  If your flash cable is connected directly to the photogate 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 of the delay unit, 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: Turn the 10-kΩ potentiometer in one direction or the other until the flash discharges spontaneously. Then back up the dial just before the point of spontaneous discharge.


If you change the distance between the PT and 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 6 inches. The larger the separation, the more care you need to take in aligning the components.



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