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Assembly and Operating Instructions for Kits


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Assembly Instructions for the Transistor Photogate on a Breadboard (TPG-BB)


Note: These instructions are for v10 kits.


Assembly instructions for other kits


While these instructions are for theTPG-BB, the photos all include a delay unit. If you are building the transistor photogate with a delay (TPG-DU-BB), see these instructions for building the delay unit as well as the photogate.


Parts List


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


Infrared phototransistor (clear case)
Infrared emitter (blue case)

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

2N2222 transistor (or PN2222A)

400-V SCR (EC103D)


1 100-Ω (brown-black-brown)

1 470-Ω (yellow-violet-brown)

100-kΩ potentiometer (brown knob)

3-ft of 2-conductor cable
3-ft of 3-conductor cable
1 3" piece 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.


The metal plate included with the breadboard isn't needed for the assembly instructions below.


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.  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 Potentiometer


The 100-kΩ potentiometer (brown 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 4J and 6J, and the rear leg over the (+) 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 2: Adding the Wires


Two short wires are needed for this step.  Strip about 1/4" of insulation off each end. One wire should join 2A to the nearby (-) column.  The other should join 5H to the nearby (+) column.


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SCR pin diagram


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

Transistor pin diagram

E = emitter (-)
B = base
C = collector (+)

Step 3: Adding the SCR and Transistor


The silicon-controlled rectifier (labeled EC-103D) is the output stage of the sound trigger. Putting in the SCR is easy since all three leads go in consecutive rows along Column B. Put the cathode into 2B, the gate into 3B, and the anode into 4B.




The transistor looks identical to the SCR but is labeled PN2222A (or 2N2222A). Its three leads go in consecutive rows along Column G.  To identify the leads of the transistor, hold it as in the diagram to the right.  Put the emitter into 3G, the gate into 4G, and the collector into 5G.



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Step 4: Adding the Resistors


Locate the brown-black-brown resistor (100 Ω).  Insert one end into 3C, and the other into 3F. Next, find the yellow-violet-brown (470 Ω) resistor.  Insert one end into 2J; the other end should reach over to the nearest hole in the (+) column.



Step 5a: Connecting to a Flash Unit


The three feet of 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.


Step 5b: Preparing and Connecting the Photogate Cable


Click here for instructions on how to assemble the photogate cable. Return here after you've assembled the cable.

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Connections shown with photogate and output cables

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Step 6a: Operation of the Photogate Trigger


For the flash: Connect the red wire of the gray 2-conductor cable to 4A and the black wire to the ground column.


For the photogate cable, connect::

  • Green wire to 4F
  • Red wire to 2F
  • Black wire to 2D

Adjusting the sensitivity: Position the PT and LED facing each other about an inch apart. Run your finger between them to make sure the flash discharges. Then turn the 100-kΩ potentiometer in one direction or the other until the flash discharges spontaneously. Back up the dial just before the point of spontaneous discharge in order to achieve maximum sensitivity. If you change the distance between the PT and LED 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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Connection from photogate outut to delay unit input

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Step 6b: Operation with a Delay Unit


If you're using the photogate with a delay unit, add a wire from 4E, the output of the photogate, to 18F, the input of the delay unit. (See the blue wire in the photo just above.) This will allow the photogate trigger to trip the delay unit. Connect the 2-conductor gray output cable in one of the following two configurations:


Output of delay unit
red wire
black wire
ground (-) column
ground (-) column


Sometimes it's necessary to adjust the sensitivity pot when the photogate is used with a delay unit.


Important: If you connect your flash to the direct output of the photogate, be sure to disconnect the wire from 4E to 18F first. If this wire is left in place, some flash units can burn out the 556 timer.



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