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Assembly and Operating Instructions for Kits


Assembly Instructions for the Crossed-Beam Photogate Kit (CBP-O/F)


Assembly instructions for other kits



The CBP-O kit contains the parts for building the electronic detection circuits, while the CBP-F kit contains the parts for building the photogate frame. The parts and assembly instructions for both the CBP-O and CBP-F are provided below. In order to use the crossed-beam photogate to actuate your camera shutter, you'll need the appropriate shutter cable. These are purchased separately. See this page if you need a shutter cable.


While the CBP-O is a necessary element of the crossed-beam photogate, the CBP-F is not. You may wish to design a frame of your own choosing. If you're drilling and cutting sections for a PVC frame yourself, then you'll need a saw (miter box recommended), small round file, a drill (drill press recommended), 13/64" and 1/4" drill bits, and rubber mallet. Instructions for cutting and drilling the pieces are given in the video tutorial.


CPB-O Parts

Optoelectronics and semiconductors

2 Infrared phototransistors (clear case)
2 Infrared emitters (blue case)

1 PN2222A transistor

1 PS2501 optocoupler



2 of 100-Ω (brown-black-brown)

1 of 220-Ω (red-red-brown)

1 of 10-kΩ potentiometer (white knob)

1 of 100-kΩ potentiometer (brown knob)

6-ft of 3-conductor cable
1 15" piece each of blue, yellow, green, and black hookup wire

Additional 6-in lengths of red, black, green, and blue hookup wire

9-V battery cable



3.5-mm female stereo connector


CPB-F Parts

4 10-inch lengths of half-inch PVC pipe (with holes pre-drilled for emitter-detector pairs)

4 half-inch PVC elbows

13-inch length 3/16" dowel

2 detector shields


Tools that you'll need to assemble the CBP-O/F

Wire stripper

Needle-nose pliers

15-30 W soldering iron and resin-core solder

Hot glue gun and glue sticks

Electrical tape


Plan of the Assembly


Carry out the assembly as follows.

  1. View the video tutorial to see the process of building the photogate framework and testing the trigger. Note that if you have the CBP-F kit, the portion of the video about cutting the PVC pipe and drilling holes will have already been done for you. In that case, after viewing the introduction of Part 1, you may advance the video to 4:15 minutes to continue viewing.

  2. Build the PVC framework and the circuit. See these instructions to complement the video.


Video tutorial, Part 1 Video tutorial, Part 2


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 Circuit


Clicking on the images will open them in a floating window. If you want the images to appear in their own tab, right click on the thumbnail and open in a new tab.

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Step 1: Adding the Potentiometers


The 10-kΩ (white knob) and 100-kΩ potentiometers (brown knob) allow you to adjust the sensitivity of your photogate. Each potentiometer has three legs, two in the front and one in the rear. Place the two front legs of the white pot over 10J and 12J, and the rear leg over the (+) column. Press the legs in firmly as far as they will go, but avoid bending them. In a similar manner, seat the two front legs of the brown pot over 5I and 7I, and the rear leg in the blue column adjacent to the (+) column.


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Step 2: Adding the Resistors


Before inserting the resistors into the breadboard, trim the legs so that the resistors will sit close to the board. Leave the legs long enough, though, that they will seat completely in the breadboard holes.


Locate one of the brown-black-brown resistors (100 Ω). Insert one end into 10H and the other into the blue column adjacent to the positive column. Next, find the red-red-brown (220 Ω) resistor. Insert one end into 2J; the other end should reach over to a hole in the (+) column. Insert the remaining 100-Ω resistor between 4C and 14C.

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Step 3: Adding the Wires


Use the 6" piece blue hook up wire. You can cut off pieces for the three sections that you'll need. Strip about 1/8" of insulation off each end of the pieces. Connect the wires as follows:

  • red column to 6H
  • 4F to 4D
  • 15A to blue column
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Transistor pin diagram

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

PS2501 optoisolator

Step 4: Adding the Transistor, Red LED, and Optocoupler


The transistor, labeled PN2222A, is illustrated to the right. Its three legs 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 4G, the gate into 5G, and the collector into 6G.


The red LED can be used to check for correct operation of the circuit even without a flash unit or photogate cable connected. It has legs of different lengths to help indicate the proper polarity. Insert the short leg into blue column and the long leg into 4A. You may wish to trim these leads so the LED sits closer to the breadboard. For future reference, note that the lip on the case has a flat on the negative side.


The optocoupler is an integrated circuit with 4 legs. Click on the photo to the right for an enlargement. Note the white dot near the lower left leg. Position the optocoupler on the breadboard in the same orientation as in the photo. The legs will be in 14 and 15 E and F, bridging the channel down the middle of the breadboard.

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Step 5: Adding the Battery Clip and Testing the Circuit


Insert the red wire of the battery clip into the positive column and the black wire into the negative column at the right side of the breadboard.


At this point, you can do an initial test of the circuit. First turn the pots to their half-way positions if they aren't there already. Connect a fresh 9-V battery to the battery clip. The red LED should light. This represents the open-circuit condition which corresponds to a blocked photogate. Since no photogate is yet connected, the circuit interprets that the same as if the photogate were connnected but blocked. Now to test the closed-circuit condition, cut one inch off of the blue hookup wire and strip the ends. Connect 4H to 5F. The LED should go out.


After testing, disconnect the wire from 4H to 5F and remove the battery.


If your circuit doesn't behave as described above, first make sure your battery is fresh. Then push down gently on the pots in case they're not seated completely. Check that the LED isn't reversed. Check all connections and jiggle the wires and components in case there's a loose connection. It's a simple circuit, so there's not much that can go wrong.

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Step 6: Connecting the Photogate Cable


In order to continue further, you'll need to have mounted the emitter-detector pairs on the PVC frame. See the video tutorial and the diagram here. After you've completed the gate, connect the photogate output cable from the gate as follows.

  • Red wire to 2H
  • Green wire to 5H (or 5F)
  • Black wire to the blue column

Step 7: Testing the Crossed-Beam Photogate and Adjusting Sensitivity


We assume at this point that you've built the crossed-beam photogate framework and connected it to the breadboard. Begin testing in a dimly-lit room. Make sure the pots are in their half-way positions. Connect your battery. If the photogate is wired correctly and the emitter-detector pairs are aligned well, the red LED should not be lit. Now move your hand through the center of the photogate frame. The red LED should light and then go out when you move your hand away.


Now turn on an overhead light or nearby lamp. This will affect the sensitivity of the gate, since it picks up light at the red end of the visible spectrum as well as infrared. In order to adjust for this, turn the brown pot counterclockwise until the red LED lights. Then turn the pot back up to the point where the LED goes out. This sets the circuit near the sensitivity threshold. Move your hand through the center of the gate again to see that the red LED lights. Outdoors where the ambient light may be particularly bright, you may need to turn the brown pot all the way counterclockwise. In that case, continue the adjustment by turning the white LED counterclockwise to find the threshold of sensitivity.


Troubleshooting: If your photogate doesn't function as described above, here are some possible causes.

  1. Your battery isn't fresh.
  2. The legs of an infrared LED or phototransistor in the PVC frame are touching each other.
  3. The PVC frame is wired incorrectly.
  4. The emitter-detector pairs are misaligned.

Items 1-3 are easy to check. For items 4 and 5, if you've waited until after this testing phase to complete the soldering and taping on the PVC frame, then you'll be able to check and possibly correct any errors.

3.5mm mono female connector soldered

Photo 1


3.5mm mono female connector soldered

Photo 2


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Photo 3

Step 8: Preparing and Connecting the 3.5mm Stereo Jumper


Next you'll need to do some soldering. Unscrew the jacket from the 3.5mm female stereo connector. Note the legs labeled red, green, and black in Photo 1. You'll connect and solder the 6-inch lengths of red, green, and black hook up wire to the corresponding legs of the connector.


Photo 2 shows the wires inserted into the holes on the legs before soldering. After soldering, crimp the tabs of the long leg around all the wires to hold them tightly. Screw the jacket on and strip the ends of the wires back 1/8 inch for insertion into the breadboard.


See Photo 3 for the connection points of the jumper to the breadboard:

  • red to 14H
  • black to 15H
  • green to 15G

For future reference, these are the functions of the three wires when the jumper is connected to the shutter cable from a camera:

  • Shutter wire: The red wire is connected to the shutter.
  • Focus wire: The green wire is connected to the camera's half-press functions, auto-focus and exposure.
  • Ground wire: The black wire is connected to ground.

Here's how the photogate circuit controls the camera:


When an object breaks the intersection of the photogate beams, the optocoupler closes the circuit between the shutter and ground, thereby actuating the shutter. Note that the green and black wires (focus and ground) are electrically connected, because they're in the same row of the breadboard. This is required by some cameras, Nikons in particular. The focus must be connected to ground before the shutter can be actuated. For Nikon models, this is true even if the camera is being operated in full manual mode. For other cameras, Canons in particular, the focus wire need not be connected when operating in manual mode. Nevertheless, it's fine to keep the focus wire connected to the breadboard.


Step 9. Triggering a Camera


The most common device to trigger with a crossed-beam photogate is a camera. When the photogate is used in daylight or shaded conditions, the camera shutter can't be left open for long periods of time as would be needed if using the open-shutter technique with a triggered, external flash. With the crossed-beam photogate, the camera is focused on the intersection of the two beams, and the output of the photogate circuit is connected to the camera shutter. A high shutter speed is used to freeze action. When a bird or insect flies through the intersection of the beams, the camera shutter is actuated.


In order to use your camera with the photogate circuit, you need to have the shutter cable for your camera. You may have purchased this with your CBP. If not, see this page. The shutter cable typically has the connector for your camera make/model on one end and a 2.5mm stereo plug on the other end. A 2.5mm-to-3.5mm adapter is included with our cables so that you can plug them into the stereo jumper that you prepared in the previous step. Before plugging in your camera, though, do the following:


  1. Set your camera for full manual operation. The more automatic functions you can turn off, the better. This will reduce shutter lag and give your camera the most rapid response. This is needed so that a fast-moving subject passing through the photogate won't be out of the zone of acceptable focus before the shutter is actuated.

  2. Turn off the camera before plugging the shutter cable into the stereo jumper on the breadboard. Be sure that the two pieces snap together completely. It's a good idea to have the camera turned off when connecting to or disconnecting from the jumper.

  3. Now turn the camera on. For a test, switch the camera to auto-focus operation. The camera should auto-focus. This is because the focus and ground wires are shorted together on the breadboard. After this test, put your camera back in manual focus mode.

  4. Disconnect the red shutter wire and momentarily connect it to 15I. The shutter should actuate. What you did was short the shutter to ground. This step and the previous one serve as tests that your camera is correctly connected through the shutter cable and jumper.

  5. Put the shutter wire back in 14H. Now run your hand through the photogate beams. The red LED should light momentarily and the camera shutter actuate. If so, everything is working correctly.

Note about disabling of the image review function for some cameras: For some camera makes, notably Nikons, the focus wire must be disconnected from ground before you can review the image on the camera's LCD. In such cases, you will need to disconnect this wire in order to see your images. Then you will need to reconnect it in order to take more photos. For Canons, the focus wire need not be disconnected for image review to function.

Using the Crossed-Beam Photogate


Adjusting sensitivity

If you're using the crossed-beam photogate outdoors throughout the day and/or the ambient light level is changing, you'll need to monitor your setup to make sure the gate continues to function. This involves adjusting the sensitivity of the trigger circuit. This was discussed in Step 7. Here's a typical scenario if, say, you set up early in the morning and take photos all day. Let's suppose you've adjusted the sensitivity for pre-dawn darkness. Now, as the ambient light intensity increases, the sensitivity will drift. You'll need to turn the brown pot (coarse control) counterclockwise to compensate. If the light gets particularly bright, turn the brown pot all the way counterclockwise and make fine adjustments with the white pot. If the sun goes in and out of the clouds, check the adjustment frequently. As the light intensity decreases, toward late afternoon and dusk, the sensitivity will drift the other direction, and your camera shutter will actuate spontaneously. You'll need to turn the brown pot clockwise.


If you take pictures in darkness, then you won't need to monitor the sensitivity once you've set it.


Hints for successful operation

  • Set your camera for complete manual operation. Turn off any lens VR control. The fewer operations your camera needs to perform before opening the shutter, the faster it will respond (shorter shutter lag).

  • If you photograph outdoors, set up in an area that will be shaded throughout the day if possible.

  • If you're going to leave the gate set up for long periods of time, power your devices with AC/DC adapters if you're within reach of an outlet. The trigger circuit can be powered with a 9-V AC/DC adapter such as the one shown here. Similarly, power your flash unit(s) and camera with AC/DC adapters.

  • When the light level goes down and the sensitivity drifts, your camera may take a large number of shots spontaneously. Use a large memory card so that you don't fill it quickly with useless shots. Monitor your set up closely in late afternoon/early evening and whenever the ambient light level is changing significantly.

  • Prefocus your camera on the center of the crossed-beam gate. Use a small aperture to increase depth-of-field. Flying birds and insects will quickly move out of the plane of the gate after triggering. Your camera's shutter lag, even if small, is still large enough that some subjects will leave the area of sharpest focus before the shutter opens.

  • Don't use a delay unit with the photogate trigger if you don't need one. Even with the time delay set as small as possible, there will still be a small delay that may be enough to give the subject time to leave the region of sharp focus.

  • Use more than one flash unit both to balance the lighting and also to provide more light on the subject. Trigger additional flash units off a main unit. You'll need to use your units on low power in order to freeze wing motion.

  • The background will be dark even in daylight conditions if you're using a small aperture. If you want the background to show, reduce the shutter speed. This will take some trial and error. If you reduce the shutter speed too much, the subject will show ghosting as a result of movement. Something else you can try to bring out the background if it's not too far away is to use flash--triggered off the main unit--to illuminate the background only. Then you can set your camera for the fastest shutter speed that will synch with your main flash.

  • Finally, be patient. In many of your shots, the subject will be cut off by the frame, out-of-focus, or in an uninteresting position. If you get a few good shots a day, that's par for the course. Some examples are shown below.


Addendum: Using the Crossed-Beam Photogate to Trigger a Flash Unit, Wireless Transmitter, or Delay Unit


The optocoupler output of the crossed-beam photogate will trigger devices other than a camera. Two things are required:

  1. It must be possible to trigger the device with a simple short circuit.
  2. Flash units may not have a terminal voltage greater than 70 V. Higher voltages may burn out the optocoupler.

The first requirement is met by most flash units, wireless transmitters, and the delay circuit. In order to connect any of these devices to the optocoupler output, you would need to prepare a 2-conductor cable to connect the device to 14H and 15H on the breadboard. In this case, the stereo jumper would be removed.




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