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.
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
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
15-30 W soldering iron and resin-core solder
Hot glue gun and glue sticks
of the Assembly
Carry out the assembly as follows.
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.
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
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
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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.
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.
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
Transistor pin diagram
E = emitter
B = base
C = collector (+)
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
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.
Step 5: Adding the Battery
Clip and Testing the Circuit
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.
Step 6: Connecting the Photogate
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.
Your battery isn't fresh.
The legs of an infrared LED or phototransistor in the PVC frame are
touching each other.
The PVC frame is wired incorrectly.
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.
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
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:
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.
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.
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.
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.
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
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
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
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
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:
It must be possible to trigger the device with a simple short circuit.
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 HiViz.com 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.