Operating Manual for Crossed-Beam Photogate (CBP2)
|Figure 1. Crossed-beam photogate control box|
Figure 2. 10-inch photogate frame (painted black) with cable, connected to control box
Connecting the Photogate to the Control Box
The crossed-beam photogate consists of the two parts shown in the photos above: control box (Fig. 1) and photogate frame with cable (Fig. 2). For operation, insert the quarter-inch plug into the jack labeled PG on the control box. Be sure that the plug is seated completely. Extra force may be required to snap it into the jack.
Powering the Unit
The photogate frame is powered through its connection to the control box. The control box is powered either with a 9-V battery or an optional AC-to-9V DC adapter. In order to install a battery, remove the 4 screws on the corners of the lid of the control box. Lift the lid carefully to expose the battery holder. Clip a 9V battery into the battery holder. Then reassemble the box. Note that if a battery is installed but the AC adapter is plugged in, the battery is automatically disconnected internally so that it doesn't drain. If you plan to use the unit for long periods of time, it's recommended that you power the unit with the AC adapter.
When the photogate and flash or camera cables are connected, flip the on-off switch to the right to turn on the unit. Note that the LED will not light at this point. The function of the LED will be described in the next section.
Testing the Operation of the Photogate
It's recommended that you test the photogate on an indoor tabletop before setting it up outside. During the test, you may have the room lights on. However, avoid having bright lights shining directly on the photogate frame. It's not necessary to connect a flash unit or camera for this test.
Turn both the fine and coarse sensitivity knobs about half the way clockwise from their furthest counterclockwise positions. Place your hand in the middle of the photogate frame to block the invisible infrared beams. The LED on the control box should be lit as long as both of the infrared beams are broken. You can use a finger to accurately locate the intersection of the beams. If the LED doesn't light, the room lights may be too bright. In that case, turn the coarse sensitivity knob counterclockwise to reduce the sensitivity of the unit.
Adjusting for Maximum Sensitivity
With the beams unblocked, turn the coarse sensitivity gradually counterclockwise. At some point, the LED will light even though the beams aren't blocked. From this point, turn the knob back clockwise to the point where the LED goes out. This is the position of maximum sensitivity. You can fine tune it by going through the same process with the fine sensitivity knob. Turn it counterclockwise until the LED comes on even with the beams unblocked. Then turn it back clockwise until the LED goes out.
Learning to adjust the sensitivity critically as described above is most important for using the photogate in relatively bright ambient light conditions. As lighting changes throughout the day, you may find that you need to adjust the sensitivity correspondingly. For particularly bright conditions, you may need to turn the coarse sensitivity all to way counterclockwise and use only the fine sensitivity knob for adjustments. For indoor photography or photography at night, the sensitivity adjustment is generally not critical.
Connecting the Output Device
The crossed-beam photogate can be used to discharge a flash unit, actuate a camera shutter, or trigger a wireless transmitter. In most applications, though, a camera is actuated and the camera in turn discharges a flash unit. Before connecting a flash or camera to the CBP control box, it's best to have the control box and the flash or camera turned off. Turn on the equipment after you've made connections.
In order to actuate a camera using the CBP, connect the shutter cable for your camera to the 3.5mm stereo jack labeled OUT on the control box. If your shutter cable doesn't have a 3.5mm stereo plug on the end, you'll need to connect one first. If you want to purchase a shutter cable that is ready-to-use with your camera and the CBP control box, see http://hiviz.com/kits/shutter_cables.htm.
In order to discharge a flash unit or trigger a wireless transmitter, simply connect the device to OUT using a 3.5mm stereo plug. When triggering a flash unit with the CBP, the response is quite rapid; the delay between breaking the beam and the start of flash discharge is less than 50 microseconds. Use this mode when you need the fastest possible response and you can work in a very dark environment. The reason for the latter is that you would need to keep the shutter of the camera open on bulb in readiness for the flash discharge.
Caution: If your flash unit has a high-voltage synch circuit (>80 V), don't connect it to the CPB control box as this would burn out the optocoupler. In the event that you do burn out the optocoupler, a replacement part is available here.
In most situations, you'll be actuating a camera shutter. In this mode, you can work in ambient light and your flash discharge will be synchronized by the camera. The response time is affected primarily by the shutter lag of the camera. Turn off as many automatic operations on the camera as possible, as these operations tend to increase the shutter lag time. Note that the control box has a Focus switch. This switch is off when flipped to the left. The switch is required for the operation of some cameras such as Nikon models. You'll need to flip the FOCUS switch to the right before the control box can actuate the shutter. This is true even when using the camera in manual focus mode. Then you'll need to flip the FOCUS switch off in order to review your images. For Canon models operated in manual focus mode, the FOCUS switch may be either on or off.
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.
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. In case of situations where the use of a delay unit is warranted, see A Note about Triggering a Delay Unit below.
Use more than one flash unit both to balance the lighting and also to provide more light on the subject. Slave additional units off a master. 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--slaved off the master--to illuminate the background only. Then you can set your camera for the fastest shutter speed that will synch with your master 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.
Triggering a Delay Unit
When using the crossed-beam photogate, a fast response is typically needed in order to capture the subject at or near the intersection of the beams where the camera is prefocused. In the event that the path and speed of the subject are predictable, you may want to focus on a position in front of or behind the intersection of the beams. In such a case, you'll need to delay the flash discharge or actuation of the camera shutter. It's possible to trigger a HiViz delay unit (or the external input of a Multi-Trigger) by connecting the output of the CBP control box to the input of the delay unit.