The HiViz.com WorkShop
Crossed Beam Photogate Breadboard Kit, unassembled
The kit comes unassembled. Assembly instructions are here.
The view below shows an assembled crossed-beam photogate triggering on the passage of a tufted titmouse. Flash units (not provided) are positioned to either side of the gate.
Click on the photo for a larger view.
The photo below shows the circuit used to detect the passage of an object through the center of the gate.