These are small beam-break modules, about 30 years old, originally used in (my best guess) vending machines. They are open-collector output, and as such are extraordinarily easy to interface to an Arduino. Simply connect 5V or 3.3V, GND, and the output to a digital input pin on your Arduino. Set the mode of that input pin to INPUT_PULLUP. When the beam is broken, the pin will go HIGH. If the beam is not broken, it will be LOW.

Here's a diagram showing the dimensions and pinout of the board + beam break sensor.

Specs:

• Operating voltage: 1.6V - 5.5V (may be capable of higher voltages, but one of the resistors can get pretty hot at higher voltages)
• Current draw: 22mA @ 3.3v, ~38mA @ 5V
• Pins: 0.1" pitch
• Output mode: open collector (requires a pull-up for "TTL" operation)

I drew the dimensions shown above, and so I can give you a CAD model of the module upon request.

Please note: There are a couple colors of silkscreen that these were made with - yellow (bare FR4?) and green. I will send them randomly.