The optointerrupter consists of a light emitting diode (LED) and a phototransistor with a 3.18 mm (1/8″) slot between them.

It comes with the following parts:

• 3 ft of hook-up wire (1 piece or precut pieces)
• Flat opaque disk with four blades (you need to cut it out)
• Resistor 270 Ohm (used to limit the current through the LED)
• Resistor 4.7 K (to be connected between the emitter of the phototransistor and ground/negative wire)
• 2 sequins