A photogate sensor is a timing device used for very precise measurements of high-speed or short-duration events. Typical applications include free fall acceleration, the period of a pendulum, or the velocity of a ball rolling down a track. A photogate consists of a light source and a light detector. Whenever an object moves through and blocks the beam of light between the source and the detector, a signal is produced which can be detected by a Vernier interface to start and stop timing operations.

Design Objectives

  • Build a photogate using an infrared phototransistor and LED
  • Construct a simple voltage-divider circuit
  • Use a Vernier Digital Breadboard Cable to make the necessary connections between the photogate circuit and the lab interface
  • Use Logger Pro to time an event


  • infrared LED
  • 3.3 kΩ resistor
  • 220 Ω resistor
  • breadboard
  • jumper wires for breadboard
  • multimeter (optional)

Download Project

Includes construction, software setup, calibration, and troubleshooting instructions.

Download Project

About STEM Sensor

Buid your own sensors (BYOS) give students the opportunity to build their own detectors and measurement devices. Students will assemble an electronic circuit and then use Logger Pro software to convert an electrical signal to proper measurement units.

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