Photogates

A photogate is a sensor system that uses a light source and a photodetector to establish a beam across a path. When an object interrupts the beam, the resulting change in light reaching the detector is converted into an electrical signal used to trigger timing events. Photogates are widely used in physics and engineering experiments to measure short time intervals as an object passes through one or more gates.

Most photogates consist of a transmitter, typically an infrared LED, and a receiver such as a phototransistor or photodiode. Some devices include a second beam to create two gates for timing. They may be standalone with built-in timing functions or connect to data acquisition systems, computers, or microcontrollers via USB, serial, or analog outputs. Sensitivity and gate thresholds are often adjustable, and some models offer options for reflective configurations.

Operation involves establishing a baseline when the beam is intact; when interrupted, the timer records the event. With dual gates, the time between interruptions yields velocity or time-of-flight measurements. The devices can resolve times from milliseconds to microseconds depending on the model and the response of its optical components.

Applications and limitations: Photogates are common in education labs for measuring speed, acceleration, pendulum periods, free fall, and collisions. In industry, they serve as non-contact sensors and safety interlocks on conveyors and machinery. Limitations include dependence on clear line-of-sight, sensitivity to ambient light, alignment requirements, and finite response time. Proper shielding and calibration help mitigate interference.