Photogate timers are timing devices used to determine the velocity of a passing object in physics experiments. They are useful for measuring events within milliseconds or any interval that cannot be measured by a human being.
Most photogate timers have a selectable resolution switch that gives the user the ability to select the sensitivity of the timer. Photogate timers are relativity inexpensive and easy to use, requiring standard AC power adapters or battery cells to function. They are usually used in physics labs to conduct experiments involving the measurement of instantaneous versus average velocity, the force of gravity, pendulum motion and the speed of a projectile. The device measures the velocity of objects by using an infrared diode and photocell. Timing occurs when an object interrupts the infrared beam between the diode and photocell. According to Waukesha South High School, photogates have three timing modes including gate, pulse and pendulum mode. The gate mode produces the time interval between beam interruptions to determine the speed of an object. The pulse mode measures the time between the first and second times the beams are broken. The pendulum mode measures the time between the first and third times the beams are broken.