The photogate is a timing device which is useful for measuring events which happen faster than you can time by hand.  It is also useful in determining the speed of many objects.

The photogate consists of an infrared diode and a photocell.  Timing occurs when the infrared beam between the diode and photocell is interrupted.  You can’t see the beam, but it is there.  The numbers in the display show the time of the event in seconds.

Timing Modes

• The GATE mode produces the time interval the beam was broken.  You use this mode to find out how long it took something to move through the beam.  Very useful in determining speed.
• The PULSE mode measures the time between the beam being broken until it is broken a second time. The first break starts the timer, the second break stops the timer.
• The PENDULUM mode measures the time between the beam being broken and the third time the beam is broken.  The first break starts the timer, the second break is ignored, the third break stops the timer.

Using the Photogate

To determine speed

• Set the timer in the GATE mode.
• Set-up the photogate so that the object passes through the beam at right angles to the beam.
• Measure the width of the object which is breaking the beam.  This object must have definite edges and pass through the beam at right angles.
• Since speed is distance/time, divide the width of the object by the time on the photogate to get the speed.

Measuring elapsed time of travel

• Use the photogate as a semi-automatic stop watch.
• Set the timer in the PULSE mode.
• Set-up the photogate so that the object will pass through the beam at right angles to the beam.  The timer should be at the end point of the motion.
• Press the START button on the timer simultaneously with the start of the object’s motion.  This will start the timer.
• The timer will stop when the object enters the beam.
• This method does involve some error since you are starting the timer manually, but it is better than just using a stop watch.