Timers

You will use digital timers in this course to measure intervals of time. These timers work by counting the oscillations of a quartz resonator, which oscillates at a steady and accurately known rate. Depending on the timer and mode, the resolution of the timer display may be 1 ms, 0.1 ms, or 0.01 ms (and if Glen gets the new ones finished, 1 ms). The timer may report its resolution in units of Hz (1 Hz = 1 oscillation per second). So the clock frequency will be between 1 kHz and 1 MHz.

You may safely assume that the displayed time accurately represents the time between start and stop signals received by the timer to a "1" in the last decimal place. That is, if the timer reports 1.032 s, you may assume that the true value is between 1.031 s and 1.033 s. It can be useful to use your watch to confirm in a rough way the values reported by the digital timer, and to be sure you do not make a factor-of-ten error.

Modes

Timers are usually started and stopped with a photogate or photogates. They may be run in different modes according to the event you are timing.

• In gate mode, the timer begins counting on a high-to-low transition (when the photogate beam is first obscured) and stops counting on the next low-to-high transition (when the beam resumes passing from the source to the detector).

• In pulse mode, the timer begins countin on a high-to-low transition, ignores the next low-to-high transition, and stops counting on the next high-to-low transition.

• In pendulum mode, the timer begins counting on the first high-to-low transition of the photogate, ignores the next low-to-high, high-to-low, and low-to-high transitions, and stops counting on the following high-to-low transition signal.

Updated 6/17/99 by Peter N. Saeta .