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The timer may switch equipment on, off, or both, at a preset time or times, after a preset interval, or cyclically. A countdown time switch switches power, usually off, after a preset time. A cyclical timer switches equipment both on and off at preset times over a period, then repeats the cycle; the period is usually 24 hours or 7 days.
The Intel 8253 PIT was the original timing device used on IBM PC compatibles.It used a 1.193182 MHz clock signal (one third of the color burst frequency used by NTSC, one twelfth of the system clock crystal oscillator, [1] therefore one quarter of the 4.77 MHz CPU clock) and contains three timers.
A typical kitchen timer. A timer or countdown timer is a type of clock that starts from a specified time duration and stops upon reaching 00:00. An example of a simple timer is an hourglass. Commonly, a timer triggers an alarm when it ends. A timer can be implemented through hardware or software.
In addition to the converters, many embedded microprocessors include a variety of timers as well. One of the most common types of timers is the programmable interval timer (PIT). A PIT may either count down from some value to zero, or up to the capacity of the count register, overflowing to zero.
It also has a timer that signals the electronic control unit after a set period of inactivity. The control unit uses this signal to activate the switch/relay to turn equipment on or off. For lighting applications, there are three main sensor types: passive infrared, ultrasonic, [8] and hybrid.
Most cam timers use a miniature mains synchronous motor to rotate the mechanism at an accurate constant speed. Occasionally more complex timers with two motors are seen. A drum sequencer is a re-programmable electromechanical timing device used to activate electric switches in repetitive sequences.
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