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Concept
Timer
Summary
A timer is a specialized type of clock used for measuring specific time intervals.
Timers can be categorized into two main types.
The word "timer" is usually reserved for devices that count down from a specified time interval called a countdown timer, while devices that do the opposite, measuring elapsed time by counting upwards from zero, are called stopwatches. A simple example of the first type is an hourglass. Working method timers have two main groups: hardware and software timers.
Most timers give an indication that the time interval that had been set has expired - such as a loud noise.
Time switches, timing mechanisms that activate a switch, are sometimes also called "timers."
Mechanical use clockwork to measure time. Manual timers are typically set by turning a dial to the time interval desired, turning the dial stores energy in a mainspring to run the mechanism. They function similarly to a mechanical alarm clock, the energy in the mainspring causes a balance wheel to rotate back and forth. Each swing of the wheel releases the gear train to move forward by a small fixed amount, causing the dial to move steadily backward until it reaches zero when a lever arm strikes a bell. The mechanical kitchen timer was invented in 1926 called a fan fly that spins against air resistance, low-precision mechanical egg-timers are sometimes of this type.
The simplest and oldest type of mechanical timer is the hourglass - which is also known as "the glass of the hour" - in which a fixed amount of sand drains through a narrow opening from one chamber to another to measure a time interval.
Short-period bimetallic electromechanical timers use a thermal mechanism, with a metal finger made of strips of two metals with different rates of thermal expansion sandwiched together, steel and bronze are common. An electric current flowing through this finger causes heating of the metals, one side expands less than the other, and an electrical contact on the end of the finger moves away from or towards an electrical switch contact.
Official source
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