Timer

A timer is a specialized type of clock for measuring time intervals. By function timers can be categorized to two main types. A timer which counts upwards from zero for measuring elapsed time is often called a stopwatch; a device which counts down from a specified time interval is more usually called a timer or a countdown timer. A simple example for this type is an hourglass. By working method timers have two main groups: Hardware and Software timers.

Some timers sound an audible indication that the time interval has expired.

Time switches, timing mechanisms which activate a switch, are sometimes also called "timers".

Mechanical timers 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 by Thomas Norman Hicks. Some less accurate, cheaper mechanisms use a flat paddle 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, 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. The most common use of this type is in the "flasher" units that flash turn signals in automobiles, and sometimes in Christmas lights. This is a non-electronic type of multivibrator.

...
Wikipedia