Timer
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A timer is a specialized type of clock. A timer can be used to control the sequence of an event or process. Timers can be mechanical, electromechanical, digital, or even software, since most computers have clocks.
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Mechanical Timers
Early mechanical timers used typical clockwork mechanisms, such as a escapement and spring to regulate their speed. Inaccurate, cheap mechanisms use a flat beater that spins against air resistance. Mechanical egg-timers are usually of this type.
More accurate mechanisms resemble small alarm clocks. The chief advantage of these is that they require no battery, and can be stored for long periods of time. The most widely-known application is to control explosives.
Electromechanical timers
Electromechanical timers have two common types. A thermal type has a metal finger made of two metals with different rates of thermal expansion (steel and bronze are common). An electric current flows through this finger, and heats it. One side expands less than the other, and an electrical contact on the end of the finger moves away from an electrical switch contact, or makes a contact (both types exist). The most common use of this type is now in the "flasher" units that flash turn signals in automobiles, or sometimes in christmas lights.
Another type of electromechanical timer (a cam timer) uses a small synchronous AC motor turning a cam against a comb of switch contacts. The AC motor is turned at an accurate rate by the alternating current, which power companies carefully regulate. Gears slow this motor down to the desired rate, and turn the cam. The most common application of this timer now is in washers, driers and dish-washers. This type of timer often has a friction clutch between the gear train and the cam, so that the cam can be turned to reset the time.
Electromechanical timers survive in these applications because mechanical switch contacts are still less expensive than the semiconductor devices needed to control powerful lights, motors and heaters.
In the past these electromechanical timers were often combined with electrical relays to create electro-mechanical controllers. Electromechanical timers reached a high state of development in the 1950s and 60s because of their extensive use in aerospace and weapons systems. Programmable electromechanical timers controlled launch sequence events in early rockets and ballistic missiles.
Digital Timers
Digital timers can achieve higher precision than mechanical timers because they are quartz clocks with special electronics. Integrated circuits have made digital logic so inexpensive that an electronic digital timer is now less expensive than many mechanical and electromechanical timers. Individual timers are implemented as a simple single-chip computer system, similar to a watch. Watch technology is used in these devices.
However, most timers are now implemented in software. Modern controllers use a programmable logic controller rather than a box full of electromechanical parts. The logic is usually designed as if it were relays, using a special computer language called ladder logic. In PLCs, timers are usually simulated by the software built into the controller. Each timer is just an entry in a table maintained by the software.
Computer timers
Most computer systems have one to sixteen electronic timers. These are usually just digital counters that are set to a number by software, and then count down to zero. When they reach zero, they interrupt the computer.
Another common form of timer is a number that is compared to a counter. This is somewhat harder to program, but can be used to measure events or control motors (using a class D digital electronic amplifier to perform pulse width modulation).
Embedded systems often use a hardware timer to implement a list of software timers. Basically, the hardware timer is set to expire at the time of the next software timer of a list of software timers. The hardware timer's interrupt software handles the house-keeping of notifying the rest of the software, finding the next software timer to expire, and resetting the hardware timer to the next software timer's expiration.