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1 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.

2 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.

3 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 systemAn embedded system is a special-purpose computer system usually built into a smaller device. An embedded system is required to meet very different requirements than a general-purpose personal computer. Examples of embedded systems automatic teller machine, similar to a watchThis page is about timekeeping devices. In naval parlance, watches are a timekeeping convention. The term in general use can mean any period of duty or responsibility, such as a hurricane watch. A watch is a small portable clock that displays the current. Watch technology is used in these devices.

However, most timers are now implemented in softwareAn embedded system is a special-purpose computer system usually built into a smaller device. An embedded system is required to meet very different requirements than a general-purpose personal computer. Examples of embedded systems automatic teller machine. Modern controllers use a programmable logic controllerPLC A programmable logic controller PLC or programmable controller is a small computer used for automation of real-world processes, such as control of machinery on factory assembly lines. Where older automated systems would use hundreds or thousands of re 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 logicLadder logic is a method of drawing electrical logic schematics. They are used widely in industrial PLC applications, where a series of complex logic checks are required before a signal is issued. Ladder logic is used when programming languages such as C,. 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.