Timer
Encyclopedia
Clock
A clock is an instrument used to indicate, keep, and co-ordinate time. The word clock is derived ultimately from the Celtic words clagan and clocca meaning "bell". A silent instrument missing such a mechanism has traditionally been known as a timepiece...
. A timer can be used to control the sequence of an event or process. Whereas a stopwatch
Stopwatch
A stopwatch is a handheld timepiece designed to measure the amount of time elapsed from a particular time when activated to when the piece is deactivated. A large digital version of a stopwatch designed for viewing at a distance, as in a sports stadium, is called a stopclock.The timing functions...
counts upwards from zero for measuring elapsed time, a timer counts down from a specified time interval, like an hourglass
Hourglass
An hourglass measures the passage of a few minutes or an hour of time. It has two connected vertical glass bulbs allowing a regulated trickle of material from the top to the bottom. Once the top bulb is empty, it can be inverted to begin timing again. The name hourglass comes from historically...
.
Timers can be mechanical
Clockwork
A clockwork is the inner workings of either a mechanical clock or a device that operates in a similar fashion. Specifically, the term refers to a mechanical device utilizing a complex series of gears....
, electromechanical, electronic
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
(quartz), or even software as all modern computer
Computer
A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...
s include digital timers of one kind or another. When the set period expires some timers simply indicate so (e.g., by an audible signal), while others operate electrical switches, such as a time switch
Time switch
A time switch is a timer that plugs into an electric socket, between the socket itself and a power plug. This physical arrangement allows the connected device to automatically receive power for a desired duration of time...
, which cuts electrical power.
Mechanical timers
Mechanical timers regulate their speed. Inaccurate, cheap mechanisms use a flat beater that spins against air resistance. Mechanical egg-timersEgg timer
An egg-timer is a device the primary function of which is to assist in timing the cooking of eggs. Early designs simply counted down for a specific period of time. Some modern designs are able to time more accurately by depending on water temperature rather than an absolute time...
are sometimes of this type.
More accurate mechanisms have mechanisms similar to mechanical alarm clock
Alarm clock
An alarm clock is a clock that is designed to make a loud sound at a specific time. The primary use of these clocks is to awaken people from their night's sleep or short naps; they are sometimes used for other reminders as well. To stop the sound, a button or handle on the clock is pressed; but...
s; they require no power, and can be stored for long periods of time. The most widely-known application is to control explosives
Time bomb
A time bomb is a bomb whose detonation is triggered by a timer. The use time bombs has been for various purposes ranging from insurance fraud to warfare to assassination; however, the most common use has been for politically-motivated terrorism.-Construction:The explosive charge is the main...
.
Electromechanical timers
Short-period bimetallicBimetallic
Bimetallic may refer to:*Bimetallism, a monetary standard in economics*Bimetallic strip, a temperature sensitive mechanical device*Binary alloy, in metallurgy, a mixture of two metals*Bimetallic coins...
electromechanical timers use a thermal mechanism, with a metal finger made of strips of two metals with different rates of thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
sandwiched together; steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
and bronze
Bronze
Bronze is a metal alloy consisting primarily of copper, usually with tin as the main additive. It is hard and brittle, and it was particularly significant in antiquity, so much so that the Bronze Age was named after the metal...
are common. An electric current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
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 automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...
s, and sometimes in Christmas lights. This is a non-electronic type of multivibrator
Multivibrator
A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices cross-coupled by resistors or capacitors...
.
An electromechanical cam timer
Cam timer
A cam timer is an electromechanical system for controlling a sequence of events automatically.-Description:An electric motor drives, through a reduction gearbox, a shaft on which is arranged a series of cams. Associated with each cam is one or more switches. The motor rotates at a fixed speed, and...
uses a small synchronous AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
motor
Electric motor
An electric motor converts electrical energy into mechanical energy.Most electric motors operate through the interaction of magnetic fields and current-carrying conductors to generate force...
turning a cam
Cam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...
against a comb of switch
Switch
In electronics, a switch is an electrical component that can break an electrical circuit, interrupting the current or diverting it from one conductor to another....
contacts. The AC motor is turned at an accurate rate by the alternating current, which power companies carefully regulate. Gear
Gear
A gear is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine....
s drive a shaft at the desired rate, and turn the cam. The most common application of this timer now is in washer
Washing machine
A washing machine is a machine designed to wash laundry, such as clothing, towels and sheets...
s, driers
Clothes dryer
A clothes dryer or tumble dryer is a household appliance that is used to remove moisture from a load of clothing and other textiles, generally shortly after they are cleaned in a washing machine....
and dishwasher
Dishwasher
A dishwasher is a mechanical device for cleaning dishes and eating utensils. Dishwashers can be found in restaurants and private homes.Unlike manual dishwashing, which relies largely on physical scrubbing to remove soiling, the mechanical dishwasher cleans by spraying hot water, typically between ...
s. 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 may still be 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 relay
Relay
A relay is an electrically operated switch. Many relays use an electromagnet to operate a switching mechanism mechanically, but other operating principles are also used. Relays are used where it is necessary to control a circuit by a low-power signal , or where several circuits must be controlled...
s 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 rocket
Rocket
A rocket is a missile, spacecraft, aircraft or other vehicle which obtains thrust from a rocket engine. In all rockets, the exhaust is formed entirely from propellants carried within the rocket before use. Rocket engines work by action and reaction...
s and ballistic missile
Missile
Though a missile may be any thrown or launched object, it colloquially almost always refers to a self-propelled guided weapon system.-Etymology:The word missile comes from the Latin verb mittere, meaning "to send"...
s. As digital electronics has progressed and dropped in price, electronic timers have become more advantageous.
Electronic timers
Electronic timers are essentially quartz clockQuartz clock
A quartz clock is a clock that uses an electronic oscillator that is regulated by a quartz crystal to keep time. This crystal oscillator creates a signal with very precise frequency, so that quartz clocks are at least an order of magnitude more accurate than good mechanical clocks...
s with special electronics, and can achieve higher precision than mechanical timers. Electronic timers have digital electronics, but may have an analog
Analogue electronics
Analogue electronics are electronic systems with a continuously variable signal, in contrast to digital electronics where signals usually take only two different levels. The term "analogue" describes the proportional relationship between a signal and a voltage or current that represents the signal...
or digital display. Integrated circuit
Integrated circuit
An integrated circuit or monolithic integrated circuit is an electronic circuit manufactured by the patterned diffusion of trace elements into the surface of a thin substrate of semiconductor material...
s have made digital logic so inexpensive that an electronic timer is now less expensive than many mechanical and electromechanical timers. Individual timers are implemented as a simple single-chip computer system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
, similar to a watch
Watch
A watch is a small timepiece, typically worn either on the wrist or attached on a chain and carried in a pocket, with wristwatches being the most common type of watch used today. They evolved in the 17th century from spring powered clocks, which appeared in the 15th century. The first watches were...
and usually using the same, mass-produced
Mass production
Mass production is the production of large amounts of standardized products, including and especially on assembly lines...
, technology.
Many timers are now implemented in software
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
. Modern controllers use a programmable logic controller
Programmable logic controller
A programmable logic controller or programmable controller is a digital computer used for automation of electromechanical processes, such as control of machinery on factory assembly lines, amusement rides, or light fixtures. PLCs are used in many industries and machines...
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
Ladder logic
Ladder logic is a programming language that represents a program by a graphical diagram based on the circuit diagrams of relay logic hardware. It is primarily used to develop software for programmable logic controllers used in industrial control applications...
. 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.
Digital timers are used in safety device such as a gas timer.
Computer timers
Computer systems usually have at least one timer. These are typically digital counters that either increment or decrement at a fixed frequencyFrequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, which is often configurable, and that interrupt
Interrupt
In computing, an interrupt is an asynchronous signal indicating the need for attention or a synchronous event in software indicating the need for a change in execution....
the processor when reaching zero, or a counter with a sufficiently large word size that it will not reach its counter limit before the end of life of the system.
More sophisticated timers may have comparison logic to compare the timer value against a specific value, set by software, that triggers some action when the timer value matches the preset value. This might be used, for example, to measure events or generate pulse width modulated waveforms to control the speed of motors (using a class D digital electronic amplifier
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
).
As the number of hardware timers in a computer system or processor is finite and limited, operating system
Operating system
An operating system is a set of programs that manage computer hardware resources and provide common services for application software. The operating system is the most important type of system software in a computer system...
s and embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...
s often use a single hardware timer to implement an extensible set of software timers. In this scenario, the hardware timer's interrupt service routine would handle house-keeping and management of as many software timers as are required, and the hardware timer would be set to expire when the next software timer is due to expire. At expiry, the interrupt routine would update the hardware timer to expire when the next software timer is due, and any actions would be triggered for the software timers that had just expired. Expired timers that are continuous would also be reset to a new expiry time based on their timer interval, and one-shot timers would be disabled or removed from the set of timers. While simple in concept, care must be taken with software timer implementation if issues such as timer drift and delayed interrupts is to be minimised.
See also
- Candle-timers
- Drip irrigationDrip irrigationDrip irrigation, also known as trickle irrigation or microirrigation or localized irrigation , is an irrigation method which saves water and fertilizer by allowing water to drip slowly to the roots of plants, either onto the soil surface or directly onto the root zone, through a network of valves,...
- IntervalometerIntervalometerAn intervalometer is a device which counts intervals of time. . Such devices commonly are used to signal, in accurate time intervals, the operation of some other device...
- Time to digital converterTime to digital converterIn electronic instrumentation and signal processing, a time to digital converter is a device for converting a signal of sporadic pulses into a digital representation of their time indices. In other words, a TDC outputs the time of arrival for each incoming pulse...
- Water clockWater clockA water clock or clepsydra is any timepiece in which time is measured by the regulated flow of liquid into or out from a vessel where the amount is then measured.Water clocks, along with sundials, are likely to be the oldest time-measuring instruments, with the only exceptions...
- CountdownCountdownA countdown is a sequence of counting backward to indicate the seconds, days, or other time units remaining before an event occurs or a deadline expires. Typical events for which a countdown is used include the launch of a rocket or spacecraft, the detonation of a bomb, the start of a race, and the...