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".
Hardware timers = Mechanical 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.
= Electromechanical timers= 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.
An electromechanical 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
drive a shaft at the desired rate, and turn the cam. The most common
application of this timer now is in washers, driers and dishwashers. 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 relays to create electro-mechanical controllers.
Electromechanical timers reached a high state of development in the 1950s and
1960s because of their extensive use in aerospace and weapons systems.
Programmable electromechanical timers controlled launch sequence events in
early rockets and ballistic missiles. As digital electronics has progressed and
dropped in price, electronic timers have become more advantageous.
= Electronic timers= Electronic timers are essentially quartz
clocks with special electronics, and can achieve higher precision than mechanical
timers. Electronic timers have digital electronics, but may have an analog or
digital display. Integrated circuits 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, similar to a watch and usually using the same,
mass-produced, technology. Many 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.
Digital timers are used in safety devices such as a gas timer.
Software timers These types of timers are not devices
nor parts of devices; they exist only in lines of code. They rely on the accuracy
of a clock oscillator usually built into a hardware device that runs the
software. = Software applications=
Nowadays when people are using more and more mobile phones, there are also timer
apps that mimic the old mechanical timer, but which have also highly
sophisticated functions. These apps are also easier to use daily, because they
are available at once, without any need to purchase or carry the separate
devices, as today timer is just a software application on a phone or
tablet. Some of these apps are countdown timers, stopwatch timers, etc. These
timer apps can be used for tracking working or training time, motivating
children to do tasks, replacing an hour glass in board games, or for the
traditional purpose for tracking time when cooking and baking.
Apps may be superior to hour glasses, or to mechanical timers. Hour glasses are
not precise and clear, and they can jam. Mechanical timers lack the customization
that applications support, such as sound volume adjustments for individual needs.
Most applications will also offer selectable alarm sounds.
Some timer applications can help children to understand the concept of
time, help them to finish tasks in time, and help them to get motivated. These
applications are especially used with children with special needs like ADHD,
Down Syndrome, etc., but everybody else can also benefit from them.
= Other types= Computer systems usually have at least
one hardware timer. These are typically digital counters that either increment
or decrement at a fixed frequency, which is often configurable, and which
interrupt the processor when reaching zero. An alternative design uses a
counter with a sufficiently large word size that it will not reach its overflow
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. One specialist use of hardware timers in
computer systems is as watchdog timers, that are designed to perform a hardware
reset of the system if the software fails.
See also Candle-timers
Countdown Drip irrigation
Hourglass Intervalometer
Staircase timer Time switch
Time to digital converter Water clock
References External links
NIST Recommended Practice Guide: Special Publication 960-12 Stopwatch and Timer
Calibrations