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Keypads for the entry of [[personal identification number|PIN]]s and for product selection appear on many devices including ATMs, vending machines, [[point of sale]] payment devices, time clocks, combination locks and digital door locks.
==Keypad technologies==
Apart from mechanical keypads,<ref>{{cite web|url=https://switchandclick.com/best-mechanical-numpads-of-2020/|title=Mechanical keypad|access-date=2023-12-17}}</ref><ref>{{cite web|url=https://voltcave.com/programmable-keypads/|title=Mechanical keypads|access-date=2023-12-17}}</ref><ref>{{cite web|url=https://www.screwfix.com/c/security-ironmongery/mechanical-push-button/cat810342?cm_sp=managedredirect-_-ironmongery-_-pushbuttonlock|title=Mechanical push-button keypad|access-date=2023-12-17}}</ref> there are a wide range of technologies that can be used as keypads, each with distinctive advantages and disadvantages. These include Resistive,<ref>{{cite web|url=https://www.farnell.com/datasheets/523847.pdf|title=Resistive membrane keypad|access-date=2023-12-19}}</ref> Capacitive,<ref>{{cite web|url=https://www.electricity-magnetism.org/capacitive-keypads/|title=Capacitive keypads|access-date=2023-12-17}}</ref> Inductive,<ref>{{cite web|url=https://www.ti.com/lit/ug/tiducr1/tiducr1.pdf?ts=1634269041396&ref_url=https%253A%252F%252Fwww.google.com%252F|title=Inductive keypads|access-date=2023-12-17}}</ref>
Piezoelectric,<ref>{{cite web|url=https://nelson-miller.com/piezo-switches-an-introduction-to-these-touch-switches/|title=Piezo keypads|access-date=2023-12-17}}</ref> and Optical.<ref>{{cite web|url=https://www.ign.com/articles/the-best-optical-keyboards-for-gaming|title=Optical keypads|access-date=2023-12-17}}</ref>
== Key layout ==
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The invention of the [[push-button telephone]] keypad is attributed to [[John Karlin|John E. Karlin]], an industrial psychologist at [[Bell Labs]] in Murray Hill, New Jersey.<ref name="NYTObit">{{cite news |title=John E. Karlin, Who Led the Way to All-Digit Dialing, Dies at 94 |date=2013-02-08 |access-date=2013-02-09 |newspaper=[[The New York Times]] |url=https://www.nytimes.com/2013/02/09/business/john-e-karlin-who-led-the-way-to-all-digit-dialing-dies-at-94.html |author-first=Margalit |author-last=Fox}}</ref><ref>{{cite news |title=Monmouth man, inventor of touch-tone keypad, dies at 94 |newspaper=The Star-Ledger |date=2013-02-09 |url=http://www.nj.com/news/index.ssf/2013/02/monmouth_man_inventor_of_touch.html |archive-url=https://web.archive.org/web/20130213085246/http://www.nj.com/news/index.ssf/2013/02/monmouth_man_inventor_of_touch.html |url-status=dead |archive-date=2013-02-13 |access-date=2013-02-09}}</ref> On a [[telephone keypad]], the numbers 1 through 9 are arranged from left to right, top to bottom with 0 in a row below 789 and in the center. Telephone keypads also have the special buttons labelled * ([[asterisk|star]]) and # (octothorpe, [[number sign]], "pound", "hex" or "hash") on either side of the zero key. The keys on a [[telephone]] may also bear letters which have had several auxiliary uses, such as remembering [[area code]]s or whole telephone numbers.
The layout of calculators and telephone number pads diverged because they developed at around the same time. The phone layout was determined to be fastest by Bell Labs testing for that application, and at the time it controlled all the publicly connected telephones in the United States.
{{further|Telephone keypad#Layout}}
== Origin of the order difference ==
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=== Separate connections ===
A mechanically-switched 16-key keypad can be connected to a host through 16 separate connecting [[lead (electronics)|lead]]s, plus a [[ground (electricity)|ground]] lead (Figure 1, left). Pressing a key will [[short circuit|short]] to ground, which is detected by the host. This design allows any number or combination of keys can be pressed simultaneously. [[Shift register#Parallel-in serial-out (PISO)|Parallel-in serial-out shift register]]s may be used to [[Shift register#More I/O pins|save I/O pin]]s.
=== X/Y multiplexing ===
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=== Charlieplexing ===
{{Main
8 leads can detect many more keys if [[tri-state multiplexing]] (Figure 1, right) is used instead, which enables (''n''-1) × (''n''/2) keys to be detected with just ''n'' I/O lines. 8 I/O can detect 28 individual keys without ambiguity. Issues can occur with some combinations if two keys are pressed simultaneously. If diodes are used, then the number of unique keys detectable is doubled.<ref>{{cite web |url=https://worldwide.espacenet.com/patent/search/family/054064687/publication/GB2540560A?q=pn%3DGB2540560A |title=Touch sensor |access-date=2023-08-04}}</ref>
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* [[Keyboard matrix circuit]]
* [[Keyboard technology]]
* [[Key rollover]]
* [[Mobile phone]]
* [[Numeric keypad]]
* [[Push-button telephone]]
* [[Rotary dial]]
* [[Silicone rubber keypad]]
* [[Telephone keypad]]
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