Various key switches in keyboard:
Types of Key switches:
1. Capacitive switch
2. Opto –electronic switch
3. Membrane switch
4. Mechanical switch
5. Rubber Dome switch
1. Capacitive switch:
In Capacitive Key switch two plates of capacitor are used, when key is pressed plates comes closer and key released plates move away. The capacitance of the switch is changed and this change can be detected by measuring the voltage change across the switch using some sense amplifier. Sense amplifier measures the voltage change across the switch, Receive one voltage when switch is in open position and another voltage when the switch is in closed position. These voltages are converted into proper logic signal (keyboard circuitry) to inform CPU. Lifespan of this switch is about 20 million keystrokes.
2. Opto – Electronic Switch:
In Opto- Electronic switch, LED and Photo transistors are used. LED generates light when proper electric power is applied and opposite to LED photo transistor is used. Photo transistor allows the current as long as light is applied to it, if light falling to photo transistor is removed then current does not flow through it. When key is not pressed,light falls onto photo transistor, so the current flows through the photo transistor and produces very low voltage at the output V out.When key is depressed light emitted from LED is blocked which stops current flow through photo transistor, and a different value is produced at the output V out.
3. Membrane Switch
It is multi layer plastic or rubber assembly where two rubber or plastic sheet are used as row and column conductor sheet. Row and column sheet having lines are made up of silver or some other conductor ink. These row and column sheet is separated by another sheet with holes at key top position.
When Key is pressed, it forces the row conductor sheet through the hole to touch the column conductor sheet, Row conductor lines when touches with column conductor lines, a key contact is made.
Thus the keyboard interface interprets it as key pressed condition.
4. Mechanical Switch
Two metal pieces or contacts are used and are kept in open position and moved into closed position when the switch is depressed.When key is not pressed (normal position) contact is in open position and
when key is depressed contact is in closed position. This contact is sensed by keyboard interface at the location „X‟. To improve lifespan of switch gold plating done on this contact. Life of this type of switch is around 1 million keystroke.
5. Rubber dome switch
Dome-switch keyboards are a hybrid of flat-panel membrane and mechanical keyboards.It uses small, flexible rubber domes, each with a hard carbon center.When a key is pressed, a plunger (spring on the bottom of the key) pushes down against the dome, and the carbon center presses against a hard, flat surface beneath the key matrix. As long as the key is held, the carbon center completes the circuit.When
the key is released, the rubber dome springs back to its original shape, forcing the key back up to its at-rest position.
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