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Keycon West 2014

HaaTa - Jacob Alexander

Actuation Sensing

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Actuation

To make (a machine or electrical device) move or operate.

- Merriam-Webster

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But…isn’t that incredibly vague?

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And…isn’t this a talk about keyboards!?

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Don’t worry, I’ll make this obvious

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Keyboard switches are sensors

Which “sense” actuation

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Common Keyboard Switch Types

  • Electrical Contact
    • Cherry MX
    • Most Alps compatible (e.g. Matias)
    • IBM Model M Buckling Spring
    • ...and most switches you can think of

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  • Capacitive
    • Topre
    • IBM Model F Buckling Spring
    • IBM Beam Spring
    • Foam’n’Foil (BTC, Keytronic, etc.)

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Electrical Contact Switches

  • Pretty much everywhere (Cherry MX)
  • Easy to understand

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OFF

ON

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Electrical Contact Switches

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  • Pros
    • Simple
    • Cheap

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  • Cons
    • Requires “debouncing”
    • NKRO requires either diodes or 1xN matrix
    • Susceptible to corrosion

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Aside - Debouncing

  • Metal contacts do not make a perfect connection instantaneously
  • Causes the sense line to bounce on and off before the line settles
  • Debouncing is an issue on press and release

http://madscientisthut.com/wordpress/tag/switch-debouncing/

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Capacitive Switches

  • Common...in touch screens!
  • Understanding requires knowledge of capacitors
  • Senses the distance to a dielectric material
    • e.g. your finger

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Capacitor

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Capacitive Switches - Capacitive Sense Pads

No dielectric present

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Dielectric present

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Capacitive Switches

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  • Pros
    • No diodes
    • Simple
    • Corrosion resistant
    • Cheap

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  • Cons
    • Requires “debouncing”
    • Sensing circuit is moderately complicated (ADC)
    • Susceptible to dirt, humidity and temperature

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Exotic Switch Types - Magnetism

  • Hall Effect Sensor
    • Honeywell
    • Tesla (Soviet Bloc Company - Czech Republic)
  • Magnetic Reed
    • Alps
    • Fujitsu
    • Honeywell
    • Many others (Soviet, German, Mexican, etc.)
  • Magnetic Valve
    • ITW
    • ADI

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Exotic Switch Types - Hall Effect

Sam/dorkvader is doing a presentation on this

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  • Pros
    • Analog
    • Impervious to environment
    • Schmitt triggered - debounced
  • Cons
    • Expensive
    • Requires a moving magnetic field (e.g. magnet)
    • Less effective underwater :P
    • Each switch must have at least 3 pins
      • Vcc,Gnd,Sense

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Exotic Switch Types - Magnetic Reed

  • Uses a magnetic field to connect the two reeds
  • Usually filled with a gas to prevent corrosion
    • And often high voltage
  • Patented in 1941 by W. Ellwood
  • Very common in old Calculators

LiveThinLine

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Exotic Switch Types - Magnetic Reed

  • Pros
    • Resists corrosion
    • Impervious to environment
    • Cheap
  • Cons
    • Requires a moving magnetic field (e.g. magnet)
    • Less effective underwater :P
    • Requires debouncing

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Exotic Switch Types - Magnetic Valve

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Exotic Switch Types - Magnetic Valve

  • Pros
    • Resists corrosion
    • Impervious to environment
    • Cheap
    • Some variants require a magnetic field
  • Cons
    • Some variants require a magnetic field
    • Requires a ferrite
    • Less effective underwater :P
    • Requires debouncing

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Exotic Switch Types - Optical

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  • Single Photoresistor
    • Burroughs

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  • Encoder
    • Burroughs
    • Collimation
    • Many more (but I don’t have them :P)

Ebay pic...

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Exotic Switch Types - Optical

  • Encoder vs. Single Photoresistor
    • Similar to 4x4 vs. 1x16 matrix
    • 1x16 and Single Photoresistor is more expensive
  • Photoresistor changes its resistance depending on the light detected
    • Usually of a specific wavelength, often infrared
  • Simple blockers or complex polarizers

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Exotic Switch Types - Optical

US4379968

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Exotic Switch Types - Optical

  • Pros
    • Little debouncing
    • Conceptually simple
    • No corrosion
    • Simple sense circuit

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  • Cons
    • Requires “debouncing”
    • Susceptible to dust/dirt
    • Light source has a limited life
    • NKRO can be complicated (with encoders)

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Exotic Switch Types - Inductive

  • HP
    • Only one I have
  • Very common, but not common in keyboards
  • Often used instead of pressure sensors for cars at stop lights
  • Complicated to understand
    • Requires knowledge of an “inductive short”

Panasonic Tech Insights

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Exotic Switch Types - Inductive

  • Detects the proximity of metals
  • Different metals (like dielectrics) have different sense levels
    • Uses a pulsed signal to generate a magnetic field
    • The field is then affected by the nearby metal
    • This affects the voltage output of the inductor pad
      • On the pcb

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Exotic Switch Types - Inductive

  • Pros
    • No diodes
    • Cheap
    • Corrosion resistant
    • Impervious to environment
    • Analog

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  • Cons
    • Requires “debouncing”
    • Sensing circuit is moderately complicated (ADC)
    • Sensitivity is defined by size/length of each inductor

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Exotic Switch Types - Acoustic

  • Yep

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Exotic Switch Types - Acoustic

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Exotic Switch Types - Acoustic

  • Uses TDOA - Time Difference of Arrival
  • Each key hits a position on an isolated transfer bar
  • Similar to a guitar string, but faster

US4378552

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Exotic Switch Types - Acoustic

US4376469

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Exotic Switch Types - Acoustic

  • Cons

Complicated mechanically

Sound transfer bar must be isolated

Needs two pulses for press/release

NKRO is very tricky

Patented by Smith-Corona in the early 1980s

  • Pros
    • Was cheap in the 80’s?
    • Corrosion resistant
    • Presses don’t need to be debounced
    • Impervious to environment*

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That’s it!

Questions?

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anything I missed?