Light Sensors

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Light Sensors

• Prepared by Micah Pack
• ECE 5320 Mechatronics
• Assignment 1

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Outline

• References
• To Explore Further
• Major Applications
• Basic Working Principle
• Typical Sample Configuration in Application
• Major Specifications
• Limitations
• How to Choose
• Cost Information
• Where to Buy

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References

• Mechatronics Handbook edited by Robert Bishop pg.
  20-119 thru 20-136 ISBN 0-8493-9257-8
• Practical Electronics for Inventors by Paul
  Scherz pg. 201 thru 212 ISBN 0-07-058078-2

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To Explore Further

• Mechatronics Handbook edited by Robert Bishop pg.
  20-119 thru 20-136 ISBN 0-8493-9257-8
• Practical Electronics for Inventors by Paul
  Scherz pg. 201 thru 212 ISBN 0-07-058078-2
• Wikipedia search Light detectors

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Major Applications

• Light Detection
• Color Detection
• Distance Sensing
• Motion Sensing
• Digital Image Creation
• Fiber Optics
• Quality Control
• Ect.

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Basic Working Principle

• A light detector converts the radiant power
  it absorbs into a change of a device parameter
  such as resistance, surface charge, current, or
  voltageLight detectors can be divided into two
  main types, thermal or photon devices.

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Thermal Detectors

• Thermal detectors or Pyroelectric detectors use
  a ferroelectric ceramic material as the detection
  medium.
• As radiation falls on the detector its
  temperature rises and the charge on the surface
  changes
• A transient current flows through RL (which has a
  resistance somewhere around 1011 ohms) , and the
  JFET reduces the impedance to R

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Photon Detectors

• Photon Detectors are also separated into two sub
  categories
• Photoresistors
• Junction Detectors

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Photon Detectors

• Photoresistors are made from a semiconducting
  material whose resistance goes down as light
  within a certain wavelength band strikes the
  detector. When the light strikes it frees bound
  electrons creating holes in the material. These
  electrons contribute to the current flowing
  through the device, and lower the resistance of
  the detector.

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Photon Detectors

• Junction Detectors are also sub-categorized into
  two main categories.
• Photodiodes
• Phototransistors

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Photon Detectors

• Photodiodes are built by sandwiching two
  different types of semiconducting material
  together one of the layers being relatively thin.
  The light passes through the thinner layer. The
  light photons will dislodge electrons in the p
  layer and create holes if these electrons are
  close enough to the interface, they will cross
  over into the n region and create a potential
  difference, and thus a current.

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Photon Detectors

• Silicon solar cells are a special type of
  Photodiode with an extra large light collecting
  surface. This large collection surface makes them
  more sensitive to incoming light. They can also
  produce much greater output than normal
  Photodiodes. Typical solar cells can produce as
  much as 0.5 Volts and can supply up to 0.1 Amps
  of current.

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Photon Detectors

• Phototransistors operate in a similar manner to
  photodiodes. The P region is very large for
  collection of photons. The photons free some
  electrons which transfer into the lower N region.
  If the leads are connected, these electrons then
  flow into the upper N region and back into the P
  region thus creating a current.

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Typical Sample Configuration in Application

• Light activated relay using a photoresistor (or
  pyroelectric) detector to trip the relay.

• A solar array (photodiode) used to recharge the
  NiCd batteries.

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Typical Sample Configuration in Application

• A light activated relay using phototransistors
  to switch the relay.

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Major Specifications

• Major specifications for photodiodes include
  reverse voltage, max dark current (the small
  current that flows even when the detector in is
  in the dark), min. light current, power
  dissipation, rise time, and typical detection
  angle.
• Major specifications for phototransistors
  include collector to base voltage, max collector
  current, max dark current , min. light current,
  power dissipation, typical response time.

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Limitations

• Many light detectors are limited in the frequency
  range of the light that they can detect. With
  photoresistors having the largest range in
  general.
• Most light detectors have to have a certain
  minimum light intensity in order to operate.
• Response time varies among detectors

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How to Choose

• The different kinds of light sensors are almost
  endless. The choice of which one to use is based
  on the application for which it is intended.
• The first choice is most commonly the spectral
  response, or the type of light which is being
  detected. Pyroelectric devices have a wide
  spectral response in the visible and infrared
  spectrums, while photon detecting devices
  generally have a narrow spectral response.
• Many companies sell sensor/source combinations
  pre-assembled for use as sensors in automation
  applications.

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Cost Information
Digikey Cost info
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Cost Information

• Digikey Cost info

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Cost Information

• Digikey Cost info

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Cost Information

• Mouser Cost info

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Cost Information

• Mouser Cost info

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Cost Information

• Mouser Cost info

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Where to Buy

• Mouser
• Digikey