Light Sensors

1. Light Sensors Prepared by Micah Pack ECE 5320 Mechatronics Assignment #1

2. 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

3. 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

4. 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

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

6. 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 voltage…Light detectors can be divided into two main types, thermal or photon devices.”

7. 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 1011ohms) , and the JFET reduces the impedance to R

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

9. 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.

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

11. 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.

12. 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.

13. 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.

14. 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.

15. Typical Sample Configuration in Application • A light activated relay using phototransistors to switch the relay.

16. 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.

17. 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

18. 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.

19. Cost Information Digikey Cost info

20. Cost Information Digikey Cost info

21. Cost Information • Digikey Cost info

22. Cost Information • Mouser Cost info

23. Cost Information • Mouser Cost info

24. Cost Information • Mouser Cost info

25. Where to Buy • Mouser • Digikey