Transducers and Sensors I

1. Transducers and Sensors I Friday, March 18, 2005 ISAT 253

2. Learning Objectives • Know each element and its primary function in a measurement system • Differentiate between a sensor (detector) and a transducer • Identify signal domains as electrical or nonelectrical • Give an example of a common sensor/transducer system and describe how it works • Give examples of several sensor/transducers and the chemical and physical principles on which they are based • Be able to calculate resistance, stress, strain, and other common sensor measurements ISAT 253

3. Key Concepts List Transducer Sensor Thermocouple RTD Thermistor Strain gage Photoconductive cells Photodiode ISAT 253

4. Definitions Transducer: a device that changes (transduces) the signal to or from an electrical domain as a voltage or current. Sensor: also called detector, a device that senses a physical or chemical stimulus and converts it into a signal that can be electrical, mechanical, or optical. ISAT 253

5. The Sensors We’re Born With! • stimulus and signal? • sensitivity? • detection limit? ISAT 253

6. The Sensors We’ll Study • Optical sensors • Thermal sensors • Chemical sensors • Strain gage sensors ISAT 253

7. Sensors and Data Domains • Dunn identifies data domains as: • Chemical • Electrical • Magnetic • Mechanical • Radiant • Thermal • Need to add: • Social ISAT 253

8. Optical Sensors • respond to ultraviolet, visible, and infrared light • non-electrical sensor: photographic film • photon transducers • Photoconductive cells • Photodiodes ISAT 253

9. Wavelength range for various optical detectors ISAT 253

10. hf semi- conductor I Vout Photoconductive Light Detectors Photoconductive light detectors are made from semiconductor materials (commonly, Pb and Cd sulfides or selenides). The figure shows a piece of semiconductor with two wires attached.

11. hf semi- conductor I Vout Photoconductive Light Detectors If photons have a high enough energy (Ephoton= hf), they will release electrons from their chemical bonds, making them free to move. This will reduce the resistivity (r) of the semiconductor.

12. hf semi- conductor I Vout Photoconductive Light Detectors Since Rsemiconductor = rl/A , the resistance of the semiconductor sample will decrease when r decreases.

13. Photoconductive Light Detectors A typical photoconductive cell: (a) cutaway view (b) symbol.

14. Photoconductive Light Detectors Characteristics of a typical CdSe photoconductive cell: (a) resistance versus illuminance, and (b) spectral response.

15. Applications for Photoconductive cells • Light meter in camera • Daylight sensor • Elevator safety stop • Garage door safety stop • Lots of other applications – look for them! ISAT 253

16. Photodiodes: Junctions and Depletion Layers “reverse bias” Apply a voltage to the pn junction to cause a depletion layer to form Electrical current ONLY flows when light strikes the diode with sufficient energy to release electrons in the depletion layer ISAT 253

17. Photodiodes vs Photoconductive Cells • Photodiodes convert light energy (infrared, visible, or ultraviolet) into electrical energy. • Photoconductive light detectors do not. This is because: • Photodiodes have a built-in electric field that pushes around the charges that are released from their chemical bonds by light. • Photoconductive light detectors do not.

18. anode (metal) antireflection coating p-type silicon p-n junction n-type silicon cathode (metal) Photodiodes • The structure of a silicon photodiode (cross section, not to scale).

19. Can manufacture in arrays for imaging applications + p V I - n (a) (b) Photodiodes • (a) Typical silicon photodiodes, and (b) symbol.

20. Electric Current Produced Optical Power Photodiodes • Photodiodes are usually characterized by giving their responsivity. This quantity is also sometimes called sensitivity. • The responsivity gives the ratio of the electric current produced by the photodiode to the optical power incident on the device. • Responsivity = Â= Rl=

21. Photodiode Spectral Responsivity 0.7 0.6 0.5 0.4 Responsivity (A/W) 0.3 0.2 0.1 0 300 400 500 600 700 800 900 1000 1100 1200 Wavelength (nm) Photodiodes • The responsivity of a photodiode depends on wavelength. • This dependence is shown for a typical silicon photodiode.

22. B: CdS photoconductive cell D: CdSe photoconductive cell F: Silicon photodiode G: PbS photoconductive cell H: Thermocouple Which sensor is best for ultraviolet light? Which one gives best response to 1100 nm wavelength light?

23. Photodiodes : An Application • A square array of InSb photodiodes is shown. The array consists 128 ´ 128 (= 16,384) photodiodes. The individual photodiodes are too small to be seen in this figure. Since these photodiodes can detect wavelengths up to 5400 nm = 5.4 mm, such an array can be used as an infrared-sensitive camera for night vision. Arrays of light detectors (photodiodes, photoconductors, or photocapacitors) can be made from a variety of materials. Silicon camera arrays consisting of 4096 ´ 4096 = 16.8 ´ 106 detectors, or more, are available.

24. Photodiodes: Another Application Light source Smoke detector Photodiode location Images from howstuffworks.com ISAT 253