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CIE – A2 Level, Nov 2009




                     Applications of Physics

                      Direct Sensing – Part 1
            Mukesh N. Tekwani

            Email:    mukeshtekwani@outlook.com
            Facebook: www.facebook.com/mukeshnt
Direct Sensing

   A Sensor is something that tells something
    about its environment
   Electronic sensor – Gives information about
    its environment by generating an electrical
    signal.
   This electrical signal must change with
    changes in environment.
       E.g., if sensor measures temp, then electrical
        signal must change with changes in temp
Examples of Sensors

   Sensors in AC to measure room temperature,
    humidity
   Sensor in certain TV sets to measure
    ambient light and adjust brightness / contrast
    accordingly
   Remote control
   Infrared detectors used in motion sensors
   Magnetic proximity sensor
   Sound controlled devices
Electronic Sensor
   Parts of a sensor:
    Sensing                Processing                   Output
     Device                   Unit                      Device

Sensing Device –
Example:
LDR (Light Dependent Resistor) – to measure changes in light
Strain gauge – to measure strain experienced by a material
Important – Some physical property of the sensing device MUST
change so that it can detect changes in whatever is to be monitored.
Electronic Sensor
   Parts of a sensor:
    Sensing                 Processing                    Output
     Device                    Unit                       Device

Processing Unit
Any change in a physical property of the sensor must be processed.
(measured / converted / amplified) so that it can be indicated by an
output device.
Output Device
The output device may be a simple lamp or a digital meter that
indicates a voltage or a current – basically any device that can
respond to a change in voltage.
So: Connect the sensing device to the output device via a processing
unit
Electronic Sensor
Light Dependent Resistor (LDR)
   An LDR is a resistor whose resistance
    changes with intensity of light falling on it.
   Construction: A thin film of cadmium sulphide
    sandwiched between two metal electrodes.
Light Dependent Resistor (LDR)

   LDR is sensitive to changes in light intensity
   BUT – change in resistance with change in
    light is NOT linear.
   Normally the resistance of LDR is very high ~
    100 M (in dark)
   In sunlight, its resistance falls to about 100
    ohms
Characteristics of LDR

   Most LDRs will respond to light of 500 nm
    wavelength (yellow to green in colour)
   What is the mechanism by which an LDR
    changes its resistance with changing light
    levels?
       Photons interact with the CdS molecules
       Photons have sufficient energy to remove
        electrons
       These electrons then allow a current to flow
LDR Characteristics
LDR Characteristics
•The previous graph is not easy to draw.
• We have used a linear scale for variation of light
intensity.
• But this linear scale for light intensity is very large
      Light Source                Illumination
      Moonlight                   0.1 lux
      60 W bulb at 1 m            50
      Fluorescent lamp            500
      Bright sunlight             30,000 lux
LDR Characteristics
   So we draw a graph of resistance vs log(I)
   A log scale does not go from 0, 1, 2, 3, 4, 5…
   A log scale goes like this: 100 , 101 , 102 , 103, ..
LDR
   Limited amount of current can flow else it will
    burn out
LDR
An LDR has a resistance of 15 ohms at a certain very
high light level. What value of protection resistor is
needed if a current of no more than 10 mA is to flow
when the supply voltage is 9.0 V?

   Current through LDR = 10 mA = 0.01 A
   Voltage across LDR = 0.01 A 15 Ω = 0.15 V
   Voltage across protection resistor = 9 – 0.15
    = 8.85 V
   Resistance = 8.85 V 0.01 A = 885 ohms
Potential Divider Circuit




An output voltage Vout is obtained from a junction between the two resistors.
Potential Divider circuit
   If the output current is zero, the current
    flowing through R1 also flows through R2,
    because the resistors are in series.
   So we can use Ohm’s Law to say:
But Vout = IR2


  So, Vout =




So, the output voltage is the same fraction of the input voltage
as R2 is the fraction of the total resistance.
LDR Problem
              What is the
              output
              voltage of
              this
              potential
              divider?
LDR Problem




              4.4 V
LDR

   Will CdS LDR respond to infrared light?
       No. Since infrared light does not have sufficient
        energy, they cannot knock off electrons.

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A2 Direct Sensing

  • 1. CIE – A2 Level, Nov 2009 Applications of Physics Direct Sensing – Part 1 Mukesh N. Tekwani Email: mukeshtekwani@outlook.com Facebook: www.facebook.com/mukeshnt
  • 2. Direct Sensing  A Sensor is something that tells something about its environment  Electronic sensor – Gives information about its environment by generating an electrical signal.  This electrical signal must change with changes in environment.  E.g., if sensor measures temp, then electrical signal must change with changes in temp
  • 3. Examples of Sensors  Sensors in AC to measure room temperature, humidity  Sensor in certain TV sets to measure ambient light and adjust brightness / contrast accordingly  Remote control  Infrared detectors used in motion sensors  Magnetic proximity sensor  Sound controlled devices
  • 4. Electronic Sensor  Parts of a sensor: Sensing Processing Output Device Unit Device Sensing Device – Example: LDR (Light Dependent Resistor) – to measure changes in light Strain gauge – to measure strain experienced by a material Important – Some physical property of the sensing device MUST change so that it can detect changes in whatever is to be monitored.
  • 5. Electronic Sensor  Parts of a sensor: Sensing Processing Output Device Unit Device Processing Unit Any change in a physical property of the sensor must be processed. (measured / converted / amplified) so that it can be indicated by an output device. Output Device The output device may be a simple lamp or a digital meter that indicates a voltage or a current – basically any device that can respond to a change in voltage. So: Connect the sensing device to the output device via a processing unit
  • 7. Light Dependent Resistor (LDR)  An LDR is a resistor whose resistance changes with intensity of light falling on it.  Construction: A thin film of cadmium sulphide sandwiched between two metal electrodes.
  • 8. Light Dependent Resistor (LDR)  LDR is sensitive to changes in light intensity  BUT – change in resistance with change in light is NOT linear.  Normally the resistance of LDR is very high ~ 100 M (in dark)  In sunlight, its resistance falls to about 100 ohms
  • 9.
  • 10. Characteristics of LDR  Most LDRs will respond to light of 500 nm wavelength (yellow to green in colour)  What is the mechanism by which an LDR changes its resistance with changing light levels?  Photons interact with the CdS molecules  Photons have sufficient energy to remove electrons  These electrons then allow a current to flow
  • 12. LDR Characteristics •The previous graph is not easy to draw. • We have used a linear scale for variation of light intensity. • But this linear scale for light intensity is very large Light Source Illumination Moonlight 0.1 lux 60 W bulb at 1 m 50 Fluorescent lamp 500 Bright sunlight 30,000 lux
  • 13. LDR Characteristics  So we draw a graph of resistance vs log(I)  A log scale does not go from 0, 1, 2, 3, 4, 5…  A log scale goes like this: 100 , 101 , 102 , 103, ..
  • 14. LDR  Limited amount of current can flow else it will burn out
  • 15. LDR An LDR has a resistance of 15 ohms at a certain very high light level. What value of protection resistor is needed if a current of no more than 10 mA is to flow when the supply voltage is 9.0 V?  Current through LDR = 10 mA = 0.01 A  Voltage across LDR = 0.01 A 15 Ω = 0.15 V  Voltage across protection resistor = 9 – 0.15 = 8.85 V  Resistance = 8.85 V 0.01 A = 885 ohms
  • 16. Potential Divider Circuit An output voltage Vout is obtained from a junction between the two resistors.
  • 17. Potential Divider circuit  If the output current is zero, the current flowing through R1 also flows through R2, because the resistors are in series.  So we can use Ohm’s Law to say:
  • 18. But Vout = IR2 So, Vout = So, the output voltage is the same fraction of the input voltage as R2 is the fraction of the total resistance.
  • 19. LDR Problem What is the output voltage of this potential divider?
  • 20. LDR Problem 4.4 V
  • 21. LDR  Will CdS LDR respond to infrared light?  No. Since infrared light does not have sufficient energy, they cannot knock off electrons.