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Direct Sensing

Electronic Sensors
  • An electronic sensor consists of a sensing device and a circuit that provides an output voltage
  • The information collected by the sensing device is communicated to the output device through the processing unit
  • The changes in the physical property of the sensing device are converted into corresponding changes in voltage by the processing unit
  • The output device responds to variations in voltage
Light Dependent Resistor (LDR)

  • A semi-conducting device whose resistance changes with intensity of light falling on it
  • As intensity of light increases, resistance decreases
  • It can be used to monitor variation in light intensity
  • Variation in resistance with light intensity is non-linear
Thermistor

  • Resistance of a thermistor with negative temperature coefficient decreases rapidly with an increase in its temperature
  • It can be used to monitor variation in temperature
  • Variation in resistance with temperature is non-linear
Strain Gauge

  • It consists of a fine metallic wire of uniform cross-sectional area sealed in a small rectangular piece of plastic
  • When the plastic is stretched, the wire also gets stretched and hence its length increases while the cross-sectional area decrease slightly (consider negligible)
  • This will increase its resistance because R = ρL/A
  • Strain is directly proportional to change in resistance: ΔR ∝ ΔL
Piezo-Electric Transducer
  • A transducer is a device that converts energy from one form to another
  • A piezo-electric crystal (such as quartz) is an example of a transducer that consists of a lattice of negative and positive ions
  • Under normal conditions, the centers of the positive and negative charges coincide
  • When the crystal is stressed, the centers of the positive and negative charges will be displaced causing a voltage to be generated across the crystal – the piezo-electric effect
  • The magnitude of the voltage generated depends on the magnitude of the pressure applied
  • Application in a Simple Microphone:
    • When a sound wave is incident on a quartz crystal, it will be subject to a pressure changing between maximum and minimum alternately
    • It will generate an alternating voltage across the crystal
    • If a transducer is used as a sensing device, a processing unit is not required since variations in its property are directly converted into variations in voltage generated
Processing Unit
  • A processing unit converts variations of resistance of sensing devices into corresponding variations in voltage
  • A potential divider circuit can be used as a processing unit
  • Output voltage is given by;
    • V = R/F + R × E
  • Thus, as R increases, the output voltage also increases
  • The output can be connected across F and the opposite would occur
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