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CIE – A2 Level, Nov 2009
Applications of Physics
Direct Sensing – Part 1
Mukesh N. Tekwani
[email protected]
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

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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
Device
Processing
Unit
Output
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
Device
Processing
Unit
Output
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)


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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
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
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?
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

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
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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?
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Current through LDR = 10 mA = 0.01 A
Voltage across LDR = 0.01 A × 15 V = 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
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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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