Applications of Physics

Direct Sensing – Part 1Mukesh N. Tekwani

Email: mukeshtekwani@outlook.com

Facebook: www.facebook.com/mukeshnt

CIE – A2 Level, Nov 2009

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 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) 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

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

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?

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.