CIE – A2 Level, Nov 2009 Applications of Physics Direct Sensing – Part 1 Mukesh N. Tekwani Email: firstname.lastname@example.org 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 DeviceSensing Device –Example:LDR (Light Dependent Resistor) – to measure changes in lightStrain gauge – to measure strain experienced by a materialImportant – Some physical property of the sensing device MUSTchange so that it can detect changes in whatever is to be monitored.
Electronic Sensor Parts of a sensor: Sensing Processing Output Device Unit DeviceProcessing UnitAny change in a physical property of the sensor must be processed.(measured / converted / amplified) so that it can be indicated by anoutput device.Output DeviceThe output device may be a simple lamp or a digital meter thatindicates a voltage or a current – basically any device that canrespond to a change in voltage.So: Connect the sensing device to the output device via a processingunit
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•The previous graph is not easy to draw.• We have used a linear scale for variation of lightintensity.• 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
LDRAn LDR has a resistance of 15 ohms at a certain veryhigh light level. What value of protection resistor isneeded if a current of no more than 10 mA is to flowwhen 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 CircuitAn 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 voltageas R2 is the fraction of the total resistance.
LDR Problem What is the output voltage of this potential divider?