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 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 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 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 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.