light sensor

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Introduction : 

Introduction To be useful, systems must interact with their environment. To do this they use sensors and actuators Sensors and actuators are examples of transducers A transducer is a device that convertsone physical quantity into another examples include: a mercury-in-glass thermometer (converts temperature into displacement of a column of mercury) a microphone (converts sound into an electrical signal). We will look at sensors in this lecture





Light energy : 

Light energy For a sensor, we’re interested in the light power that falls on a unit area, and how well the sensor converts that into a signal. A common unit is the lux which measures apparent brightness (power multiplied by the human eye’s sensitivity). 1 lux of yellow light is about 0.0015 W/m2. 1 lux of green light (50% eff.) is 0.0029 W/m2. Sunlight corresponds to about 50,000 lux Artificial light typically 500-1000 lux

Light sensors : 

Light sensors Simplest light sensor is an LDR (Light-Dependent Resistor). Optical characteristics close to human eye. Can be used to feed an A/D directly without amplification (one resistor in a voltage divider). Common material is CdS(Cadmium Sulphide) Sensitivity: dark 1 M,10 lux 40 k,1000 lux 400 .

Slide 8: 

Semiconductor light sensors include: photodiodes, phototransistors, photodarlingtons. All of these have similar noise performance, but phototransistors and darlingtons have better sensitivity (more current for given light input). Phototransistor:1 mA @ 1000 lux Photodarlingtonsup to 100x this sensitivity.

Light sensors – high end : 

Light sensors – high end At the cutting edge of light sensor sensitivity are Avalanche photodiodes. Large voltages applied to these diodes accelerate electrons to “collide” with the semiconductor lattice, creating more charges. These devices have quantum efficienciesaround 90% and extremely low noise. They are now made withlarge collection areas andknown as LAAPDs (Large-Area Avalanche Photo-Diode)

Light sensors – cameras : 

Light sensors – cameras Two solid-state camera types: CCD and CMOS. CCD is the more mature technology, and has the widest performance range. 8 Mpixel size for cameras Low noise/ high efficiency for astronomy etc. Good sensitivity (low as 0.0003 lux, starlight) CCDs require several chips,but are still cheap ($50 +) Most CCDs work in near infraredand can be used for night visionif an IR light source is used.

Light sensors – cameras : 

Light sensors – cameras CMOS cameras are very compact and inexpensive, but haven’t matched CCDs in most performance dimensions. Start from $20(!) Custom CMOS camerasintegrate image processingright on the camera. Allow special functions likemotion detection, recognition.

Light Sensor Circuits : 

Light Sensor Circuits There are many ways to assemble light sensor elements into sensor devices that can be employed on a robot. A CdS photocell is a very simple transducer that one can use for transforming changes in light intensity into changes in resistance.

How does a CdS photocell work? : 

How does a CdS photocell work? CdS is a semiconductor. When photons on light strike the semiconductor, electrons are excited into the conduction band, leaving behinds holes (vacancies in the valence band). The more intense the light, the more carriers which are created and hence the more conductive (less resistive) is the photocell.

Optical Spectrum : 

Optical Spectrum

Light Sensor Circuits : 

Copyright Prentice Hall, 2001 15 Light Sensor Circuits Single Photocell Circuit Photocell Voltage Divider Circuit Photocell element is connected to the circuit ground and the HB’s sensor input line via a voltage divider circuit Vsens , resulting sensor voltage, varies as to the ratio between 47KW and Rphoto When the photocell resistance is small (brightly illuminated), the Vsens ~= 0v When the photocell resistance is large (dark), Vsens ~= +5 v Continuously varying range between extremes Sensor will report small values when brightly illuminated and large values in the dark May invert the sense of the readings from the HB’s analog ports: int light(int port) {return 255 - analog(port);} “dark sensor”

Light Sensors : 

Light Sensors Photovoltaic light falling on a pn-junction can be used to generate electricity from light energy (as in a solar cell) small devices used as sensors are called photodiodes fast acting, but the voltage produced is not linearly related to light intensity A typical photodiode

Reflective Optosensors : 

Copyright Prentice Hall, 2001 17 Reflective Optosensors Photocells vs. Phototransistors How do you choose one type of device rather than the other? Photocells are easy to work with, because electrically they are just resistors, but their response time is slow compared to the photodiode or phototransistor’s semiconductor junction. This means photocells are suitable for detecting levels of ambient light, or acting as break-beam sensors in low frequency applications (e.g., detecting when an object is between two fingers of a robot gripper). For applications such as shaft encoding, the rapid response time of the photodiode or phototransistor is required. Also, these devices are more sensitive to small levels of light, which allows the illumination source to be a simple LED element.

Reflective Optosensors : 

Copyright Prentice Hall, 2001 18 Reflective Optosensors Interfacing Light-sensitive current source: the more light reaching the phototransistor, the more current passes through it This creates a voltage drop in the 47K pull-up resistor on HB This voltage drop is reflected in a smaller voltage on the Vsens sensor signal line, which has a level that is equal to 5 volts minus the 47K resistor’s voltage drop Smaller values than 47K may be required to obtain good performance from the circuit If transistor can typically generate currents >= 0.1 mA, then voltage drop across the pull-up resistor will be so high as to reduce Vsens to zero Solution is to wire a smaller pull-up resistor with the sensor itself The current, i, flowing through the Q1 phototransistor is indicated by the dashed line.

Light sensor : 

Light sensor Photoconductive such devices do not produce electricity, but simply change their resistance photodiode (as described earlier) can be used in this way to produce a linear device phototransistors act like photodiodes but with greater sensitivity light-dependent resistors (LDRs) are slow, but respond like the human eye A light-dependent resistor (LDR)

Photoconductive : 


photodiodes :