Sensors I Lecture is based on material from Robotic Explorations: A Hands-on Introduction to Engineering, Fred Martin, Prentice Hall, 2001.

Copyright Prentice Hall, Solar Cell Digital Infrared Ranging Compass Touch Switch Pressure Switch Limit Switch Magnetic Reed Switch Magnetic Sensor Miniature Polaroid Sensor Polaroid Sensor Board Piezo Ultrasonic Transducers Pyroelectric Detector Thyristor Gas Sensor Gieger-Muller Radiation Sensor Piezo Bend Sensor Resistive Bend Sensors Mechanical Tilt Sensors Pendulum Resistive Tilt Sensors CDS Cell Resistive Light Sensor Hall Effect Magnetic Field Sensors Compass IRDA Transceiver IR Amplifier Sensor IR Modulator Receiver Lite-On IR Remote Receiver Radio Shack Remote Receiver IR Sensor w/lens Gyro Accelerometer IR Reflection Sensor IR Pin Diode UV Detector Metal Detector

Copyright Prentice Hall, How about we add a color sensor?

Copyright Prentice Hall, Objective To be able to discriminate between objects of different colors. In this example, objects are painted different colors: red, green, blue.

Copyright Prentice Hall, BRAINSTORMING How can we determine color????? Objects possess color because they reflect light of a particular wavelength and absorb light of other wavelengths. For example, green objects reflect light with a wavelength of approximately 520 nm; other wavelengths are absorbed. A green object photographed in red light would appear almost black.

Copyright Prentice Hall, BRAINSTORMING Our plan will be to illuminate an object with red light and measure the amount of reflected light. We will then repeat using a source of green light. Finally, we will illuminate the object with blue light. When the color of the light used to illuminate the object and the color of the object match we get a large value from the sensor; when the colors do not match we get a small value.

Copyright Prentice Hall, Red Light Emitting Diode (LED) Stanley H-300L ultra-bright red LEDs (647 nm) Made from GaAlAs mcd with I F = 20 mA and V F = 1.8V Digi-Key P/N: ND Maximum I F is 50 mA.

Copyright Prentice Hall, Green Light Emitting Diode (LED) Kingbright WP7104VGC1A ultra-bright green LEDs (520 nm) Made from InGaN mcd with I F = 20 mA and V F = 4.0V Digi-Key P/N: ND Maximum I F is 30 mA.

Copyright Prentice Hall, Blue Light Emitting Diode (LED) Panasonic LNG992CFBW ultra-bright blue LEDs (470 nm) Made from ? mcd with I F = 20 mA and V F = 3.5V Digi-Key P/N: P465-ND Maximum I F is ? mA.

Copyright Prentice Hall, How do we drive an LED?

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.

Optical Spectrum

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.

PDVP-5003

Copyright Prentice Hall, Light Sensor Circuits Single Photocell Circuit Photocell Sensors with Light Shields If your photocell easily floods from ambient room light, then the next order of business is to build an optical shield to limit the amount of ambient light that is able to fall on the sensor

Copyright Prentice Hall, Using a CdS Cell to Sense Light

Copyright Prentice Hall, Calibration Adjust the potentiometer in the each of the LED circuits so the ADC values are approximately the same for each of the three colored (R, G, B) LEDs when their light is reflected from a white surface (an inch or so away from the sensor). Record the ADC value when light is reflected from a red, then a green, and finally from a blue surface. Do this for each of the three colored LEDs. Using some type of “similarity measure” you should be able to determine the color of an unknown object.

Copyright Prentice Hall, Color Sensor

Copyright Prentice Hall, FORCE SENSOR

Copyright Prentice Hall, Resistance vs Force

Copyright Prentice Hall, Force Threshold Switch

Copyright Prentice Hall, Force/Pressure Measuring Circuit