28123 Parallax Inc, 28123 Datasheet - Page 197

• Manufacturer Part Number: 28123
• Description: TEXT WHAT'S A MICROCONTROLLER
• Manufacturer: Parallax Inc
• Type: Programmingr

Specifications of 28123

• Style: Book
• Title: What's a Microcontroller?
• Contents: Whats a Microcontroller? Text
• Product: Microcontroller Accessories
• Core Processor: PIC16C57c
• Flash: 128 Bytes
• Operating Supply Voltage: 9 V
• Board Size: 31 mm x 16 mm
• Lead Free Status / RoHS Status: Not applicable / Not applicable

Chapter #7: Measuring Light · Page 189
Chapter #7: Measuring Light
DEVICES THAT CONTAIN LIGHT SENSORS
You have already worked with two different kinds of sensors. The pushbutton can be
thought of as a simple pressure sensor, and the potentiometer is a position/rotation sensor.
There are many kinds of sensors built into appliances and machines that are not as
obvious as a button or a knob. Other common sensors measure things like temperature,
smoke, vibration, tilt, and light. Although each of these different kinds of sensors can be
found in one or more devices that most people use on a daily basis, light sensors are
probably the most common.
One example of an every-day device that contains a light sensor is probably your
television. If it can be controlled by a handheld remote, it has a built-in detector for a
type of light called infrared that cannot be seen by the human eye. The handheld remote
uses infrared light to transmit information about the channel, volume, and other keys that
you might press to control the TV. Another common example is a digital camera. A
camera’s light sensors help it adjust for various lighting conditions so that the picture
looks clear regardless of whether it’s a sunny or cloudy day.
INTRODUCING THE PHOTORESISTOR
Light sensors have many different functions, and they come in different shapes, sizes, and
with different price tags. Some sensors are designed to sense a particular color of light,
such as blue, green, red, or infrared. Some sensors don’t care what color the light is
because they react to how bright the light is. Other sensors look for only special kinds of
light given off by certain chemical reactions. Light sensors also have a variety of ways to
tell a microcontroller what they see. Some sensors send a voltage, some send a sequence
of binary values, and others react to different kinds of light or light levels by changing
resistance.
Of the light sensors that react to light by changing their resistance, the photoresistor
shown in Figure 7-1 is probably the most common, least expensive and easiest to use. Its
active ingredient is a chemical compound called cadmium sulfide (CdS). This compound
changes resistance depending on how bright the light is that shines on its collecting
surface. Bright light causes low resistance values between the two leads while dim light
causes higher resistance values.