The Microcontroller Idea Book - Jan Axelson's Lakeview Research

Chapter 9
can sense light level and distance. And modern automobiles contain all kinds of sensors,
including ones to measure engine temperature, composition of exhaust emissions, oil
pressure, engine speed, and whether or not the seatbelts are fastened.
You can find a sensor to detect and measure just about any property. Some sensors are readily
available from suppliers of other electronic components. These include photodiodes and
solar cells, which respond to light, and semiconductors that respond to changes in temperature.
Surplus catalogs sometimes have good deals on sensors from failed or obsolete products—
for example, dollar-bill sensors from vending machines and motion detectors from security
systems.
Sometimes you can make your own sensors from everyday materials. The conductive foam
commonly used to hold CMOS components can double as a simple pressure sensor, since
its top-to-bottom resistance decreases as the foam is pressed. A popular homemade moisture
detector is a printed-circuit board with two interleaved but untouching copper traces. When
the board is wet, water shorts the traces together and changes the resistance between them
from very high to a few hundred ohms.
Some projects call for a specialized sensor that you just won’t find in the usual sources. A
good resource is the Sensors Buyer’s Guide, published annually by Sensors magazine. The
guide lists over 1200 companies involved with sensors, and indexes them according to
property sensed, technology used, manufacturer, and related products and services. From
the list of properties sensed, you can select the category that interests you and consult a list
of companies that offer products in that area. Most companies are happy to provide product
information and applications hints.
Choosing Sensors
To pick the right sensor for a job, you first need to specify what you want the sensor to do.
Below are some of the questions to ask about your desired sensor. The example answers
describe a temperature sensor intended for use in a controller used in processing photographic
film:
• What property do I want to measure? (temperature)
• What range of inputs do I need to measure? (60-110 degrees Fahrenheit)
• What resolution and accuracy do I need? (accurate to within 0.5 degree Fahrenheit)
• How fast must it respond to input changes? (quick response not critical for this
application)
• What kind of output do I need (analog, digital, voltage, current,...)? (8-bit digital output
would be ideal, but analog voltage or current output is OK)
154 The Microcontroller Idea Book