Designing Embedded Hardware, 2nd Edition

Light Sensor

Now we'll take a look at a light sensor. The obvious use of a light sensor is to monitor natural light levels, and perhaps use the results to control artificial-lighting systems. But combine this sensor with a directional light source (such as a bright LED enclosed in a baffle), and you have a security detector. As long as the sensor can "see" the LED, everything's fine. But when the light is interrupted, you know that someone or something has passed between.

Tip

I used the particular sensor we're going to look at on a small datalogger. One of my customers is a biologist who studies albatrosses (giant seabirds) of the southern oceans as part of an ongoing conservation program. These birds will fly for years at a time, circumnavigating the world on the ocean winds. The tiny datalogger (smaller than your smallest finger) weighs only a few grams and is attached to the bird's leg. (The attachment is designed and fitted with great care to ensure that the bird is not harmed or adversely affected in any way.) The light sensor is used to record sunlight levels that the bird experiences on its journey.

By comparing the recorded sunrises and sunsets with the reference clock aboard the datalogger, and looking at the duration of twilight, latitude and longitude can be computed. In this way, the simple recording of light levels is used to track an albatross's journey as it circumnavigates the world.

The recorded light profiles also give information about what the albatross does. You ...