Today's technology integrates a variety of sensors into our daily lives--sensors are used extensively in residential and commercial systems that we are exposed to every day. However, the details of how these sensors operate are often ignored, and the sensors are simply treated as transparent components in larger systems. Microcontrollers provide a cost-effective platform for studying sensors and sensor interfacing techniques, and make available an interactive learning environment for students in the classroom or using the World Wide Web (Cotton & Johnson, 2006).
For educational purposes it is not necessary to use expensive commercial-grade equipment to teach students about commonly used sensors. Schools frequently have limited funds for acquiring educational materials; therefore, purchasing sensors identical to those used in large-scale industry is not an option. By purchasing small-scale components that cost only a small fraction of what commercial systems may be using, schools can educate and engage students in a much broader range of authentic experiences.
The focus of the following activities is directed at interfacing commonly available sensors with simple electronic components and inexpensive microcontrollers, which aligns well with STL Standard 11, "Apply the design process," STL Standard 12, "Use and maintain technological products and systems," and STL Standard 19, "Manufacturing technologies." Only a basic understanding of electronics and microcontrollers is needed to implement these activities. The activities explore several cost-effective techniques for the study of selected sensors by interfacing them with very basic electronic circuits and easy-to-program microcontrollers. An educator can effectively direct a course of study by selecting suitable sensors and emphasizing topics appropriate to their course objectives. Computer programming techniques that control hardware, sensor theory of operation, and hardware interfacing strategies are only a few of the subject areas that could be the focal point of a course of study. Presented here are ideas for implementing a course of study with several basic but interesting operational sensor systems together with some possible alternatives or enhancements.
A good sensor is the common photoresistor. (See Figure 1.) Photoresistors are inexpensive ($0.75-$2.00), easy to work with and understand, and readily available from electronic parts suppliers (Gilliland, 2000). Photoresistors, coupled with a microcontroller and simple supporting circuitry, can be used as the basis for several illustrative laboratory experiments, like controlling a simple dusk-to-dawn yard lighting system or controlling a line-following wheeled robot.
A simple version of a dusk-to-dawn yard lighting system experiment is easily implemented by building a basic electronic circuit that uses a light-emitting diode (LED) as a simulated yard light, a photoresistor, and microcontroller. This experiment relies on changes in light intensity to represent daytime and nighttime conditions. When the light intensity varies, the resistance of the photoresistor is altered, and ultimately a voltage change is detected by the microcontroller, which will activate the yard light or LED in this experiment. This activity allows students to observe how sensors are used to monitor a common homeowner's event. Students could be challenged to enhance the yard-light activity by enabling the system to compensate for light coming from the headlights of a passing automobile or from lightning during a storm.
Another photoresistor activity that is always appealing to students is the robot line-following experiment. The activity is implemented by using a pair of photoresistors mounted on a small wheeled robot that follows a black line on a white background. Photoresistors are mounted on the front of a small robot …