on when you expose the photoresistor to more light in terms of its resistance? Is the resistance increasing or decreasing when the sensor is exposed to more light? Here’s what’s going on in the circuit. The more light hits the photoresistor, the lower its resistance. If the room is quite bright, then the LED will be quite bright. If light is low in the room, the sensor resists the current flow, which is expressed by the LED getting dimmer. The current flow through the sensor controls how bright the LED will shine. That’s because the circuit is wired so that all current to the LED must pass through the photoresistor first. The photoresistor, as you just learned, is a resistive sensor. There are many types of resistive sensors; this category of sensors is used to measure much more than luminosity. As you continue reading and you encounter new sensors, it’s a good idea to think how the stimuli are measured and especially how the output is structured. None of the sensors will output data that is in a convenient format for end-user consumption. Instead, you will need to decide how to express or format the raw sensor data output in a way that makes sense to users. Another sensor type is electro-mechanical. These sensors do not manifest changes in voltage or current, but rather by a change in their physical properties. The thermostat in your home or apartment is a great example (unless you have a digital thermostat). When the room temperature changes, a thermostat’s bimetallic coil will expand or contract depending on whether the room’s temperature increases or decreases. The sensor is actually physically expressing itself by changing shape! But even these sensors may trigger an electronic sensor (for example, a thermostat’s bimetallic coil is usually connected to a tilt switch that turns the heat on or off).
Interactive Sensor Control “Project 1: Photoresistor to Measure Light” on page 3 used a sensor in a way that didn’t directly involve interaction with a human. Sure, you were the one who changed the lighting in the room to force the photoresistor to change its resistance, but it could have just as easily been the setting or rising of the sun. There are many sensors that you’ll manipulate directly; you’ll see these in later chapters.
Going Forward In all the projects in this book, you’ll be building small systems that collect input data by taking measurements with a sensor. The systems will do some-
Sensors
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