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^ Ultrasonic motion detector: This detector generates ultrasonic waves that reflect off any objects in the room. If nothing in the room moves, the ultrasonic waves bounce back with no change. If someone or something moves in the room, the ultrasonic waves distort, and that distortion triggers an alarm. You donít really have a compelling reason to use these devices rather than the PIR detectors unless you have a special fondness for ultrasonic gadgets.
You're getting warmer: Temperature sensors
Remember when you were a kid lying in bed on a cold winter evening? Suddenly, you heard a sound! But you quickly realized that it wasnít the boogeyman coming to get you ó just the furnace turning on in your cold house. The thermostat in your wall activated the furnace because it sensed that the temperature had dipped below the preset temperature.
A thermostat uses a coiled metal strip (called a bimetallic strip) that shrinks as the temperature cools. When the coil shrinks to the point you set on your thermostat, it trips a switch and turns the furnace on. This is a simple and common type of temperature sensor that you use in certain types of gadgets, such as the thermostat. Other types of temperature sensors, including thermocouples, semiconductor temperature sensors, infrared temperature sensors, and thermistors measure changes in temperature electrically, rather than mechanically, as with the bimetallic strip.
To make your life a little easier, we suggest that you just stick to using thermistors for projects where you want to measure temperature because they are generally easier to use than thermocouples and infrared temperature sensors. A thermistor is a resistor whose resistance value changes with changes in temperature.
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114 Part II: Aisle 5, Component Shack: Stocking Up
Other ways to sense temperature
In the section "You're getting warmer: Temperature sensors," we mention several other types of temperature sensors. Here, we briefly summarize their characteristics for the curious among you:
Semiconductor temperature sensors: After thermistors, these are probably the easiest to use. The most common type of this sensor contains two transistors. This sensor's output voltage depends on the temperature.
^ Thermocouples: These sensors generate a voltage that changes with temperature. Thermocouples contain two wires (for example, a copper wire and a wire made of a nickel/copper alloy) welded or soldered
together at one point. The metals it uses determine how the voltage changes with temperature. You can use thermocouples to measure high temperatures, such as several hundred degrees or even over a thousand degrees.
^ Infrared temperature sensors: These sensors measure the infrared light given off by an object. You can use these sensors when the sensor must stay at a distance from the object you plan to measure; for example, you use this sensor if a corrosive gas surrounds the object. Industrial plants and scientific labs typically use thermocouples and infrared temperature sensors.
There are two types of thermistor:
^ Negative temperature coefficient (NTC) thermistors: The resistance of this type of thermistor decreases with a rise in temperature.
^ Positive temperature coefficient (PTC) thermistors: The resistance of this type of thermistor increases with a rise in temperature.
You should find an NTC or PTC marking on your thermistor; if you canít find this marking, you can verify which type of thermistor youíre dealing with when you calibrate it by identifying whether the value increases or decreases with a rise in temperature.
Suppliersí catalogues typically list the resistance of thermistors at 25 degrees Celsius (77 degrees Fahrenheit). Measure the resistance of the thermistor yourself with a multimeter (see Chapter 9 for more about using multimeters) at a few temperatures; these measurements give you the resistance at each temperature so that you can calibrate the thermistor. If you plan to use the thermistor to trigger an action at a particular temperature, make sure to measure the resistance of the thermistor at that temperature. Thermistors have two leads and no polarity, so you donít need to worry about which lead you have wired to +V.
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Chapter 5: Filling Out Your Parts Bin
Good Vibrations with DC Motors
Have you ever wondered what causes a pager to vibrate? No, not Mexican jumping beans: These devices usually use a DC motor. DC motors change electrical energy, such as the energy stored in a battery, into motion. That motion may involve turning the wheels of a robot that you build or shaking your pager. In fact, you can use a DC motor in any project where you need motion.
Electromagnets make up an important part of DC motors because these motors consist of, essentially, an electromagnet on an axle rotating between two permanent magnets, as you can see in Figure 5-8.
SOUTH POLE OF PERMANENT MAGNET