Active and Passive Components of Transducers

You may not know their technical name, but transducers are everywhere. The microphone and earpiece on your telephone, the speaker in your TV set, the LED (light emitting diode) on your video game console and even the pickup on your electric guitar are all forms of transducers. Technically, a transducer is any device that changes one form of energy into another. Transducers can be designed with other circuitry to measure speed, pressure and temperature, as well as to convert sound to and from electrical signals for communication and entertainment. Transducers are generally classified as being either "active" or "passive" devices.

Energy Conversion

Electricity, sound, light and heat are types of energy. A transducer converts one of these forms of energy into another. An example of a non-electrical conversion, in which a physical element is converted into another physical element, is a simple mercury-filled thermometer. Heat energy causes the physical movement of mercury to rise inside a glass column. Most transducers, however, involve electricity to perform a variety of very useful tasks.

Defining Active and Passive Transducers

Transducers that deal with electricity are said to be "active" or "passive" in their characteristics. Active transducers produce electricity themselves as a result of some external stimulation. Passive transducers change their electrical characteristics as a result of changes in their environment.

Examples of Passive Transducers

When a DC (direct current) voltage is applied to a light emitting diode (LED), the LED converts this electricity into light. You can probably account for a dozen LEDs in your home, which are typically used today as indicator lights on everything from computer printers to cordless phones and microwave ovens. Another type of passive transducer is the thermistor. These devices change their resistance value to electricity that flows through them as temperature changes. Thermistors are commonly used in circuits that measure temperature. The speakers in your TV, stereo and car radio convert a varying voltage into sound.

Examples of Active Transducers

The magnetic pickups on an electric guitar are transducers that detect the mechanical vibrations of the strings and change them into an electrical signal, which is then sent to other electronic circuitry for amplification. Sonar transducers change electric energy to a high frequency sound wave that travels through water and bounces off objects it encounters, sending the sound back to another receiving transducer. Sonar is used by submarines and even weekend fishermen. Perhaps the most common transducers in everyone's home are found in telephones. The microphone converts sound waves into electrical signals, and the earpiece does the exact opposite -- that is, it converts electrical signals into sound waves. The record and playback heads on a tape recorder or cassette recorder convert electrical signals to magnetic fluctuations that are recorded (or played back) on magnetic tape. The magnetic phono cartridge on a record player is another example of a transducer. Electric motors convert electricity into mechanical energy. A generator does the reverse, converting mechanical energy into electricity.

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