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variable capacitors - piezo transducer?

Discussion in 'Electronic Basics' started by Mad Scientist Jr, May 17, 2007.

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  1. According to this wiki page

    a transducer is a type of variable capacitor.

    Does this include the Radio Shack 273-073A Piezo Transducer?

    If so how can you measure the capacitance (uF) it puts out?

    What are some other devices that function as variable capacitors,
    using either sliders, knobs, or other funky (ie touch-sensitive, light-
    sensitive, sound-sensitive, etc) inputs?

  2. You've got it backward. A transducer converts some physical signal
    (temperature, force, motion...) to an electrical signal. Using a moving
    part to vary the value of a capacitor is one approach, but there are also
    resistive, magnetic, photoelectric and piezoelectric transducers.

    If you have a variable capacitor, you can put it in series with a resistor
    across a constant voltage, then measure the voltage across the capacitor.
    If the capacitance changes, the voltage will change. The first capacitor
    microphones worked this way. You could use the capacitor to control an
    oscillator and measure its frequency, or apply a known current to the
    capacitor and measure the time required to charge it to a given voltage,
    which is how the capacitance function in some digital multimeters works.
  3. default

    default Guest

    A transducer is a device that converts a physical variable to an
    electrical signal AND visa versa The trans in TRANSducer. Voltage to
    sound and sound to voltage.

    A magnetic loudspeaker does the same thing without significant

    Piezo devices happen to also be capacitive - as frequency goes up
    impedance (capacitive reactance) goes down - but that doesn't make all
    variable capacitors transducers or all transducers variable

    They will frequently specify the capacitance of the device or you can
    measure it - with some caveats . . . they will exhibit different
    characteristics at different frequencies (all caps do - but piezo
    devices have strong mechanical resonance's as well as electrical

    Capacitive Reactance is used in a variety of devices - touch sensitive
    screens, keyboards, level sensing, liquid sensing, dielectric sensing,
    pressure sensing, etc.. What did you have in mind?

    The piezo electric devices can be used as voltage multipliers like
    step up/down transformers - or used as spark ignition devices for gas
    BBQ grills. They can be used for a lot more than just making noise.
    Ultrasonic cleaners . . . ceramic phono cartridges . . . microphones .
    .. . micro positioning devices for mirrors and lenses etc..
  4. Don Bowey

    Don Bowey Guest

    Many transducers are bi-directional. Some are intended to only transmit or
    only receive, but will do both.

  5. default

    default Guest

    Interesting. I had to look it up. In the "old days" a transducer had
    to talk both ways, that was the definition of "transducer." Like the
    sonar transducer used for depth/object sensing. Transmits out to
    bounce an echo off an object in water or air then senses the return

    Today transducer is applied to any device that converts from one
    signal form to another form. Technically, by that definition, a
    mechanical wind vane, to sense air flow, is a "transducer."

    This is probably definition changing over time.
  6. Don Bowey

    Don Bowey Guest

    My first exposure to transducers was with depth sounders in mid-50s. They
    were bi-directional, and were base on a Nickel core or a chuck of crystal.

    My how times change.... I love it. I don't use speakers in my sound system,
    I use transducers.

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