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Piezoelectric Tweeter, 75V across terms. from 9V batt.?

Discussion in 'General Electronics' started by Christopher A. Steele, Sep 9, 2004.

  1. Below is the answer to an 8-year-old post about building a small
    ultrasonic tank (which I am attempting to do also, BTW).

    I Showed the post to a friend who has extensive experience in the
    field of electronics, and he seriously questioned -- to put it mildly
    -- the comment below about being able to modify the circuit so as "to
    read close to 75V across the tweeter's terminals... all from a 9V
    battery."

    So I have one person saying one thing is possible, and another saying
    it is NOT possible. I'm a complete novice at all this and, more than
    anything else, actually, I'm curious: who knows what they are talking
    about, here ? ? ?

    ((If you COULD get 75V at the tweeter, what pragmatic effect would
    that have? Produce a stronger output from the tweeter or what? And any
    suggestions about water-proofing the tweet are welcome.))

    THANK you!

    Christopher A. Steele
    Son of Col. M.J. Steele, US Army Security Agency/AGC, Fts: Devens,
    Rucker, Lawton, OATerm, +


    ........................................


    Question: How do I make an inexpensive ultrasonic tank?

    Answer:
    A common audio tweeter (piezoelectric) is available from Radio Shack.
    It costs $5, handles 75W of power, and is (more or less) easy to
    waterproof. Although I've never tried this, it *MIGHT* be possible
    just to enclose thbugger in a Baggie and tie the open end tightly. If
    you're more adventurousyou might try to just leave the beastie open,
    and dangle it underwater(Pure water is a great insulator (k=78.0), but
    enough impurity turns it inta great conductor, too.)

    As for the ultrasonic driver, a 555 timer works well directly. From a
    9battery, you'll get exactly 9V across the piezo's terminals. HOWEVER,
    (!you can boost that to over 75V terminal voltage by "tuning out" the
    inherent capacitance of the piezo.)

    First, approximate the frequency you'll be working with. (15kHz to
    30kHz workwell in driving people, bugs, mice, etc., absolutely crazy.
    ;^) ) A good starting value might be in that range. For now, assume
    30kHz.

    Next, measure the capacitance of the tweeter. The tweeter mentioned
    above has a series capacitance of 1.2uF. Either a cap meter or a
    'scope would work. The way I did it is by using the tweeter itself as
    a capacitor and substituting different "real" capacitors to get a
    frequency match on a 'scope. Later testing on a genuine cap meter
    verified my earlier results, so don't be afraid to experiment.

    Next, connect up a 555 circuit (or any good oscillator with decent
    output drive capability). Using the tweeter as the output, and a good
    'scope across the tweeter's terminals, check for an output at the
    selected frequency. You should get the full battery voltage across the
    terminals.

    Next, (aha!), calculate the required inductance, using the formula:

    f = 1 / ( 2 * pi * sqr( L * C ) ) (I think)

    (For 1.2uF @ 15kHz, I got about 0.5mH. I don't have a calculator
    handy, so you might want to verify the equation above. If it checks
    out within an order of magnitude, it's ok.)
    Get your hands on such an inductor. As a helpful hint, try your best
    to get one with the largest possible wire size to minimize lead
    resistance. Inductors from speaker crossovers are usually best.

    Put that inductor in series with the tweeter and fire the circuit up.
    Now, you should be able to read close to 75V across the tweeter's
    terminals... all from a 9V battery! If you want to play around, you
    can vary the frequency and watch the amplitude peak and fall off in
    either direction. The only drawback of this inductor approach is that
    it is now frequency-selective. Still, a 2:1 frequency variation
    shouldn't cause appreciable falloff.

    Next, test the circuit in actual use. Try it in a small tank with a
    dirty part. Then vary the frequency (or look it up, if possible) to
    get optimal results. When it works on a smaller scale, try it in a
    bigger tank. If you actually get around to building the bloody thing,
    you'll have a great sense of accomplishment, believe me! At the very
    least, you'll have an ultrasonic source which will keep mosquitoes
    away during the summer, mice in the winter and possibly people all
    year round, if you lower the frequency to 15kHz ;^) .
     
  2. Jupitersally

    Jupitersally Guest

    I'm not any electronics h/w design genius, but from the looks of it
    here are my 2c...
    The trick that is being used to get 75V from a 9V battery is a simple
    chopper circuit using a 555 chip. (See any reference book on power
    electronics) They've used an inductor to store charge, hence can get a
    high voltage at a specific resonance frequency. If you shift the
    oscillator frequency, the voltage output will drop drastically since
    we're exploiting resonance (that the article does say). Well, chopper
    convertors are used and not totally "black magic."

    Have fun on building the system.



    Aditya Sane
    Graduate Student - EE/Systems
    University of Southern California
     
  3. Aditya (Jupitersally):

    Thank you, Aditya/Jupitersally. Nice of you to respond, and you
    clarifiy things quite a bit for me. I'll have fun building the system,
    and you have fun in your career <G>!

    Christopher A. Steele
    Son of Col. M.J. Steele, USASA/AGC, Fts: Devens, Rucker, Lawton,
    OATerm, +
     
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