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

  • Thread starter Christopher A. Steele
  • Start date

Christopher A. Steele

Jan 1, 1970
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

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, +
[email protected]


Question: How do I make an inexpensive ultrasonic tank?

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

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

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 ;^) .


Jan 1, 1970
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

Christopher A. Steele

Jan 1, 1970
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, +