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Bare Piezo Transducers

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geowal19

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I'm trying to get some audio from a bare piezo transducer but it's strangely making zero noise. I'm driving it with a 1kHz square wave generated by an Arduino and buffered as the load in an emitter follower. I have tried a few transducers but none of them do anything. They are receiving 5Vpp.

Thanks
 
hi,
Is it an audio range sounder or 38KHz, 40KHz type.?
Do you have a datasheet.?
E
 
Hi,

I would ask a similar question.

Also, is that 5v measured right across the piezo?
Maybe you could post your emitter follower circuit too.
 
A piezo transducer is a capacitor that does not conduct so it cannot be driven directly from a transistor that does not have a resistor for its load.
Have you looked at the frequency response of a piezo transducer? It plays many resonant peaks and nulls at high frequencies and almost no sounds below 2.5kHz. Here is an example:
 

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Hi,

I was wondering that too that's why i asked him to show his circuit so we can take a look. The drive should be somewhat low impedance, for ether switch state not just one.
 
The drive should be somewhat low impedance, for ether switch state not just one.
The drive does not need to be a low impedance because a piezo transducer is a fairly high impedance.
But the drive must be push-pull which does not happen if an output transistor does not have a resistive load.
Please post your emitter-follower circuit that drives the piezo transducer.
 
A monomorph (the disk portion of a ceramic transducer of which you speak) is excited largely by the dV/dT of the driving signal - that is, it being a capacitor, it is the sudden change in voltage across the device that will cause the most current to flow and the mechanical effect. An emitter follower, by its nature, may (or may not) provide this when presented with the potentially large capacitance of one of these devices.

Many of these units are in cases with single-transistor driving circuits utilizing a small segment of the monomorph as a feedback loop - a sub-portion of the silvered surface that is electrically separated from the main: I've seen where people will mistakenly drive (or forget) that the piezo unit contains this circuit and drive it with an AC signal and get very little signal when, in fact, the unit as a whole was intended to be driven with a continuous DC signal for the contained oscillator circuit. It should be noted that the frequency of oscillation is partially set by the mechanical properties of the monomorph itself, but largely the resonant chamber consisting of the small air column above the monomorph and the small-ish hole from which the sound escapes - which is why the typical "sonalert" type unit without the internal driver circuit has such a limited frequency range over which usefully loud output may be obtained.

Driving a the "bare" monomorph directly is most easily done directly from the I/O pin of the processor itself: This is commonly done in the industry and is quite safe to do, even without a series limiting resistor. Much more output/drive may be obtained if you have two I/O pins available and can drive them in a complimentary fashion as this doubles the driving voltage.

Note that unless you use a specially designed piezoelectric "speaker" (rarely seen these days, but they used to show up in dial-up modems) their frequency range is going to be quite limited - typically in the 2+ kHz range.

If the advice of a large dV/dT signal (e.g. fast rise time such as a square wave) is heeded, they will make noise down into the Hz range, but as a series of clicks: As with the human voice, it will often not be the fundamental frequency that is perceived directly with these things, but the harmonics - until one gets into the kHz range - but the human ear and psychoacoustics being what they are, there may be the illusion - for some - of a wider range than that!

If you extracted the monomorph from an enclosure make sure that it did not get fractured due to (even slight!) bending: As you can imagine, this would render it practically inert - something that could be determined by measuring its capacitance: It should be in the many hundreds to tends of thousands of pF range, depending on the size. Finally, as you may have noticed, one driving terminal is the metal substrate on which the thin ceramic is mounted while the other is the silvered surface itself: If there is a separate feedback loop in addition to the main surface, the two (main and feedback) may be connected together.
 
A piezo beeper in a resonant plastic chamber with a little hole to let out the beep is different from a bare piezo transducer that I posted the horrible frequency response. The bare transducer is small and stiff so its resonances are at fairly high audio frequencies. It has no enclosure so the low frequency sounds from its rear comes around it and cancels the low frequency sounds from its front.
 
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If the bare monomorph is held near the ear, or (better yet) pressed against a flat, somewhat flexible surface (thin metal, cardboard) even very low-level drive signals should be easily heard.

Because these devices operate in a thickness-shear mode (I believe) the momentum of their change in thickness will offer reasonable transfer of energy to said surface, even if there is no countering mass: Adding such a mass so will often reduce the efficacy of such mechanical energy transfer - usually due to absorption - and risk damage to the substrate and/or the metallic surface.

The fact that geowal19 could not hear anything at all when driving the device indicates one of several things:

- It is a complete piezo buzzer with built-in drive circuit. (That seems unlikely - and even in that case, I'd expect even a weak response).
- There is no drive signal at all (e.g. emitter follower is not working as expected. I would recommend driving the device directly between the MCU pin and ground, for starters.)
- It is damaged due to bending and the element is fractured.
- The drive signal may be being applied between the main element and the smaller feedback loop rather than as described in my previous posting.

Having used these devices for many years (my father worked for a company that manufactured them so I had many to play with and break!) they are very easy to drive to deafening levels - even with 4000 series logic gates - and nearly bullet-proof, provided that one doesn't bend them!
 
Hi,

I was wondering that too that's why i asked him to show his circuit so we can take a look. The drive should be somewhat low impedance, for ether switch state not just one.

The drive does not need to be a low impedance because a piezo transducer is a fairly high impedance.
But the drive must be push-pull which does not happen if an output transistor does not have a resistive load.
Please post your emitter-follower circuit that drives the piezo transducer.

Hello,

So who said "low impedance" ?

I had said "somewhat low impedance", and for both polarities. In other words, the high state must be somewhat low and the low state must be somewhat low, which means it should be lower than the thing it is driving. This doesnt necessarily mean a push pull either, it just means that the impedance has to be low enough to properly drive the device it is driving.
For a few examples:
If the the impedance of the load is 10 ohms, then the driver should be maybe 1 ohm for both high and low states.
If the impedance of the load is 100 ohms, then a 10 ohm driver should be ok.
If the impedance of the load is 1k then a 100 ohm driver might be ok.

So it does not necessarily have to be push pull, it can be a single transistor with a load resistor where the load resistor is of low enough value to drive the load properly, and the gain of the transistor and its rating is good enough to drive both the load and the load resistor.

Also, it would be wise to look at the impedance of the piezo with frequency.
 
hi,
Has anyone noticed the OP/TS has not posted since his opening post.?:banghead:

E
 
The OP could not get his emitter-follower to work. He said he fed the piezo with 5Vp-p which is impossible since the emitter-follower has at least a 0.7V loss. If the emitter-follower has no emitter resistor then it has a 5Vp-p loss.
 
hi,
Has anyone noticed the OP/TS has not posted since his opening post.?:banghead:

E

Hi there Eric,

Yeah but that doesnt bother me yet because it is Wednesday and that was Monday, only a couple days so far. We'll see though.
 
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