Inductor arrangement for boosting low-voltage piezo drive

Drive an RS232 chip at the piezo resonant frequency and connect the piezo to the output.of the RS232. The charge pump in RS232 will give you a higher output voltage.

Regards,

Boris Mohar

Got Knock? - see: Viatrack Printed Circuit Designs (among other things)

snip

Try tunig it acoustically, with a small tube, this may give a few dB's gain. Another option would be to use a small step up transformer

martin

I've developed an MSP430 (TI low-power 3V uP) device that drives a small piezo in push-pull from two pins at 2KHz, and it's not loud enough. The device has a requirement of very low quiescent power, and although it flashes an LED at the same time, should be audible to an older person by the side of a noisy road, so far as possible. I realise that frequency range is a problem, hence the LED :-). I haven't finalised selection of the piezo, but I see different ones with capacitances ranging around 0.02uF. I don't want to produce a higher drive voltage all the time, as they don't beep very often and at present the thing idles with the RTC running at under 1uA.

I was thinking of trying the following arrangement:

Connect each end of the piezo by a series L (SMT inductor?) to +3V, and each end also to the collector of an NPN BJT in common emitter mode. Drive the base of each transistor in push-pull, and let the inductors ring the voltage, hopefully up to 6V or so, as in a boost-mode power supply.

It doesn't seem crucial to me to get the thing running at resonance, but if possible, I guess I'd calculate the frequency using half the piezo's capacitance and the L, aiming for the natural resonance of the piezo - does that sound right?

Anyhow, I'm confident that this drive arrangement will work, but is there a better way? Am I missing something? is it sensible and possible to get SMT inductors in a suitable size?

Clifford Heath.

There are piezo sounders that only need a voltage across them to make a lowd sound. Is there a reason to re-invent that wheel?

In fact,

page 8 lists "Crosswalk Audible Signal Devices" - these are presumably loud enough, there are a couple at corners near me, that can be heard from across the street; and are listed at 80 dbA at 2 ft, but they're for 120 VAC.

There are others, like the "Loud" ones, that can put out 95 dbA at 16 VDC in.

I vote for a Sonalert. :-)

Cheers! Rich

I suggest driving your piezo element from an audio squarewave generator. You can tune around and see if frequency makes much difference.

You don't say what kind of transducer you have. If it is a bare brass plate with piezo on top, it will not be very efficient. You can get elements with a little resonator box on top, and these are much more efficient - the resonator performs acoustic impedance matching. You have to get the frequency right for each model of resonator transducer. Squarewave drive is OK for these.

Roger

I measured a CUI (www,cui.com) model CEP-1136 at 79db at 3.0 volts with a measured current of 7.7ma. I have this device in a medical instrument and have been very happy with the low voltage performance. I have used phase synchronized sine waves, 180 out of phase, to drive these devices when higher voltage was required. It is not elegant but does the job.

D> > There are piezo sounders that only need a voltage across them to make

So power down the RS232 chip if you are not beeping. If you use the internal shutdown of the MAX220-249... the leakage is 0.1 uA typ.

It may also be possible or good to use an external switch that powers on your whole circuitry only for a wake-up with a 1 or 0.1 % duty cycle.

In case of alarm this would change to a continuous power-on.

Thomas

I can't find one at a low price that will make a loud sound at 3V, is all. The self-drive and internal-circuit type don't use any voltage booster AFAIK, so aren't louder than my direct push-pull drive. I don't need 80dbA, as the person is only 2' away. I don't want to scare them witless - but there must be no mistake about what they've heard despite noise and poor hearing. Push-pull gives 6V P-P and is almost passable, I just figure that the boost inductors might give me 12V P-P. Guess I'll have to try it.

Boris Mohar wrote: > Drive an RS232 chip...

I've done that with 40KHz sonar, but I'm worried that the quiescent current will get me. The device must last for the shelf life of the

2 AA batteries, as it's sealed and will likely be replaced when the batteries fail. I was actually go> I vote for a Sonalert. :-)

The only Sonalert that will produce 80db at 3V (the SC307N) is $19, more than my entire BOM cost :-).

Roger Lascelles wrote: > If it is a bare brass plate...

It's a little ABS resonator case with the brass plate inside.

Clifford Heath.

Those inductors will be big and you need two drivers. If it's a voltage boost you're after then use any of the modern low voltage boost regulators with electronic shutdown to produce the voltage you're after. Some of them have shutdown quiescent current in the nA range, and operate at high frequencies- meaning small filter caps, inductor, and footprint. See Linear Technology micropower boost regulator section for a good sampling of what's possible.

Don,

Thanks, that looks like it would suit well - almost too big, but I think it'll fit my chosen case. If it's really that loud I can dispense with extra drive (for which I think the '232 chips would have been best). Did you tune the frequency, or just use close to the typical resonance?

A question about these self-drive piezo's - it looks to me as though they could be easily switched by leaving power connected and controlling the base drive (220K resister in the recommended drive circuit). Will the piezo suffer from having 3V across it forever? I suspect not, and that way my micro only has to switch uA. Not that 8mA would be a problem, except perhaps for EMI.

Clifford.