Re: Inductor arrangement for boosting low-voltage piezo drive

On Fri, 03 Jun 2005 07:01:27 +0000, Robert Baer wrote:

> Clifford Heath wrote:
>
>> 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, http://www.mallory-sonalert.com/Mallory%20Sonalert%20Catalog.pdf
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

.

Relevant Pages

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