Inductor arrangement for boosting low-voltage piezo drive

[Clifford Heath]

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.

 

[martin griffith]

On Thu, 02 Jun 2005 23:21:11 +1000, in sci.electronics.design Clifford
Heath <no@spam.please.net> 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.
snip
Clifford Heath.

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

 

[Mebart]

Use short pulses, which are louder and sound raspier, I'm assuming you
want an alarm type noise level to draw attention. The square waves
don't drive them hard enough to produce much output.

You will probably need a higher output voltage also, 3 volts just
isn't much, we run some of ours at 20 volts peak to peak for short
periods.

M

On Thu, 02 Jun 2005 23:21:11 +1000, Clifford Heath
<no@spam.please.net> 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.

 

[Boris Mohar]

On Thu, 02 Jun 2005 23:21:11 +1000, Clifford Heath <no@spam.please.net>
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.

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) http://www.viatrack.ca

 

[Robert Baer]

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?

 

[Roger Lascelles]

"Clifford Heath" <no@spam.please.net> wrote in message
news:429f07c7$0$20631$afc38c87@news.optusnet.com.au...

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.

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

 

[Rich Grise]

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

 

[Clifford Heath]

Robert Baer wrote:

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?

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 going to go with a CR2430 or similar
lithium battery, before it was pointed out that it needs an LED to
back up the beeper. What's the lowest quiescent current you can get
in a 3V RS232 driver chip?

Rich Grise wrote:

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.

 

[Zak]

Clifford Heath wrote:

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.

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.

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 going to go with a CR2430 or similar
lithium battery, before it was pointed out that it needs an LED to
back up the beeper. What's the lowest quiescent current you can get
in a 3V RS232 driver chip?

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

 

[Don Baker]

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.

Don

"Clifford Heath" <no@spam.please.net> wrote in message
news:42a15704$0$5744$afc38c87@news.optusnet.com.au...

Robert Baer wrote:
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?

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 going to go with a CR2430 or similar
lithium battery, before it was pointed out that it needs an LED to
back up the beeper. What's the lowest quiescent current you can get
in a 3V RS232 driver chip?

Rich Grise wrote:
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.

 

[Fred Bloggs]

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.

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.

 

[Clifford Heath]

Don Baker wrote:

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.

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.