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'Semi-[OT] Driving peizo transducers directly from'
1998\08\29@134313 by Ansel Sermersheim

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I'm contemplating a project which will use (ultrasonic) peizo
transducers.  I don't want to go to all the trouble of implementing
FSK in hardware, so I'd like to generate the already-modulated square
wave in software.

That's not a problem; I'm a decent programmer.  However, these xducers
take some odd drive circuitry.  Has anyone figured out a circuit to
drive these from a output pin?  I'm going to be running in the range
of 38-42 kHz.

If not, I'll have to learn quite a bit more about linear electronics
before I can implement this, I think.

Thanks very much,
-Ansel
--
I used to be convinced that MicroSquish shipped crap because they simply
didn't give a flying fuck as long as the sheep kept buying their shit.
Now, I'm convinced that they really do ship the best products they are
capable of writing, and *that's* tragic.
  - John C. Randolph, about MS quality control.

1998\08\29@153208 by Peter L. Peres

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imho, you can obtain 20 usec loops relatively easily with a PIC at 4 MHz
for transmission ( == 50 kHz), but for a decent receiver at that frequency
you'd need to use a faster clock. What are your trying to modulate ? 4-bit
voice can be passed through such a channel but don't tell me about the
quality (you'd use crude ADPCM at 8 kHz). I've tried to do a simplex codec
with two PIC 16C71s once and it works but MAJOR work is required to make
the sound reasonable w/o external hardware.

Peter

1998\08\29@163746 by Ansel Sermersheim

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>>>>> "Peter" == Peter L Peres <spam_OUTplpTakeThisOuTspamACTCOM.CO.IL> writes:

> imho, you can obtain 20 usec loops relatively easily with a PIC at 4
> MHz for transmission ( == 50 kHz), but for a decent receiver at that
> frequency you'd need to use a faster clock. What are your trying to
> modulate ? 4-bit voice can be passed through such a channel but
> don't tell me about the quality (you'd use crude ADPCM at 8

I'm using FSK to pass digital data.  I'm only going to be using about
2400 bps, with two filters to receive.  I can easily poll at 415 usec
to read that.

However, it would be a lot easier if I don't have to rig something
similar to that for xmit.

> Peter
-Ansel
--
I used to be convinced that MicroSquish shipped crap because they simply
didn't give a flying fuck as long as the sheep kept buying their shit.
Now, I'm convinced that they really do ship the best products they are
capable of writing, and *that's* tragic.
  - John C. Randolph, about MS quality control.

1998\08\29@165030 by cousens

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part 0 954 bytes
In one job where I needed to make a smart alarm to interface with an
analog pressure sensor
I used a micro (small) peizo siren, removing the existing pcb (35mm x
29mm)
fitting a 16f84 on the equivalent size perf-board

I kept the peizo drive circuit components which were

A 100 Ohm resistor to the base of a 2N5551, emitter to ground
colector to the stepup (20 to 1 ?) transformer(ferrite) pin 1
The stepup transformer common, pin 2 connects to +
and the peizo connects to + and the third pin

The main advantages of modifying a siren is that you get the case,
mounted peizo and transformer, but if you want to do it yourself
then I sugest you try this circuit with a trigger transformer
the type used for Xenon tubes the ratio is around 50 to 1

--
Peter Cousens
email: .....cousensKILLspamspam@spam@her.forthnet.gr  phone: + 3081 380534
snailmail:  Folia, Agia Fotini, Karteros, Heraklion  Crete, Greece.

Attachment converted: wonderland:pdrive.gif (GIFf/JVWR) (00015505)

1998\08\29@170315 by Chip Weller

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Ansel Sermersheim wrote:


>>>>>> "Peter" == Peter L Peres <plpspamKILLspamACTCOM.CO.IL> writes:
>
>> imho, you can obtain 20 usec loops relatively easily with a PIC at 4
>> MHz for transmission ( == 50 kHz), but for a decent receiver at that
>> frequency you'd need to use a faster clock. What are your trying to
>> modulate ? 4-bit voice can be passed through such a channel but
>> don't tell me about the quality (you'd use crude ADPCM at 8
>
>I'm using FSK to pass digital data.  I'm only going to be using about
>2400 bps, with two filters to receive.  I can easily poll at 415 usec
>to read that.
>
>However, it would be a lot easier if I don't have to rig something
>similar to that for xmit.
>
>> Peter
>-Ansel


The piezo and film devices normal require a high voltage, but very little
current. The current requirements from the raw supply normal requires you to
use an off chip transistor (NPN or N-Channel MOSFET) to switch the current.
Use this to drive a transformer single ended. Tune the transformer leakage
inductance to be complementary to your capacitive piezo device at your
center frequency, this can provide a large voltage boost at high
effieciency. It is not hard to produce 600Vpp this way.

If I was trying to receive FSK I would use a simple bandpass filter wide
enough for both frequencies and then feed that into TMR0 or TMR1. Every
1/(3*2400) seconds read the change in the counter and just compare against
the center frequency to determine if the input is a 0 or 1. To synchronize
with the start pulse you would expect to see the mean frequency on the pulse
which contains the edge, if it closer to the 1 level then shift your phase
back in time, and if it is closer to the 0 level then shift it forward in
time.

Chip Weller

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