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PICList Thread
'[OT]: Ultrasonic Sensors'
2000\12\15@133430 by Dan Michaels

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Fred Bailey wrote:
>I have built a few interfaces from PIC's to ultrasonic sensors. Basically, you
>ping (trigger) the transducer using a FET and a good sized inductor (1uH). You
>have to wait for the ping ring to dampen down before you can see the echo
>(Pong).


Hi Fred,

Were you using a Polaroid transducer, or the ceramic kind? I found the
following simple ckt on the robohacking sites. Any comments?

             +5v
              |
             22K  <--- probably too large
              |
              +--------------+
              |              |
              c           U/S xmtr
PIC---1K-----b    NPN         |
              e              |
              |             gnd
             gnd
===========


>The echo amplifier is done with four or more op-amps in series to get about 10K
>gain in total. Many stages are used to keep the frequency bandwidth high.


Uh oh. Guess I would learn this soon as I breadboarded the ckt.
The ckts I've seen had fewer stages with more gain, but you are
certainly right. Hmmm.
========


>Try puting the sensor on a stepping motor and map in all directions.
>

Tom has stepper motors for everything :). My bot will probably
just turn its whole body [initially].

thanks,
- dan michaels

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2000\12\15@215329 by Dan Michaels

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At 03:57 PM 12/15/00 -0500, you wrote:
>The sensors I used were ceramic (see Digikey P/N 270-1000-ND) and required the
>inductor to voltage spike it to get the ping. For a piezoelectric, your
stimulas
>circuit will not work without an inductor and diode.
>

Digi-Key Part Number 270-1000-ND Quantity
Unit Price 65.19000

oy!

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2000\12\15@215348 by Dan Michaels

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Rich Ottosen wrote:
>Dan:
>I think you are making a wrong assumption about the resistor that you
>labeled "probably too large".
>
>What controls the size of this resistor is not the energy of the
>transmitted ultrasound! The transmitted power is determined by how fast
>you discharge the capacitance of the transducer.
>
..........
>If I remember correctly, the capacitance is about 0.01uf for an
>ultrasonic transducer. 22k ohm times 0.01uf is a 220us time constant.
>


Hi Rich, this is precisely why I thought the R was too large.
Since it's a 40 khz device, I figure you need to recharge in
less time than 1/40khz = 25 usec. So for 0.01uF, R should
be maybe 1-2K.

I am assuming it doesn't so much "resonate" at 40 Khz, but that
you need to "stimulate" at 40 khz. OTOH, it's an assumption.
This weekend I'll try stimulating my transducer, and see what
really happens. [Jinx, no wisecracks, mister].

- danM

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2000\12\16@102435 by Matt Bennett

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Fred Bailey wrote:
>
> I have found that it is not the capaictance of the sensor but the physical
> diameter of the sensor that determines how long it rings. It's surface flexes
> when the voltage spike is applied. The ringing is electromechanical and not just
> due to capacitance. The echo is the reverse and it will have a similar ring down
> time.
>
> Best Regards, Fred

I'd have to agree with Fred- the ringing of an ultrasonic sensor is a
mechanical result of the excitation voltage.  The large polaroid sensors
rely on this mechanical resonance for a *very* powerful pulse.
Unfortunately, a pulse that rings for a long time will make your
distance resolution poorer than what it could be.  If your transmitter
rings for a long time, so will any response from your target, which
makes it that much harder to distinguish an object.

You can reduce the ringing by applying a load to the transmitting sensor
immediately after you send the pulse out.  In my experiments I tried a
bunch of different resistors, but then found it was most effective to
just short it out.  Since I've used separate transmit and recieve
transducers, I just used a transistor to short out the recieve
transducer so that I could resolve close-in targets.

The polaroid and other ultrasonic sensors use the high initial pules and
measure off the leading edge of the response- which is fine if you just
want a single measurement, but you lose S/N that could be used to
resolve quieter targets.  I built an ultrasonic sonar
<http://web.hazmat.com/~mjb/projects/picsonar/> that gives a continuous
time response.  If you are familar with radar terminology, I can get
what is called an "A-scope" which the response is measured in distance
on the horizontal axis, and amplitude on the vertical axis.  You can
wrap this into a circle by rotating your sensor, and you can get what is
called a "PPI" display (Plan Position Indicator) which is essentially
the traditional radar scope that most people expect.

The biggest problem with ultrasonic sensors is that the angular
resolution is poor.  The small sensors I've been using have a beam width
of roughly 45 degrees between 3dB points.  The Polaroid sensors, with
their large size with respect to the frequency, can get a pretty good
angular resolution, but you lose the ability to resolve small objects
behind a big reflector.  I would like to build a phased array radar, but
all the transducers available are just too big- you need to get the
spacing between the sensors to about .7 of the wavelength for it to be
effective, otherwise, you get *huge* sidelobes which will obscure your
target.  The other method I have rolling around for increasing the
angular resolution is a "synthetic aperture" sonar, which may be
possible with my sonar, since the transducers move in a semi-circle
about a central point.  But this project is on hold, since usually once
I get to a place where I'm pretty sure I could succeed, I lose interest,
and start tilting at the next windmill (a pic-controlled walking robot
at this point).

Mechanically focusing the sonar is a possibility too (with a parabolic
reflector), but that has its own mechanical and time response problems.
I've never gotten to that part of my project.

Matt

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2000\12\16@115852 by Dan Michaels

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Matt Bennett wrote:
>
>I'd have to agree with Fred- the ringing of an ultrasonic sensor is a
>mechanical result of the excitation voltage.
...........
>You can reduce the ringing by applying a load to the transmitting sensor
>immediately after you send the pulse out.  In my experiments I tried a
>bunch of different resistors, but then found it was most effective to
>just short it out.
............


Hi Matt,

I looked at your imaging sonar page. Pretty cool. It sounds like
you are the guy with the info on the U/S sensors that I haven't
been able to find. I bought some 40 khz transducers from Jameco but
there is no info about the best way to hook them up/etc. I want to
use them on the PIC minibot control board I am designing for range
sensing, and want the minimal circuitry [it's always that way].

In the bot literature, they use pretty simple ckts. An NPN inverter
with 22K pullup R, U/S xmtr from collector to gnd, and base driven
by 40khz oscillator thru 1K resistor. Receiver is simple high-gain
opamp circuit.

Could you comment on this simple lashup, vis-a-vis your experience?
This would be regarding ranging, and not imaging.

Also, looking at your sonar imaging stuff, I was wondering if you
couldn't improve the image quality somewhat by using simple
edge-enhancement techniques? Take 3 ranges a few degrees apart
in azimuth, and subtract a weighted version of the outer 2 from
the center.

thanks and best regards,
- Dan Michaels
Oricom Technologies
http://www.oricomtech.com
=========================

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2000\12\16@132829 by rottosen

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Dan:
See my comments below -- and you come to the 6502 Group meeting Tuesday
so we can talk about this in much more detail.

Dan Michaels wrote:
{Quote hidden}

Actually, it is better to recharge at a slower rate than 40KHz. The
amplifier should be tuned to 40KHz so that it rejects the recharge
change. If you recharge too fast, the ac amplifier will amplify the the
recharge voltage and possibly saturate.

Another note on the amplifier. You should have some clamping/clipping to
keep from overdriving the amplifier during the transmit pulse and
transducer ringdown periods.

One other thought. There was an article in Ciarcia Circuit Cellar
Magazine some time back showing how to use a 40KHz IR receiver module
for a ultrasonic transducer receiver by replacing the photodiode with
the transducer. The one big drawback is that you are forced to use
separate transmitting and receiving transducers. I tried to pulse the
transducer connected to the receiver input and only succeeded in making
a (permanently) nonfunctioning receiver.  :-((


>
> I am assuming it doesn't so much "resonate" at 40 Khz, but that
> you need to "stimulate" at 40 khz. OTOH, it's an assumption.
> This weekend I'll try stimulating my transducer, and see what
> really happens. [Jinx, no wisecracks, mister].

No, the transducer will mechanically resonate all by itself. In fact,
the higher the resistor value the higher the Q of the resonant circuit.
The resonation (?) is started by the discharge of the transducer
capacitance which causes the piezoelectric material to change in
thickness. If you discharge at a rate that is at least as fast as the
40KHz resonance, the transducer will be properly excited.

By the way, most transducers do not like having a DC voltage applied to
them continuously. Consider capacitor coupling into the transducer.


-- Rich



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2000\12\16@143713 by Dwayne Reid

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At 09:53 PM 12/15/00 -0500, Dan Michaels wrote:
>Rich Ottosen wrote:
> >Dan:
> >I think you are making a wrong assumption about the resistor that you
> >labeled "probably too large".
>
>Hi Rich, this is precisely why I thought the R was too large.
>Since it's a 40 khz device, I figure you need to recharge in
>less time than 1/40khz = 25 usec. So for 0.01uF, R should
>be maybe 1-2K.

I hadn't really thought about it that much, but wouldn't you want to send
just *one* pulse at very high amplitude?  Multiple pulses just make it
harder to detect echoes.

If that were the case, R @ 22K would be just fine, but I'd expect V to be
quite high (100V plus).

dwayne



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2000\12\16@151005 by rottosen

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Dwayne:
See Comments below.

Dwayne Reid wrote:
{Quote hidden}

I think so. The multiple pulses only extend the "ringdown" and therefore
the length of the wavefront thus "smearing" the distance information.


>
> If that were the case, R @ 22K would be just fine, but I'd expect V to be
> quite high (100V plus).


Yes. However the maximum voltage allowed is dependent on the transducer.
Some only allow a few volts where others allow hundreds of volts.

Looking in my Digi-Key Book of All Knowledge I find the following:
Panasonic ceramic transducers only allow 20vrms.
ITC transducers allow 50vp-p and 100vp-p depending on the model.

From memory, the Polaroid transducers allow about 400vp-p. Their circuit
clamps with 200 volt zener diodes.


-- Rich


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2000\12\16@160708 by Jinx

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> This weekend I'll try stimulating my transducer, and see what
> really happens. [Jinx, no wisecracks, mister].

Dan, there are some complete and utter transducer stimulators
around. You aren't one of them

Thanks for the thread btw, I might need a U/S detector soon. Does
any company offer a chip like National's obsoleted LM1812 ? It
had a 12W o/p and could range to 20 in air, 100ft in water  (as an
aside, I saw a tech piece on TV at the w/e about measuring the
temp of water by timing the travel of a sonic pulse through it - sound
travels faster in warm water than in cold)

There's a functional description in Nat's Linear Book 3, covering
ringing, amp gains etc which may help. Perhaps you could back-
engineer one

I did a quick search for LM1812 and pulled up this

http://www.wizard.org/sonardoc.html

which has a quite a long description and 16C84 code

also this

http://www.seattlerobotics.org/encoder/apr97/sonar.html

which is a circuit using discretes

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2000\12\16@175551 by John Mullan

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Can I assume this circuit would work for most xmiter units?  I originally
assumed this type of circuit for turning it on.  Also, is there a circuit
snippet for using the receiver unit?

I purchased the matched set offered at Alltronics.  Haven't started using
them yet.

John Mullan


{Original Message removed}

2000\12\16@224123 by Matt Bennett

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Dan Michaels wrote:

> I looked at your imaging sonar page. Pretty cool. It sounds like
> you are the guy with the info on the U/S sensors that I haven't
> been able to find. I bought some 40 khz transducers from Jameco but
> there is no info about the best way to hook them up/etc. I want to
> use them on the PIC minibot control board I am designing for range
> sensing, and want the minimal circuitry [it's always that way].

It depends on your range- see what you need.  If you're just looking for
really short range for a robot- you don't need too much excitation.  You
can get an effective 10V p-p by differentially driving a transducer that
is directly across 2 pins of a pic.  Like a mini H-bridge.  For my
sonar, I wanted as much power as possible, so I did some level shifting
and drove some CMOS inverters powered by 10V to get 20V p-p effective
across the transducer.

> In the bot literature, they use pretty simple ckts. An NPN inverter
> with 22K pullup R, U/S xmtr from collector to gnd, and base driven
> by 40khz oscillator thru 1K resistor. Receiver is simple high-gain
> opamp circuit.

I just used the PIC to create the oscillator.  As you can see it works
pretty well, and you can control how many cycles go out.  Th reciever
will be the biggest problem.  Since the transmitter and reciever will be
pretty close to each other, you will get a *very* loud signal into the
reciever, which will cause ringing, and make a minimum usable range.
Remember that the power of the response you get falls off by a factor of
r^4 (transducer voltage will fall off by a factor of r^2).  If you're
just trying to range- don't worry too much about an amplifier, you would
probably want some sort of non-linear amplifier.  You could use an
amplifier, use a low-pass filter to get rid of anything but 40 KHz
(nominally the resonant frequency of your transducer).  Bias a
transistor just barely on, some feedback to keep it just on the "off"
side of on for anything but a high frequency signal.  A little signal
should cause the output to saturate.  Don't worry about linearity.  You
don't need it for your application.

> Could you comment on this simple lashup, vis-a-vis your experience?
> This would be regarding ranging, and not imaging.
>
> Also, looking at your sonar imaging stuff, I was wondering if you
> couldn't improve the image quality somewhat by using simple
> edge-enhancement techniques? Take 3 ranges a few degrees apart
> in azimuth, and subtract a weighted version of the outer 2 from
> the center.

I'm not familar with that- my gut feel is that it won't work, but I'm
willing to look at it.  Do you have any resources?  Subtracting a
weighted version would seriously reduce the amount of return power from
the target... unless you're thinking of the "synthetic aperture" method,
where you use the difference in time from a target as the transducer
moves, and cancel out some of the side-lobes.

Check out this page, it looks like what you want:
<http://www.hamjudo.com/sonar/> very simple.

Matt

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2000\12\17@132933 by Dan Michaels

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Wisecracker wrote:
>> This weekend I'll try stimulating my transducer, and see what
>> really happens. [Jinx, no wisecracks, mister].
>
>Dan, there are some complete and utter transducer stimulators
>around. You aren't one of them
>

[awwww, now I'm gonna cry].
===========

>
>http://www.wizard.org/sonardoc.html
>

Yeah, I've seen "Brat Wizard's" page before, and several other
robo-pages regarding ultrasonic sonars - most are derivative of
info in "The Robot Builder's Bonanza" by Gordon McComb. This
is the basic ckt I described previously with the NPN inverter
wanging the transducer to gnd at 40 Khz rate.

However, this goes counter to what Matt Bennett and Rich Ottosen
were saying about transducer ringing, tuned ckts, etc. Matt's
ckt is a little too large for me - about 5X bigger than the
entire PICoBot - guess I'll have to do my own stimulation.
[note to self - slip schedule on SBC40-BOT a few more weeks].
===============

>
>http://www.seattlerobotics.org/encoder/apr97/sonar.html
>

thanks, another variation.

best regards Mr J,
- danM

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2000\12\17@133555 by Dan Michaels

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Rich Ottosen wrote:
.........
>> Hi Rich, this is precisely why I thought the R was too large.
>> Since it's a 40 khz device, I figure you need to recharge in
>> less time than 1/40khz = 25 usec. So for 0.01uF, R should
>> be maybe 1-2K.
>
>Actually, it is better to recharge at a slower rate than 40KHz. The
>amplifier should be tuned to 40KHz so that it rejects the recharge
>change. If you recharge too fast, the ac amplifier will amplify the the
>recharge voltage and possibly saturate.
>

Hi Rich,

Thanks for the info. Gives me more to think about when I get to
breadboarding the ckt.
==============

>One other thought. There was an article in Ciarcia Circuit Cellar
>Magazine some time back showing how to use a 40KHz IR receiver module
>for a ultrasonic transducer receiver by replacing the photodiode with
>the transducer.

Hmmm, interesting idea. Sure would save on real estate. Double hmmm.
==================

>
>No, the transducer will mechanically resonate all by itself. In fact,
>the higher the resistor value the higher the Q of the resonant circuit.
>The resonation (?) is started by the discharge of the transducer
>capacitance which causes the piezoelectric material to change in
>thickness. If you discharge at a rate that is at least as fast as the
>40KHz resonance, the transducer will be properly excited.

More to think about. This seems to go counter to the ckts shown in
"The Robot Builder's Bonanza" by Gordon McComb, upon which many
of the robo-hack-sonars on the web are based --> lots to play with.

thanks and best regards,
- danM

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2000\12\17@151230 by Dan Michaels

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Matt Bennett wrote:

>>
>> Also, looking at your sonar imaging stuff, I was wondering if you
>> couldn't improve the image quality somewhat by using simple
>> edge-enhancement techniques? Take 3 ranges a few degrees apart
>> in azimuth, and subtract a weighted version of the outer 2 from
>> the center.
>
>I'm not familar with that- my gut feel is that it won't work, but I'm
>willing to look at it.  Do you have any resources?  Subtracting a
>weighted version would seriously reduce the amount of return power from
>the target...


Hi Matt,

Thanks for the additional info about your system. I'm still
gathering info. I like the idea Rich mentioned, am already looking
at cannabalizing my radio shack IR detector - trivial to hack, but
as usual, no info about the BP freq.

Regarding the image enhancment business, above, the idea is that
by subtracting weighted versions of the surrounding scans you
enhance what makes the center scan "different" from the surrounds.
This is basically how the retina sharpens up images produced by
photoreceptors with fuzzy receptive fields. Here's a visual
overview of retinal function:

http://www.spectrum.ieee.org/publicaccess/9605teaser/vis3f1x.html

The bipolar cells are the ones to look at. They receive input from
the center of the visual field and subtract input from the surround,
thus enhancing the difference. Strictly speaking their receptive
fields look like so:

         +---+
         |   |
---+     |     |      +---
  +-----+     +------+

due to adding this

---+                  +----
  +------------------+

to this
         +---+
        |     |
       |       |
--------+       +-----------

x[distance in 2D]---->

best regards,
- Dan Michaels
Oricom Technologies
http://www.oricomtech.com
=========================




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2000\12\17@154808 by James R. Cunningham

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This sounds like unsharp-masking.  Sould work quite well.

Jim

Dan Michaels wrote:

> Matt Bennett wrote:
>
> >>simple
> >> edge-enhancement techniques? Take 3 ranges a few degrees apart
> >> in azimuth, and subtract a weighted version of the outer 2 from
> >> the center.
> >
> >I'm not familar with that- my gut feel is that it won't work, but I'm
> >willing to look at it.

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2000\12\17@222306 by Scott Beatty

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Hello

   I am interested in trying a project with ultrasonic sensors.  How does the
circuit account for Dopler effect and what is the greatest range that a
transmitter-sensor pair have?  Also how would other mediums other then air effect
the signal (i.e. liquid)?

Scott

Dan Michaels wrote:

{Quote hidden}

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2000\12\17@234332 by Dan Michaels

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Scott Beatty wrote:

>    I am interested in trying a project with ultrasonic sensors.  How does the
>circuit account for Dopler effect and what is the greatest range that a
>transmitter-sensor pair have?  Also how would other mediums other then air
effect
>the signal (i.e. liquid)?
>

The ckt mentioned is apparently too simple-minded to account
for much of anything. I'm a stranger here myself, and just saw
it in a book on bots. I would suspect, however, that if you
send the amplified pulses straight to a PIC, and actually
determine the frequency of the return pulses, then you could
measure velocities directly. Sounds fairly easy [this is called
"bait" - any nibbles?].

- danM

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2000\12\18@001602 by rottosen

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Scott:
The main effect of the Doppler shift that I can see is that it might
shift the received echo outside of the amplifiers bandwidth. I suspect
that this will normally not be a problem but have done no calculations
to prove it.

If I remember correctly, the Polaroid systems could do tens of feet.


The speed of sound in air is about 335 m/s (1100 ft/s). Water is about
1540 m/s (5000 ft/s). So, in the same amount of time the sound travels
about 41/2 times as far in water as in air. The measurement will
therefore have to be 4 1/2 times faster.

Also the sound is attenuated differently in water. Now for a weird
number from the far recesses of my brain... The attenuation of sound is
about 1db/cm/MHz (round trip, I believe). Oh, that hurt. :-) The brain
is too sore to convert this to 40KHz type of ultrasound frequencies
right now.

For water some kind of sealed transducer will be required. Maybe a fish
finder one?


-- Rich


Scott Beatty wrote:
{Quote hidden}

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2000\12\18@002409 by Dale Botkin

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On Sun, 17 Dec 2000, Richard Ottosen wrote:

> If I remember correctly, the Polaroid systems could do tens of feet.

They claim their module will measure out to 35'.  I'm invilved in a
project now that uses a Polaroid module; anything past 25' is proving to
be a challenge.

Dale
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2000\12\18@125119 by Don Hyde

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Last year I programmed a PIC on an ultrasonic-transducer-based project.
Someone else did the transducer code, which used a DSP and got pretty
sophisticated.  They had a lot of trouble with the mechanical ringing of the
transducer, and after lots of sophisticated electronic hacks at trying to
force it to ring down faster, wound up with a workable hack by packing the
transducer in plumber's putty.  It seems that the slow ringdown is due to
energy leaking from the desired mechanical flex that makes the sound into
some other motion of the transducer (which does not radiate acoustically).
This energy then leaks back into the working mode that radiates sound, thus
creating the slow ringdown.

Electronic hacks like feeding out-of-phase energy only made the problem
worse, merely feeding more energy into the parasitic mechanical resonances.

Strategically-placed plumber's putty was able to absorb much of the energy
in the parasitic resonances without much affecting the desirable mode that
makes the sound.  Last I heard, they'd kicked it back to the mechanical guys
who were supposed to find some elastomeric plastic that wouldn't dry out
with time the way they said the plumber's putty would.

It seems the transducer people know all about this stuff, and would be happy
to sell an inproved model transducer that fixes the parasitic resonances.
But not for anywhere close to the under-US$10-in-quantity budget for the
sensor on that project.

> {Original Message removed}

2000\12\18@203008 by cflat

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> There are 41 messages totalling 2568 lines in this issue.
Hi all,
With all the talk of ultrasonic transducers, I thought I'd check here
and see if any of you all can offer info.  I am interested in
building a circuit to use a transducer (the kind used on fish
finders) to tell the distance to the bottom in a body of water.  I
would prefer it to use a PIC but would be greatful for any info
whether its a complete project or just links to info that will get me
started.  Thanks for your time.
Happy Holidays,
Charles

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