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From: Pedro on 7 Nov 2008 14:48
Please could someone advise me where I can get a kit or even instructions to
build a device that emits a very high frequency 17.5-18.5 kHz tone.

I am not an experienced electronics expert but could manage a simple
project.

Thanks


From: Jasen Betts on 8 Nov 2008 05:50
On 2008-11-07, Pedro <invalid(a)invalid.invalid> wrote:

> Please could someone advise me where I can get a kit or even instructions to
> build a device that emits a very high frequency 17.5-18.5 kHz tone.

18 kHz(+/-3%) is not VHF, not even technically ultrasonic.

> I am not an experienced electronics expert but could manage a simple
> project.

.-------------------------------------------------------------.
| This is an ascii schematic, if the diagram appears garbled |
| try switching to a fixed-pitch font (courier works well) |
| pasting it into notepad works well on ms-windows. |
| or in google groups "show original" (in "more options") |
`-------------------------------------------------------------'


S E M I - P R E C I S I O N D O G A N N O Y E R


+6V -+------------------+----------+--
| | |
R1 `-[100Ω]-. . . . .|. . . . |
\ | . VCC(8) . |
/ | . . |
\<---------+---DIS(7) RES(4)----'
2K / . NE555 . |
\ +----------TH(6) OUT3(4)--------+--------|
R2 / | . . | | piezo speaker
| +----------TR(2) CV(5)--. | .---|
| | . . | | | |
+---+ . GND(1) . | | |
| . . . .|. . . . | | |
| | | `----|---[10KΩ]---+
C1 === | | | |
| 22nF | === 10nF +---[1KΩ]----+---->
| NP0 | | | TO 3.5mm plug
0V -+----------------+--------+----------+----------------->

the chip should be LM555 or NE555, C1 should be an NP0 ceramic type
R2 should be a multi-turn trimpot.

build this then plug it into your computer's line in jack and view
the wave on a program like audacity adjust R2 to get as close to
18Khz as you can.

Bye.
Jasen
From: Bob Masta on 8 Nov 2008 08:32
On Fri, 7 Nov 2008 20:48:27 +0100, "Pedro"
<invalid(a)invalid.invalid> wrote:

>Please could someone advise me where I can get a kit or even instructions to
>build a device that emits a very high frequency 17.5-18.5 kHz tone.
>
>I am not an experienced electronics expert but could manage a simple
>project.
>

If you'd like to test the feasibility of your
project before you get into circuit construction,
you might want to try generating the tone with
your computer's sound card. You can use the
signal generator in Daqarta to do this. (It's
free.) This will allow you to see what sort of
frequency and waveform work best.

One thing to be aware of is that tweeters
(especially piezo tweeters) tend to have *very*
irregular frequency responses, with serious dips
and peaks. Changing the frequency by a few kHz
can mean a huge change in output level... easily
12 dB or more. That's a factor of 4 difference in
the required amp drive voltage. So it would make
sense to tune the system to find an optimum
frequency for the tweeter you are going to use,
before building anything.

And I mean literally the same physical speaker,
not just one of the same model number... the
device variability is huge in this frequency
range.

Another issue is whether you can hear in this
range, in order to tune for best response. Most
adults have poor response here, but you might be
able to make a relative determination by getting
your ear next to the speaker. Better yet would be
to get a kid who can still hear these frequencies
to help you.

You can also use Daqarta to measure the response
of the speaker, though that part is only free if
you can do it in 30 sessions or 30 days, otherwise
US$29 for a license. The problem for a one-off
design (like I presume your project is) is that
you need a microphone with a known frequency
response, but calibrated mics are expensive. If
you wanted to get involved, you could build a
pretty good one yourself from an inexpensive
Panasonic WM-61A mic capsule (couple of bucks,
from Digi-Key). You wouldn't need to calibrate to
get absolute sensitivity, and the response is
fairly smooth in the frequency region you need, so
you could make relative measurements to find the
best frequency. But for starters I recommend just
doing it "by ear".

Best regards,


Bob Masta

DAQARTA v4.51
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, Sound Level Meter
FREE Signal Generator
Science with your sound card!
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