Subject:
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RE: sonar (fwd)
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Mon, 18 Nov 1996 17:27:50 GMT
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Original-From:
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Chuck McManis <CMCMANIS@saynotospamFREEGATE.NET>
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Viewed:
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980 times
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I believe somebody wrote:
> So...someone correct me if I'm wrong...the Digikey things are missing the
> driving circuitry. As an analogy to the Polariods, you only get the
> "speaker" and "microphone" part. Thus you have to build this yourself
> which will cost some $.
This is correct, but it isn't too expensive. Scott Edwards has a PIC that
will do all of the computation for you.
I built a cheesy ultrasonic sensor using a Ramsey Electronics Tone Detector
and a modified Rainbow electronics IR-3 board. (total cost $15).
The actual theory is simple, you send out a tone on the transmitter and then
you wait until you hear the tone on the receiver, but of course it isn't that simple.
You have to send out a "loud" tone, to do that you need a buffer amplifier (a
single transistor stage will work) to put about 15 - 20 volts across the transducer.
Then you need a sensitive receiver, a 12 - 15db gain amplifier is ok, a 24db gain
one is hard to keep from false triggering. I found some Motorola amplifiers in
potted epoxy that were little 6db units and ganged a few together, but there are
lots of circuits that you could use. Remember that you want the gain to be at
40Khz and so be careful if you use an audio amplifier (they lose big above 15Khz
usually).
Finally you need some way to detect the tone. The Ramsey kit uses the LM567
which works but can take 8 - 12 complete cycles before it outputs a detect signal.
For 40khz IR, 4 cycles plus or minus is 1.3" plus or minus. In practice I had a
hard time keeping the stability under 3". However, the other way to detect the
incoming frequency is to do a simple FFT on the input an threshold the 40Khz
component. When it passes the threshold output a detect signal. Much more
reliable at the cost of additional hardware.
--Chuck
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