Could you briefly describe to me how this sort of device, which I think =
is just a microphone "tuned" to 40KHz, works? I mean, do I put +5 on one =
side and ground on the other and stick in some capacitors or what?  What =
is this "gain" stuff that I need a lot of?=20

Is there some electronics book that I should perhaps read?  (Besides =
Forest Mimms's book which I have studied to the point where it is =
falling apart at the seams.)

Thanks!
Lindy

-----Original Message-----
From: pic microcontroller discussion list =
[mailto:PICLIST@MITVMA.MIT.EDU] On Behalf Of michael brown
Sent: Sunday, May 23, 2004 18:36
To: PICLIST@MITVMA.MIT.EDU
Subject: Re: [PIC:] Newbie question about Sonar Receiver

From: "Spehro Pefhany"


> You need a lot of gain, depending on the distance to be covered and
the
> transmitter signal. You may even need a nonlinear time-variable gain
> amplifier.
> You may have trouble using a breadboard due to noise issues (signals
in the
> mV range). I'm trying not to be negative, but I think this is not a
really
> a beginner project, IMO. If you have an oscilloscope, look at the
> transducer output and you'll see just what you are dealing with.

I did this several months back on a solderless breadboard and it worked
very well.  I used an ordinary LM741 as the amplifier stage and a 393
comparator as the second stage to clean up the signal.  I just biased
one of the comparator inputs a few millivolts higher than the idling
output of the 741 (which was fed into the other input).  The echoes give
me a real nice square wave out of the comparator perfect for feeding
directly into a digital input of a PICs CCP module (no ADC required).
The transducers are very insensitive to frequencies that are not
ultrasonic so normal room noises had very little effect.  I was
surprised that I didn't need to apply any audio filtering to get decent
reliability.

I found that about 5-6 cycles directly into the sending transducer
(driven directly from the PIC) gave decent results without allot of
ringing decay of the transducer.  A small blanking interval during and
after transmission and you're ready to capture the echoes.

michael brown

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