Hi Tony,

Wow,this sounds like a great project. I am especially interested because I
am involved in projects to help the disabled (my current one,which has been
going on and off between school and work, is a talking color identifier for
the blind).

Well, I only had a little time right now to take a look at your
schematic,so I may be misunderstanding things,but here are a few possible
problems I see:

#1) There seems to be a lack of audio frequency bypassing. When you have
digital circuits combined with audio amplifiers and audio circuits, it is
critical that you have a large amount of capacitance (even a few 100 uF
might not be overkill) NEAR THE DIGITAL power pins. Especially duing
recording, it is the digital circuits (and any others which are
switching,etc.) that are likely to need the extra power. I would place a
few 100uF caps between the power and ground pins of the PIC, the ISD chips,
and any other digital ICs,as well as right at the microphone bias
connection (right at the bias pin).
You have some bypassing, but 0.1uF caps don't do alot at audio freqs. The
4.7uFs will help,but might not be enough.

#2) You might try shutting down the PIC and other unneccesary stuff duing
recording. You can do this by having the PIC enter sleep and waking it up
(by interrupt on port pin change or RB0/INT pin) when recording has been
finished. This should reduce noise to a minimum.

#3) BTW, do you have the watchdog timer disabled? If not, this could be the
whole problem. You might think that the PIC is just looping and waiting for
3.5 seconds,when in fact it is resetting and starting to try to send data
to stuff.

#4) You should use either ground planes or star points for both power and
ground distribution. For example, run one wire from your power supply to
all the digital ICs, BUT then run a separate wire to each analog point,
especially the microphone bias, etc. Do the same for ground.

#5) There are a few parts of your circuit that I don't understand. Why do
you have a transistor across your regulator? It seems to me that it would
increase the output voltage if the current drain went above a certain
value. Also, what is that three transistor circuit up at the top right for?
It looks sorta like a delay circuit, but with my quick look, I couldn't be
sure that it wouldn't oscillate. If this is some kind of oscillator and it
is on during the recording (or playback for that matter), this could be the
problem.
Also, is it part of the recommended circuit to ground the mic thru a 10K
resistor?

#6) The MAX232 has an on board oscillator which might be in the audio
range. Try turning it and all other unecessary ICs off (even if you do it
manually just to see if it is the cause).

I would probably investigate #3 first, then try #5 and #6. If they don't
work, #1,#2,and #4.

I hope this works out for you soon!

Sean

At 05:20 AM 7/11/99 PDT, you wrote:
>Hi.
>I am currently putting together a project to design a flexible facilitated
>communication device to help disabled people communicated - those suffering
>from autism, for example. This will end up an open source design and
>software. Anyway, Ive hit a snag and hope a few people will take a look for
>me.... take a look at
>http://members.xoom.com/tonynich/problem/index.html
>I have a deep, deep suspicion it is something silly. By the way, I knew
>nothing about PIC's or microcontrollers when I started this, 6 months ago.
>It is only thanks to this list and certain individuals who I met via the net
>that its got this far. Many thanks.
>
>Tony Nichols
>South Australia
>
>tonynich@hotmail.com
>or
>tnichols@bedford-ind.com.au
>
>
>______________________________________________________
>Get Your Private, Free Email at http://www.hotmail.com
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|
| Sean Breheny
| Amateur Radio Callsign: KA3YXM
| Electrical Engineering Student
\--------------=----------------
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