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From: Kevin McMurtrie
Subject: Re: why does IR circuit oscillate?
User-Agent: MT-NewsWatcher/3.2 (PPC Mac OS X)
Date: Thu, 26 Sep 2002 21:59:38 GMT
NNTP-Posting-Date: Thu, 26 Sep 2002 14:59:38 PDT
>The open circuit output of the IR receiver has a little noise, about a
>microsecond pulse ever ms or so, but when I connect it to the transistor
>buffer, it oscillates nonstop. The power to the IR receiver is clean, I don't
>see any noise on it at all. 'scope was connected with the IR receiver's ground
>as it's reference.
I'm guessing that you don't want to run the output of the IR sensor
through a transistor follower. Connect the IR output directly to the
7555 RST line. Don't use any series resistors.
Loose the 10K resistor on pin 7 of the timer. Pin 7 is a duplicate of
pin 3's signal but with no pull-up power.
Eliminate the visible LED driver circuit. Power it from the IR driver
transistor. You can use a diode and capacitor to keep it from blinking
and causing feedback.
The is one more minor problem. The 7555 cycles the timing capacitor
between 1/3 and 2/3 of V+. Using RST pulls your timing capacitor to 0V.
Now there's an extra delay in charging the capacitor to 2/3 of V+ that
will degrade the signal slightly. I'm not sure if it's enough to matter.
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