From: John Popelish
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Subject: Re: why does IR circuit oscillate?
Date: Thu, 26 Sep 2002 23:42:32 GMT
NNTP-Posting-Date: Thu, 26 Sep 2002 19:42:32 EDT
> In article <3D9369B8.B4361D3C@rica.net>, John Popelish wrote:
> >The.Central.Scrutinizer.firstname.lastname@example.org wrote:
> >> http://www.pobox.com/~the.central.scrutinizer/irrepeat.png
> >> http://www.pobox.com/~the.central.scrutinizer/irrepeat.gif
> >> 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.
> >The 555 is connected as an oscillator (output fed back to threshold
> >and trigger through a resistor to a grounded capacitor). Any time
> >reset is not pulled up, the oscillator will run.
> I meant at the IR receiver's output. It should be the IR signal with the 40khz
> signal removed. When there's an actual IR signal then it looks ok. But when
> there's no IR, the receiver's output is noisy as hell, like it is in oscilation.
> It's mostly clean when the IR receiver's output is isolated.
Sorry I didn't read as well as I should have.
The receiver includes a limiter amplifier, which has lot of gain. The
internal 40 kHz bandpass filter is supposed to reject this noise, but
at some level, some will get through. If you block light from
reaching the sensor, does the output noise stop? If it does not, then
I suspect that output energy is going back into the supply pin and
turning the bandpass filter into an oscillator. Try adding a bypass
capacitor between pins 2 and 3. Another one between the anode of the
LED and the emitter of the driver transistor is also a good idea.