On 18 Apr 2012 at 7:30, David Duffy (AVD) wrote:

> I had someone ask me the other day about "dust / smog" detection.  They=20
> wanted to automatically close some windows when the outside air quality=20
> was poor (lots of particulates).
>=20
> My initial thought was a simple IR LED + photo diode pair to sense a=20
> reduction in emission.  I then pulled a photo electric smoke alarm apart=
=20
> to look at the optical chamber.  It uses an IR LED and photo transistor,=
=20
> but set at 90=B0 to each other.  The smoke particles cause a reflection a=
s=20
> opposed to beam interference.

That's actually quite interesting to think about... assuming narrow beam an=
gle=20
source and receiver at right angles to each other makes it more sensitive t=
o light=20
reflected from smoke particles at the optical intersection point, less sens=
itive to dust=20
on the surfaces of the sender/receiver.

--=20
Brent Brown, Electronic Design Solutions
9 Titoki Place, Pukete,
Hamilton 3200, New Zealand
Ph: +64 7 849 0069
Fax: +64 7 849 0071
Cell: +64 27 433 4069
eMail:  brent.brown@clear.net.nz




--=20
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