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'[PIC]: Sssssssssssmokin''
2000\12\19@140621 by Lawrence Lile

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Should have very few insects inside a hot oven.  I'll also probably build
some hysteresis into the thing to ignore bugs or smoke for a short while
(many incompetent cooks like to burn their food just before pulling it out.
Blackened redfish?  Blackened cheese sandwich? )  but finally hit the panic
button if it persists.   I am mostly looking for the case where the oven is
unattended, the food is left for way too long, and we need an emergency
shutdown.

Right now I am working on punching more current into my IR diode.   Looking
through my junk box trying to figure out what a low ESR capacitor looks
like.  I figure dumping current out of a low ESR capacitor using a low RDSon
FET should do the trick nicely.

May have to break down and order some today.  Junk box is running thin.

--Lawrence
>
> They are, but they are somewhat less sensitive to smoke than the
> radioactive types.
> The crossed path detection mechanism is correct, the fun is in screening
> out small reflective insects, without screening out the smoke :)
>

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2000\12\19@140828 by Lawrence Lile

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Uh oh.

Optek has been telling me that IR is the way to go.  This is bad news.

I actually found that my flashlight set off the detector quite well, it is
sensitive to visible light as well as IR.  I think it is biased toward the
red end because a BLUE led will not set it off at all.

Maybe all I need is a stupid flashlight bulb instead of a fancy LED.   Or
just a red LED.  I'll try one.

--Lawrence


{Original Message removed}

2000\12\19@142702 by David VanHorn

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> Right now I am working on punching more current into my IR diode.   Looking
>through my junk box trying to figure out what a low ESR capacitor looks
>like.  I figure dumping current out of a low ESR capacitor using a low RDSon
>FET should do the trick nicely.

Generally, a higher voltage cap at a given uF rating, will be lower ESR.


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2000\12\19@142908 by David VanHorn

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At 01:09 PM 12/19/00 -0600, Lawrence Lile wrote:
>Uh oh.
>
>Optek has been telling me that IR is the way to go.  This is bad news.


IR works well to sense smoke. IR isn't attenuated as much (or reflected by,
as much) as visible light, but it's pretty easy to exclude near-IR pollution.

I'd definitely want to dump that module. I don't think it's buying you
anything.



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2000\12\19@143132 by M. Adam Davis

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Of the various designs I've seen (most of which were copyrighted - sigh)
the one that sticks had a tube which the smoke passed through.  On one end
was the sensor (or on both ends) and inside the tube there were holes
(actually smaller tubes connected to the larger one) which would shine IR
into the tube.  the tube interier was flat black, so you'd get a good bit
of IR on the sensor only when smoke was present.  But this required
several IR leds.  Rather than pulsing one LED, why not pulse several high
power IR leds?

Alternately you could use a fine particle filter, filter a bit of the
outgoing air and weigh the filter continuously - Ala the chicken weighing
problem.  Warm up the filter so there's no condensation and viola!  ;-)

-Adam

Lawrence Lile wrote:
{Quote hidden}

> {Original Message removed}

2000\12\19@144118 by David VanHorn

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>Alternately you could use a fine particle filter, filter a bit of the
>outgoing air and weigh the filter continuously - Ala the chicken weighing
>problem.  Warm up the filter so there's no condensation and viola!  ;-)

That sounds like a cute idea for a micromachine application.

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2000\12\19@155340 by James Paul

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Lawrence,

You may want to look into the PHOTOFLASH QUALITY low ESR caps.
Dumping that much energy from any general purpose cap will
cause failures after a short time because of the flexing of the
plates against the end caps and connections.  At least thats
what we found in the oil industry anyway.  Just food for thought.
Granted, I know you're just developing right now, so any cap should
be sufficient for that, but when production come around, go for the
better cap for more reliability.

                                             Regards,

                                               Jim



On Tue, 19 December 2000, Lawrence Lile wrote:

{Quote hidden}

spam_OUTjimTakeThisOuTspamjpes.com

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2000\12\19@160805 by David VanHorn

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At 12:54 PM 12/19/00 -0800, James Paul wrote:
> Lawrence,
>
> You may want to look into the PHOTOFLASH QUALITY low ESR caps.
> Dumping that much energy from any general purpose cap will
> cause failures after a short time because of the flexing of the
> plates against the end caps and connections.  At least thats
> what we found in the oil industry anyway.  Just food for thought.
> Granted, I know you're just developing right now, so any cap should
> be sufficient for that, but when production come around, go for the
> better cap for more reliability.

He really shouldn't have to go to these extremes.
I've seen it done with very simple circuitry. (I used to install them)

Synchronus detection, or at least sending a pulse train will help, but I've
seen it done with DC.

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2000\12\19@173753 by Terry
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Lawrence, since the oven is hot i assume you're placing the electronics and
IR emitter/detecter somewhere below 70 degrees C? Are you by chance passing
the IR through any sort of glass? Cos if you are, that would explain why
your flashlight and a red LED seems to work better. IR gets attenuated
through most glass and other optically clear medium. Blue LEDs have little
or no wavelengths close to the IR spectrum that's probably why it won't work.

Another thought is the heating element in the oven. It'll be emitting heat
and some measure of IR as well. That being the case, why not try the
reverse of what you're trying to do? Have a light source illuminate a
detecter (end to end) through a horizontally mounted tube. Drill a series
of holes in the tube to allow a free flow of smoke to pass through the
light beam and detect a drop in light level.

If costs permits, i would use a microcontroller for better reliability.
Instead of just having a preset light level threshold, i'd incorporate a
power on self test/calibration to compensate for soot/grease/baked insect
build-up in the tube. The light source and detecter's performance  will
also suffer from ageing and heat over a period of time. A bypass switch
would be a nice touch for those who sneak in to reflow solder PCB's when
the wife's away.

Terry



At 01:09 PM 12/19/00 -0600, you wrote:
{Quote hidden}

>{Original Message removed}

2000\12\20@191655 by Lawrence Lile

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On the advice of most everybody I converted the smoke detector to a photoLED
( Optek OP999)  in reversed biased mode.  Hook this up to an op-amp, and
compare the result to a constant DC voltage.  Reverse biased Opto LED's leak
more if they are exposed to light, according to theory.

This one has 4.7 megohm in series with the reverse biased LED.  Supposed to
produce about 60 nanoamps dark current (which would drive the whole thing to
about 4.7 volts)  and maybe 1 microamp light current (driving the thing to
about .3 volts)   The op amp compares this with a DC signal produced by a
pot.  Way too high impedances to put into a PIC directly.

The IR emitter and detector is arranged in reflective mode, with the
detector and emitter 90 degrees from each other, so a reflective object
would set them off.

In practice, no such thing happens.  It makes a great solid object sensor
detecting fingers, paper,  etc.  Won't detect smoke.

Had a chat with the OPTEK engineer I'm working with, and this is the
arrangement he recommended.  Said that the reversed biased LED was much more
sensitive than the photologic module I was working with earlier.  So far,
you can't prove it by me.

-- Lawrence "Smokey" Lile

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2000\12\21@100913 by Lawrence Lile

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The ratio of dark to light current is hard to measure, but I can infer I'm
getting about 150 nanoamps dark current, and 595 nanoamps with a solid
object in front of the detector.  What light current I get with reflected
light from smoke I don't know since I've never been able to trigger it with
smoke yet..

Remember this arrangement is based on pulsing the IR emitter at high amps
for brief periods.  I tried an arrangement with a phototransistor and an LED
powered with DC yesterday, still no luck.  The DC led was probably not
bright enough to do any good.   All of these have worked fine in detecting
solid objects.


{Original Message removed}

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