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PICList Thread
'[OT] Humidity Sensors'
1998\10\28@161547 by Sean Breheny

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Dear piclisters,

I work part time here in one of the libraries at Cornell and I need to
help advise them on which temp/R.H. dataloggers to buy. There seems to be
a wide range of prices (about $150 to $800), without much difference in
specs. I was wondering if anyone could give me some advice. We don't have
very rigorous needs. in particular, we need to be able to measure temp
between 40 and 90 F or so to about 3 deg accuracy. We need R.H. from
about 10% to 90% with about 5% accuracy. The main concern is that the
battery life be long (several years), that it can store at least a
month's worth of data, with a sample every half hour. We would also like
to get ones which have the option(with extra equipment, of course) of
being connected via modem or ethernet to a computer and automatically
downloading the data automatically, rather than our having to send
someone around with a laptop.

One question I had in particular: what kind of reliability issues are
there with the sensors used? I ask this because it seems that the lower
end units use monolithic R.H. sensors and the higher priced ones use
thin-film polymer cap R.H. sensors. I think that both use regular
themistors for the temp.

of course, this would be a great PIC project, but I doubt that they are
willing to let me develop something like this on their time.

Thanks very much,

Sean


'[OT] Humidity sensors'
1999\10\25@174445 by Mark Willis
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Hi, all - need a cheap, cheap, cheap "fog detector" sensor, to tell when
a piece of glass is getting fogged up.  I've thought of using a
LED/photodiode pair, which has possibly problems;  Could use an
enclosed/open thermistor pair, I suppose.  Anyone have ideas?

What I need is a signal "Glass is fogged, or getting slightly fogged" /
"Glass isn't fogged", it'd be nice if this is $1 or so in price, I want
to turn a fan on & prevent fogging of the glass, if I can.

Good ideas on how to read said sensor with a PIC would be wonderful, as
well, as analog is NOT my forte'.  Digital, I can handle, and op amps
etc., just too rusty on transistor biasing and so forth <G>

Looked over the Archives, there is a Philips P/N 2322 691 90001 sensor
that is "low cost", but no numbers on what "low" is, etc.

(The usual cheap rock salt crystal / clothespin switch won't work here
<G>)

If anyone has something like a resistive sensor that picks up water &
changes resistance, that'd be fine, even if it has to periodically be
washed or something.  This is a $30 per unit project, so obviously $15
parts are not usable <G>

 Mark

1999\10\25@175111 by Dave VanHorn

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> Hi, all - need a cheap, cheap, cheap "fog detector" sensor, to tell when
> a piece of glass is getting fogged up.  I've thought of using a
> LED/photodiode pair, which has possibly problems;  Could use an
> enclosed/open thermistor pair, I suppose.  Anyone have ideas?

Use the optical sensor, but set it to pick up the diffuse reflection.
Most are designed to maximize the specular reflection, which will be exactly
what you don't want.

I designed a sensor like this to pick up paper behind a glass window. The
omron sensors had a contrast ratio (target / no target) of 3/1 at best. The
one I designed was 7600+ /1

1999\10\25@175734 by jamesp

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Mark,

What about a piece of PC board with a couple of small traces
on it.  Use thin stuff and mount it directly to the glass so
that it takes on about the same temperature as the glass.
If fog starts on the glass, it also will start on the pc board.
You could read it digitally as part of a voltage divider or
you could read it with an A/D converter input on one of the
PIC's.  Or if the application permits, put two traces of
adhesive backed copper tape directly on the glass to be
monitored.  Read it the same as the PC board sensor.  I used
a sensor similar to this for detecting rain and it worked
like a champ.  Let me know if this will do it.

                                       Regards,

                                         Jim




{Quote hidden}

1999\10\25@185535 by Mark Willis

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I wrote:
> > Hi, all - need a cheap, cheap, cheap "fog detector" sensor, to tell when
> > a piece of glass is getting fogged up.  I've thought of using a
> > LED/photodiode pair, which has possibly problems;  Could use an
> > enclosed/open thermistor pair, I suppose.  Anyone have ideas?

Jim Paul wrote:
{Quote hidden}

That will be my backup plan - I may be making this PCB myself, and I
cannot make 10 mil traces on 10 mil centers yet <G>  The idea's to flip
the fan on the instant fog starts to perform, to improve vision (The
more I talk about this, the more uses I think of for this circuit, you
know?  <G>)  Thanks!

Dave VanHorn wrote:
> Use the optical sensor, but set it to pick up the diffuse reflection.
> Most are designed to maximize the specular reflection, which will be exactly
> what you don't want.
>
> I designed a sensor like this to pick up paper behind a glass window. The
> omron sensors had a contrast ratio (target / no target) of 3/1 at best. The
> one I designed was 7600+ /1

Yes!  I think you mean, shine the LED at the glass from (say) 45 degrees
from normal, and have the Photodiode at ANY angle other than 45 degrees
from normal to the glass on the far side?  THAT makes good sense, can
have both aiming in parallel at 45 degrees to normal at the same patch
of glass/plastic, something like (Digikey p. 575, Q994 catalog) the QT
Optoelectronics H23A1QT-ND (IR Interrupter module) at $0.73 would be
great, possibly?  This'd detect the first bit of fogging, exactly what I
want, sensor position will be the thing to take care on here <G>  Or is
Visible light better to use, here?  (Most everything bounces off small
water drops fairly well, I thought?)

Hmm, do I need an ADC or can I read this photosensor directly off a
12C509A, I wonder?  <G>

Thanks, folks!

 Mark

1999\10\25@203150 by Jinx

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Any chance a thin slice of conductive foam could be used somehow ?

Joe

> Hi, all - need a cheap, cheap, cheap "fog detector" sensor, to tell when
> a piece of glass is getting fogged up.  I've thought of using a
> LED/photodiode pair, which has possibly problems;  Could use an
> enclosed/open thermistor pair, I suppose.  Anyone have ideas?
>
> What I need is a signal "Glass is fogged, or getting slightly fogged" /
> "Glass isn't fogged", it'd be nice if this is $1 or so in price, I want
> to turn a fan on & prevent fogging of the glass, if I can.

1999\10\25@204638 by Mathew Cohen

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Try using a dew sensor from a VCR. these devices are used to detect
condensation in a VCR to avoid damage. They are a small device and I dont
think they should cost to much.

Mathew Cohen
{Original Message removed}

1999\10\25@211758 by hris Fanning

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> > Hi, all - need a cheap, cheap, cheap "fog detector" sensor, to tell when
> > a piece of glass is getting fogged up.  I've thought of using a
> > LED/photodiode pair, which has possibly problems;  Could use an
> > enclosed/open thermistor pair, I suppose.  Anyone have ideas?
>
> Use the optical sensor, but set it to pick up the diffuse reflection.
> Most are designed to maximize the specular reflection, which will be exactly
> what you don't want.
>
> I designed a sensor like this to pick up paper behind a glass window. The
> omron sensors had a contrast ratio (target / no target) of 3/1 at best. The
> one I designed was 7600+ /1

That sounds good, but what about dirt?  Is that an issue here?  If it
is, the only reliable way I can think of doing this is to have two
temperature sensors and a hygrometer:

  1 temperature sensor monitoring ambient temperature
  1 hygrometer measuring humidity
  1 temperature sensor monitoring the glass surface temperature

If the temperature of the glass gets close to the dew point, then
kick the heater/fan on.

Not $1 but you could set it so the glass would never fog...

Chris

1999\10\25@212829 by Mark Willis

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Hmmm, first I've heard of those - Any idea who makes them, etc.?
(Haven't done much VCR repair work.  I can ask locally, come to think of
it, for one.)

 Mark

Mathew Cohen wrote:
>
> Try using a dew sensor from a VCR. these devices are used to detect
> condensation in a VCR to avoid damage. They are a small device and I dont
> think they should cost to much.
>
> Mathew Cohen
> {Original Message removed}

1999\10\25@213911 by Erik Reikes

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At 09:16 PM 10/25/99 -0400, you wrote:
>
>That sounds good, but what about dirt?  Is that an issue here?  If it
>is, the only reliable way I can think of doing this is to have two
>temperature sensors and a hygrometer:
>
>   1 temperature sensor monitoring ambient temperature
>   1 hygrometer measuring humidity
>   1 temperature sensor monitoring the glass surface temperature
>
>If the temperature of the glass gets close to the dew point, then
>kick the heater/fan on.
>

I know there must be some reason you can't do this or somebody would have
said it already...

Why not just leave the fan/heater on all the time?

Or at worst cycle it to lengthen the life...

-Erik Reikes

1999\10\25@214130 by Mark Willis

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Chris Fanning wrote:
{Quote hidden}

Anything more than $1, and it eats profits, though.  The consumer, in
this case, knows to clean the unit if it gets dirty (Same as a furnace
manufacturer just tells you to replace your furnace filter regularly,
they cannot MAKE you do it, but if you don't do it, it's not their
problem.)  So long as it works when the glass is fairly clean, customer
will be happy!

 Mark

1999\10\25@220403 by Mark Willis

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Erik Reikes wrote:
{Quote hidden}

Noise, heat conservation, battery life, and the fact that heat
generation (in this case) comes in big bursts, that cannot be really
predicted - My 3rd choice here is to run a 12v fan at 9V or so, which
works, and gives a nice slow fan speed that overdoes it (but you can see
through the glass/plastic window when you want to!)  Also could just
have a user-operated switch, even cheaper.  Having it just handle the
problem, and then stop, would be a really good solution, though.

 Mark

1999\10\25@232023 by Anne Ogborn

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>> > Hi, all - need a cheap, cheap, cheap "fog detector" sensor, to tell
when
>> > a piece of glass is getting fogged up.  I've thought of using a
>> > LED/photodiode pair, which has possibly problems;  Could use an
>> > enclosed/open thermistor pair, I suppose.  Anyone have ideas?


Put a collimated beam of light through it at right angles.
Put a light detector at an off angle. (obviously all this needs light
sheilding from the outside world).

When the glass is clear the beam will pass straight through.
When the glass is fogged, the beam will be reflected at random
angles off the little spherical dew particles.

Another way -
put a beam of light through the glass to a detector. Where it passes through
the
glass, put four resistors in a square as a heater loop.
At intervals (I don't know how often you need to do this measurement)
measure the light transmitted, turn on the heater for a minute, and measure
again.
If the light has increased, you've heated a local area and cleared it.
You can do your measurement by bumping a reference voltage up until
it matches the output voltage, then doing a differential compare.

Why not just pass light through and compare to a reference?  Probably not
sensitive enough considering the long term noise fluctuations.

1999\10\25@234906 by Dave VanHorn

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> Yes!  I think you mean, shine the LED at the glass from (say) 45 degrees
> from normal, and have the Photodiode at ANY angle other than 45 degrees
> from normal to the glass on the far side?  THAT makes good sense, can
> have both aiming in parallel at 45 degrees to normal at the same patch
> of glass/plastic, something like (Digikey p. 575, Q994 catalog) the QT
> Optoelectronics H23A1QT-ND (IR Interrupter module) at $0.73 would be
> great, possibly?  This'd detect the first bit of fogging, exactly what I
> want, sensor position will be the thing to take care on here <G>  Or is
> Visible light better to use, here?  (Most everything bounces off small
> water drops fairly well, I thought?)

Should be. What we're doing is actually catching the specular reflections
off the drops, but missing the big one from the plate.

I used emitter direct at the target, and sensor at 45 degrees, with some
baffles to keep the sensor from seeing the emitter. LEDs emit ALL OVER.

> Hmm, do I need an ADC or can I read this photosensor directly off a
> 12C509A, I wonder?  <G>


Depends on your ckt, photo transistors output current, not voltage.
Me, I'd run the LED hot, and make the phototransistor "numb" to avoid
problems with other light getting in (possible?)

1999\10\26@003501 by Mark Willis

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Thanks, William and Anne, also <G>  One of the things I'm realizing as I
look for parts here is that fans have gone UP in price, fun fun fun <G>

Dave VanHorn wrote:
{Quote hidden}

I think those IR interrupters are in a package like a rectangular LED;
Put the LED behind the detector, and slightly higher, bend it to aim it;
should do the job.  (Need to figure how to go through the grille there,
I'll handle that, it's a soft plastic <EG>  "Drill one .250 inch hole"
should handle this.)

> > Hmm, do I need an ADC or can I read this photosensor directly off a
> > 12C509A, I wonder?  <G>
>
> Depends on your ckt, photo transistors output current, not voltage.
> Me, I'd run the LED hot, and make the phototransistor "numb" to avoid
> problems with other light getting in (possible?)

Probably just use the phototransistor & a pull-up, I'm thinking, and see
how that works.

Planning to design it so other light isn't seen here - I would look
through the glass/plastic at a piece of the upper mounting frame, should
handle external light.  (If it sees glare & runs the fan a little extra,
that's not TOO bad, the idea is to just not run it TOO much, if it bugs
the user they can turn it off for a while.)  Part of why I wanted to use
IR was that I planned to add a wrap of electrical tape, to screen off
visible light (If I can block possible sun as well, I will!)

 Mark

1999\10\26@005348 by Dave VanHorn

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Our sensor is sold by opto technology, I forget the part number.
It's an emitter/detector pair in a plastic case. Should be about $1 or less.

1999\10\28@085855 by fernteix

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Hi
I would use a reflective type of optocoupler and choose an optimal position
to get a "total" reflection without fog. In this position the intensity of
reflected light with and without fog should be very different.
A OPA as comparator will do the rest, I believe. Some small Pics have
comparators inside.

fernando

{Original Message removed}

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