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'[PIC]: Relative Humidity Sensor'
2000\12\29@004957 by Allan

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Does anyone have a favorite PIC compatible humidity sensor?  I was
looking for something inexpensive, and low accuracy (5 or 10%).  I want
to replace the mechanical humidistat on my humidifier with a PIC based
solution that I can connect into my home automation system.
Thanks...
Allan Dobler

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2000\12\29@012248 by Robert Francisco

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I just briefly read a catalog from MSCDirect.com ( an industrial supply
dist). It's a temp and humidity sensor/readout with BCD output. The humidity
sensor is temp compensated. It's not a discrete sensor but a complete unit.
p.n. rr86499159

Robert

{Original Message removed}

2000\12\29@054030 by Marcelo Yamamoto

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Check http://www.honeywell.com
They are not so inexpensive but have a conditioned output you can
use with a built in AD PIC.

Marcelo Y.


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2000\12\29@095301 by Lawrence Lile

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Another guy suggested honeywell sensors. They are great, but at $500 to
$1000 a crack, and 1% accuracy, you probably can't afford one.

Here's how you do a cheap and dirty humidity sensor.  The most basic
humidity sensor is a dry bulb/wet bulb sensor.  It senses the dry bulb
temperarature with a standard temperature sensor, then also senses the
temperature insite a wetted cloth sack (the wet bulb).  A wet cotton sock
might work OK for this.

I built a controller for a $30,000 environmental chamber using precision wet
and dry bulb thermocouples.  This method can be very accurate and precise,
it is not just for hackers.

Now there is a complex formula relating wet and dry bulb to humidity, but
you don't care about it.  All you need is a lookup table for a few
setpoints.  You _might_ even get away with just measuring wet bulb
temperature if the air you are measuring is relatively constant dry bulb
temperature.  I've got references (ASHRAE handbook) that I can give you if
you want to look into this method further.

The last problem is keeping the wet bulb wet.  You are using a humidifier,
so you already have a source of water handy.  Shouldn't be too hard to hack
together.  You also have to keep air moving over the wet bulb, to evaporate
water.  This also shouldn't be too hard in a heating duct.  Be careful about
corrosion - use plastic parts in contact with the water.  Maybe an epoxy
encapsulated Dallas temperature sensor chip would work nicely.

I can send you a photo of my wet/dry bulb setup offlist if you are
interested.

-- Lawrence Lile


{Original Message removed}

2000\12\29@132659 by Tom Handley

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  Allan, check out General Eastern's G-CAP line:

     General Eastern: http://www.geinet.com/

  - Tom

At 10:53 PM 12/28/00 -0700, Allan wrote:
>Does anyone have a favorite PIC compatible humidity sensor?  I was
>looking for something inexpensive, and low accuracy (5 or 10%).  I want
>to replace the mechanical humidistat on my humidifier with a PIC based
>solution that I can connect into my home automation system.
>Thanks...
>Allan Dobler


------------------------------------------------------------------------
Tom Handley
New Age Communications
Since '75 before "New Age" and no one around here is waiting for UFOs ;-)

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2000\12\29@155147 by Allan

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Thanks for the links guys, that helps out a lot
Allan


>
> > Does anyone have a favorite PIC compatible humidity sensor?  I was
> > looking for something inexpensive, and low accuracy (5 or 10%).  I want
> > to replace the mechanical humidistat on my humidifier with a PIC based
> > solution that I can connect into my home automation system.
> > Thanks...
> > Allan Dobler

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2000\12\30@230109 by Dale Botkin

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On Fri, 29 Dec 2000, Tom Handley wrote:

>    Allan, check out General Eastern's G-CAP line:
>
>       General Eastern: http://www.geinet.com/

Which look really nice, and if anyone has a way to get just a couple at a
reasonable price I'm interested.  I have yet to find a similar RH sensor
that I can obtain one or two of for a few bucks.

Dale
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2000\12\30@231623 by Robert A. LaBudde

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At 10:00 PM 12/30/00 -0600, you wrote:
>On Fri, 29 Dec 2000, Tom Handley wrote:
>
> >    Allan, check out General Eastern's G-CAP line:
> >
> >       General Eastern: http://www.geinet.com/
>
>Which look really nice, and if anyone has a way to get just a couple at a
>reasonable price I'm interested.  I have yet to find a similar RH sensor
>that I can obtain one or two of for a few bucks.

I bought an evaluation kit from them with 5 sensors. They'll probably send
you a free sensor or two.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: spam_OUTralTakeThisOuTspamlcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
824 Timberlake Drive                     Tel: 757-467-0954
Virginia Beach, VA 23464-3239            Fax: 757-467-2947

"Vere scire est per causas scire"
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2000\12\30@233430 by Allan

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Hi Dale,
I have been scouring the net for a week, and I can't find anyone that
will sell one or two RH sensors either.  They are usually 1000 lot
minimum, or they cost $100 ea.  I have seen $25 RH/temp battery powered
desktop units that display to a 7 segment LCD.  I may have to
cannibalize the sensor out of those.  That seems like a waste though.

Allan

Dale Botkin wrote:
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2000\12\30@235304 by Dale Botkin

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On Sat, 30 Dec 2000, Allan wrote:

> Hi Dale,
> I have been scouring the net for a week, and I can't find anyone that
> will sell one or two RH sensors either.  They are usually 1000 lot
> minimum, or they cost $100 ea.  I have seen $25 RH/temp battery powered
> desktop units that display to a 7 segment LCD.  I may have to
> cannibalize the sensor out of those.  That seems like a waste though.

I know, my $29 Radio Shack thermometer/hygrometer has a sensor in it that
could not possibly have cost more than $.50 in any sort of quantity.  All
I want is to sense temp and humidity in my humidor, feed it to a PIC and
keep it at the right temperature and humidity with a Peltier element.  is
that asking so much??  ;-)

Dale
---
The most exciting phrase to hear in science, the one that heralds new
discoveries, is not "Eureka!" (I found it!) but "That's funny ..."
               -- Isaac Asimov

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2000\12\31@001231 by Roman Black

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Dale Botkin wrote:
> I know, my $29 Radio Shack thermometer/hygrometer has a sensor in it that
> could not possibly have cost more than $.50 in any sort of quantity.  All
> I want is to sense temp and humidity in my humidor, feed it to a PIC and
> keep it at the right temperature and humidity with a Peltier element.  is
> that asking so much??  ;-)

Hi Dale, I'm good at cheap solutions! ;o)
Go see your local TV/VCR repairer, ask or beg for a
"dew" sensor from just about ay old VCR. If he is friendly
he will find one from an old scrapped VCR and probably
donate it to you, or just charge a couple of bucks.
They are resistive, about 1200 ohms at normal room
humidity if I remember right.
-Roman

PS. If you can get an old scrapped VCR I can tell you
where the sensor is, heck you can even pull the sensor
from your own VCR and replace it with a resistor, most
of the newer VCRs don't bother with a sensor anyway. It
is one of those older "quality" features.

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2000\12\31@004111 by Allan

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Hi Roman,
do those dew sensors measure RH or condensed moisture (wet tape)?
Allan

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2000\12\31@051912 by John De Villiers

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Right, I have a spare parts VCR that has a humidity sensor in it. Its a
Mitsubishi HS-337EZ. Problem is the moisure light never goes off ( hence the
allocation to the spare parts bin ). Could be the sensor. Any idea what it
looks like and what val resistor i can replace it with?

John

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2000\12\31@101258 by Roman Black

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Allan wrote:
>
> Hi Roman,
> do those dew sensors measure RH or condensed moisture (wet tape)?
> Allan

They appear to be printed substrate on a small
ceramic plate about 8mm square. The resistance
varies depending on how much moisture in the air,
ie breathing on it changes the resistance. They
are mounted near the head drum on a vertical metal
plate on most older VCRs. The National/Panasonic
ones are notorious for going off value and causing
the dew indicator fault in VCRs about 12-15 years
old. The Sharp ones seem to be more reliable. :o)
-Roman

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2000\12\31@101924 by Roman Black

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John De Villiers wrote:
>
> Right, I have a spare parts VCR that has a humidity sensor in it. Its a
> Mitsubishi HS-337EZ. Problem is the moisure light never goes off ( hence the
> allocation to the spare parts bin ). Could be the sensor. Any idea what it
> looks like and what val resistor i can replace it with?
>
> John

Ha ha! Try a 1200 ohm resistor, if that doesn't work
go to 2200 ohms. Replace the sensor completely with
the resistor. The sensor is a little white ceramic
thing mounted on a vertical metal plate near the head
drum. :o)
-Roman

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2000\12\31@160241 by Peter L. Peres
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I think that RH sensors for VCRs can be bought as spare parts. Try a local
VCR repair shop. I do not know what range they have, and they are
guaranteed not to be linear, but at least there is one in each and every
vcr, dat tape, and camcorder (even dv), so there should be spare parts...

The only time I played with this I found that they go high Z when humid
(esp. condensation). I suppose that one could be calibrated. Standard
circuit seems to be 220-330K pullup from 5V followed by simple comparator
or A/D measurement.

For a single, strip a dead VCR and look in the schematics for the exact
value of the pullup resistor. I suspect that a lower resistor value will
not do it much good.

Since these go bad once in a while, it is a good idea not to rely on them
too much imho.

Peter

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