humidity sensor

[Wes]

Is there a reasonable priced humidity sensor that interfaces with pic's easily? Phanderson
has a board but I think at 43 USD, it is out of my price range. I have some one wire temp
chips on the way.

My application is to drive a cooling fan to bring in cool air into my garage (shop) at
night and exhaust air when the inside is hotter than the outside.

Since I have machine tools inside, dew point is important in the calculation. I don't
want to bring in moisture laden air if my tools are below dew point.

Thanks,

Wes

 

[DJ Delorie]

I'm using the HIH-4000 family of sensors ($15-20 at digikey) for my
home thermostats. They give out a DC voltage proportional to
humidity; if your PIC has an ADC that's all you need. They also
interface nicely with the maxim "battery monitor" 1wire chips (such as
phanderson uses), which have temperature sensing in them.

You can buy cheaper capacitive sensors for much less, but you need to
use them in an oscillator or RC circuit to let the PIC sense the
capacitance (and thus humidity). Again, if your PIC has an ADC, you
can use an RC circuit between a GPIO output and the ADC to measure the
RC constant in software.

example: http://electronicdesign.com/Files/29/9808/Figure_01.gif

 

[Tim Wescott]

On Tue, 24 Mar 2009 18:25:24 -0400, Wes wrote:

Is there a reasonable priced humidity sensor that interfaces with pic's
easily? Phanderson has a board but I think at 43 USD, it is out of my
price range. I have some one wire temp chips on the way.

My application is to drive a cooling fan to bring in cool air into my
garage (shop) at night and exhaust air when the inside is hotter than
the outside.

Since I have machine tools inside, dew point is important in the
calculation. I don't want to bring in moisture laden air if my tools
are below dew point.

Thanks,

Wes

Trying to figure out dew point from temperature and humidity can be a
thankless task -- the vapor pressure of water varies exponentially with
temperature, so anything more than a few degrees difference and you're
almost out of luck.

There are dewpoint sensors out there. It may even be fun to build one.

--
http://www.wescottdesign.com

 

[Yukio YANO]

Wes wrote:

Is there a reasonable priced humidity sensor that interfaces with pic's easily? Phanderson
has a board but I think at 43 USD, it is out of my price range. I have some one wire temp
chips on the way.

My application is to drive a cooling fan to bring in cool air into my garage (shop) at
night and exhaust air when the inside is hotter than the outside.

Since I have machine tools inside, dew point is important in the calculation. I don't
want to bring in moisture laden air if my tools are below dew point.

Thanks,

Wes
It will never happen !!

If a Differential thermostat is set to bring in COLD and moist air into
a Warm garage, the Tools will always be warmer than the external Air and
thus warmer than the Saturated Air . Only if you were to bring in Warm
MOIST Air would you get condensation on the COLD Tools.

BY wiring an external Thermostat and Fan to bring COOLER air than the
internal Temperature Thermostat; you would never have your tools below
the Dewpoint. Besides the fan would cutoff when the ROOM temperature
dropped below the ROOM Set Temperature.


Yukio YANO