Re: Humidity sensing

[hjohnson@pluto.njcc.com (Herbert R Johnson)]

On Thu, 11 May 2000 17:55:46 -0400, "Bob Dubner" <rdubner@compuserve.com> wrote:
*>
*>Attach a polished piece of metal to the cold side of a Peltier cooler.
*>Attach a thermometer to it.  Bounce a light source off the metal into a
*>sensor.  Ramp the TEC current up until frost begins to form on the polished
*>metal, interfering with the reflected light. Read the dew point off of the
*>thermometer.

Or, instead of a metal surface, etch up a small plate with a pair of
twisting conductors, winding around the board, BUT NOT CONNECTED.
Say you etch a small circuit board like this:

                  =========
                  |========
                  ========|
                  |========

If moisture or ice condenses on this surface, the resistance will change.
Measure the resistance. Don't have an analog input? OK, put a capacitor
in series with the high resistance, pulse it, and measure the time it takes
for the capacitor to discharge to some threshold. Feed
the R/C junction into a transistor or op amp or comparator. A BASIC
stamp can pulse an output, change the lead into an input,
and measure the change from 1 to 0. Or use a comparator, you get the idea.

After all, you do not *really*  need to know the relative humidity in
the camera is 47% at -20C; you just need to know "do I have condensation?"
If you wish you can put a Peltier on the device: make sure you can run it
both HOT and COLD, as you will want to melt the ice occasionally. Or,
mount it near the CCD Peltier for cold, and put a resistor on it and
run current through it for hot.

If you prefer the capacitive sensor solution, the idea of measuring
capacitance rather than resistance is still the same. If you don't
want to write a pulsing program, install a single-chip oscillator
with the sensor like a mini-555 oscillator, and just monitor the pulse
width (this is how the IBM PC used to measure joysticks). You
could probably mount the chip on the same card, they have some
nice SMT micro-packages these days.

I'll bet Maxim Semiconductor has some solutions for this. Check their
Web site, (maxim-ic.com? maxim.com?)

Herb Johnson

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