At 07:12 AM 8/29/97 -0600, Harrison Cooper wrote:
>I was looking thru all my old mail, and thought I had seen some
>information on these types of sensors.  I believe that motorola makes
>some barometric sensors, but what about humidty ?  any website pointers
>would be appreciated.

   Harrison, Low-cost sensors behave like capacitors that vary with humidity
to change the frequency of an external oscillator. Typical relative humidity
ranges from 10 - 90%. Philips makes a low-cost sensor (P/N 2322 691 90001).

   For my project, I needed better accuracy and I wanted to reduce the
complexity of the support circuitry. I ended using HyCal sensors which
provide an output within 0-5V and require minimal support. The outdoor
sensor is an IH-3602L which comes in a T0-39 can with a slotted cap and for
the indoor sensor, I used the IH-3605 hybrid element. Both sensors operate
from 0 - 100% and provide an output from around 0.8V - 3.9V with a 5V
supply. They require a simple low-pass filter and, as with most sensors,
need to be shielded from sources of bright light. You do need to factor in
temperature compensation so you need to measure ambient temperature near the
sensor.

   For barometric sensors, I use a 0 - 15psi pressure sensor. Motorola and
Sensym have low cost sensors. I have a pic 16C74-based weather station that
uses the Sensym SCX15ANC temperature-compensated sensor and I've had
excellent results over the last year. I use a 10V reference to supply the
bridge excitation voltage and I connect the output to an Analog Devices
ADC620 instrumentation amp with a gain of 50. That goes to a 12-bit A/D with
a span of 0 - 4.096V. I scale and offset the data in software. The A/D is a
MAX186 which has 8 channels and an SPI interface.

   For more info, contact:

      Humidity Sensors:
      HyCal          : (818) 444-4000
      Philips        : (817) 325-7871
      General Eastern: (800) 225-3208

      Pressure Sensors:
      Sensym  : (408) 954-1100
      Motorola: http://design-net.com/senseon/pressure.html

   - Tom