I've not used that particular sensor and circuit but I've used similar ones
so, here are some tips.

Be sure that the 555's are CMOS (TLC555) NMOS 555's won't work.

The Op-Amp should either run on split supplies or be a very good rail-rail
type.

The senor only changes by 40pF over the 0% to 100%RH range so, put conformal
coating over the PCB area that is before the final 555 output (but not on
the sensor ;-). This will keep the capacitance of the PCB traces from
changing significantly with %RH.

If using this outdoors, watch out for the temperature dependency
(-0.15%RH/:C =~ 13%RH reading variation outdoors).

Check the final 555's output with an oscilloscope. You should see a 25kHz
pulse train with pulse width dependent on %RH.

Final tip, other manufactures %RH sensors are much easier to use although
they usually cost more than a bare capacitive sensor.

Paul

>-----Original Message-----
>[mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Johan Baarman IT 00
>Sent: Wednesday, January 07, 2004 1:48 PM
>
>Hello
>
>Anybody used the smartec smths10 for humidity measurement? I'm having
>problem calibrating the sensor on a circuit that i have built. The circuit
>is built off the schematics in the smths10 datasheet. The problem is that
>the output varies between GND and Vcc thou it should be between GND and
>0.5*Vcc, and it doesn4t seem to be linear neither. What could I have done
>wrong. Maybe the sensor is damaged?! The circuit is quite simple so I
>don't think I've miss wired it.  Any help, comments is appreciated.
>
>Regards
>Johan Baarman

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