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Measuring Humidity

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I am attempting to design a multi-display clock, with also shows the date, temperature and humidity. Can anyone tell me, the best way of detecting/measuring humidity? I would like to output a numerical figure (1 - 100%). I'm aware there would be some form of calibration involved, but I don't really have any ideas what sort of componants to use or how to go about it! Is there a module that can be purchased that detects it and O/Ps a BCD string or some such?

Any suggestions or comments would be most welcome.

Thanks.
 
I've done it.

Go on Digikey and look for "Humidity".

But let me save you some time. For about $20, you can get the Humirel HS1100 module with a voltage output. For about $10, you can get the Humirel HS1101 sensor alone, with a variable capacitance output. Now Humirel spec sheet has excellent data on making a well specified precision 555 circuit with a variable freq output. You can then use an interrupt pin instead of an ADC pin, no additional ADC errors, and has all the nonlinearity mapped out so you can compensate for it in the uC. I like that sensor and 555 circuit myself.
 
Oznog, thank you very much for the information! I have successfully found the sensor and read through the datasheet. The 555 circuit is as straightforward as they get, but could you tell me how you converted the frequency O/P from it, to a numerical O/P (assuming you did!!!)

I can think of several ways of achieving it, with ever increasingly large component counts! I have PIC programming knowledge, but not the kit to do any programming. So a non-PIC solution would be more appreciated, although anything would be welcome!

Thanks again.
 
Put the 555 output onto a PIC interrupt pin.
That pin's interrupt routine increments a 16 bit counter.
Configure a timer so that you run an interrupt routine over a long period- maybe every 1/2 sec-1 sec (it need not happen on an even number, fractions will be accomodated on the lookup table). It copies the counter value and resets it to 0.

I used a lookup table myself to get a humidity percentage out of the counter value. It has table entries for maybe every 10% and interpolates intermediate values.
 
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