AVR-Chat

On Mar 30, 2006, at 1:48 AM, aila_alizadeh wrote:

> hi everybody:
> does anybody have information about humidity sensors with 2 pin ?
> I just know that they act as a capacitor ,but I don't know how I
> should measure it ,and how I should do that with avr .
> whould you mind sharing your experience about that with me ?
> If you send me any useful information about measuring & calibrating
> these sensors and build a hygrometer ,I'll be very

The best bet for measuring capacitive sensors is to use one of the  
capacitance-to-analog or capacitance-to-digital converters that are  
sold for that purpose.

QProx QT301
http://www.digikey.com/scripts/DkSearch/dksus.dll? 
KeywordSearch&Stock=1&Keywords=qt301

AD Capacitance to digital converters
http://www.analog.com/en/subCat/0,2879,760%255F1077%255F0%255F%255F0% 
255F,00.html

You can use other techniques (like for instance a CD4046 PLL) to  
measure the capacitance, and then use a counter to measure the  
resultant frequency.

Depending on the capacitance, you might also be able to do a really  
simple converter that charges the cap through a resistor (fed from a  
digital output pin) and then uses the analog comparator to detect  
when a threshold voltage has been reached. The time that the cap  
takes to charge to that level then gives you the capacitance.

I don't recommend using the schmitt trigger inputs for this, as their  
threshold is not necessarily too stable or predictable.

If you added an analog switch you could compare the unknown  
capacitance with a known reference; this helps cancel out  
microcontroller-based errors.

-- 
Ned Konz
ned@bike-nomad.com