Hi guys... I've designed a low cost humidity sensor based on previous suggestions using a carbon resistor with its coating removed. Here's the schematic I want to build:
Has anybody done this before? Do I need to introduce other stabilizing parts like temperature compensation, etc.? The output it not very linear, so I plan to calibrate it using programming to linearize its output.
Great Idea! Unfortunately I haven't made something like that but I want to.
I prefer AVR instead of PIC, I can use AtMega8 which has a pretty good ADC.
But how to do the calibration? You'll need a working very accurate humidity meter and controlled environment without air currents and a way to gradually change the humidity... Now that'll be difficult. You (and I) will also need to use accurate resistors, say, like, 0.5, max 1%
Carbon film resistors change resistance considerably with temperature changes.
There were experiments done using 0.1% metal film resistors, but after some time the measurement becomes unreliable because of oxydation of the metal film.
Philips (NXP) offers a reasonably priced (€10) humidity sensor using a gold cap of 122pF initially changing capacitance 0.4pF/%RH. This also non linear.
Carbon film resistors change resistance considerably with temperature changes.
There were experiments done using 0.1% metal film resistors, but after some time the measurement becomes unreliable because of oxydation of the metal film.
Philips (NXP) offers a reasonably priced (€10) humidity sensor using a gold cap of 122pF initially changing capacitance 0.4pF/%RH. This also non linear.