Capacitive Humidity sensor

pwillard

I've been looking for alternatives to the $30 Humidity sensors since I need to scan 6 individual enclosures for humidity and the costs rather skyrocketed for this once-off project.

What I've been examining are the Honeywell capacitive sensors that vary from 310-350pF with 0-100% humidity.

Reading such a small variance in capacitance seems tricky. The datasheet shows only a test setup with an LCR meter... so that's not giving a good design clue.

What comes to mind is a part like a 555 in Astable mode with the Humidity Cpacitance driving the frequency of oscillation.· Converting the frequency to voltage with something like an LM2917 maybe and then reading that voltage at an SX pin with analog in?

Ok, that sounds really complicated... but it's all I've come up with in 5 minutes of leaning back in my chair and smoking my pipe.

I'm thinking that there must be an easier way.· ideas?

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There's nothing like a new idea and a warm soldering iron.

Phil Pilgrim (PhiPi)

I think your best bet would be a 556 configured as two astables: one using the hygrometer cap and another using a fixed cap with similar temperature characteristics. This is to cancel any change in frequency caused by changes in temperature alone. (You could even use a second hygrometer cap in a hermetically-sealed enclosure with some silica gel as a reference.) Then just program your SX to read the frequencies of the two astables and compute the humidity from there.

-Phil

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'Still some PropSTICK Kit bare PCBs left!

pwillard

Whoosh... that's that sound of my letting the solution whizz by my head...· Of course just reading the frequency with the SX...·· Smack I could have had a V-8 too!

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There's nothing like a new idea and a warm soldering iron.

Sparks-R-Fun

SX/B has an RCTIME command that may help you read the capacitance variation using just two support resistors.


- Sparks