HCH-1000 Humidty Sensor

[Gav1985]

Link to datasheet. http://www.farnell.com/datasheets/1683363.pdf

Question, it states that the Vrms for this device is 1Vrms @ 20kHz. I want to use a humidity sensor in my project with a MCU (PIC18f4550) without lugging around a signal generator. If I use a digital pulse at 20KHz 2Vpeak, will that allow the sensor to work optimally?

Thanks.

Edit, I know 20KHz is quite a high frequency for a MCU output. There are other sensors that operate at a more reasonable 1KHz 1Vrms so the principal of the question is the same.

 

[BobK]

The 20 KHz is just the frequency used to characterize the device. If you look at the specs, it can operate from 1KHz to 100KHz.

How are you planning on measuring the capacitance?

Bob

 

[Gav1985]

I haven't looked at that yet. I was merely hoping that using a digital frequency would work as well as an analogue frequency.

 

[(*steve*)]

I would recommend you use the sensor as a capacitor in an RC oscillator.

Because of the low capacitance, you probably want to use something like a CMOS Schmitt trigger inverter oscillator.

Use your uC to measure either a pulse duration, or the number of pulses in a given period to determine the humidity.

How accurate do you want this to be?

 

[Gav1985]

Reasonably accurate. But as It's for my college project I'm going to be picking components on price to try and keep the project under budget. So I don't mind sacrificing accuracy to make it simpler and cheaper.

Thanks.

I shall have a look at your suggestion.

 

[Gav1985]

I found this circuit on good old Wikipedia, Is this the type of circuit you were alluding to Steve?

Replacing the capacitor with the the Humidity sensor would give a circuit with a varying capacitance depending on the humidity? And therefore a varying oscillation?

Thanks.

http://en.wikipedia.org/wiki/File:OpAmpHystereticOscillator.svg

 

[(*steve*)]

I was talking about something much simpler

 

[Gav1985]

Ahhhh, Genius. So the oscillation at Vout will vary according the capacitance of the humidity sensor?

 

[BobK]

The frequency varies with humidity.

The micro could count cycles over a fixed period, say 1 second then perform some math to get the humidity.

Bob

 

[Gav1985]

Magic. Thanks for the help.

Gav.