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HCH-1000 Humidty Sensor

Discussion in 'Sensors and Actuators' started by Gav1985, Mar 12, 2013.

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  1. Gav1985

    Gav1985

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    Mar 12, 2013
    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.
     
  2. BobK

    BobK

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    1,688
    Jan 5, 2010
    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
     
    Last edited: Mar 12, 2013
  3. Gav1985

    Gav1985

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    Mar 12, 2013
    I haven't looked at that yet. I was merely hoping that using a digital frequency would work as well as an analogue frequency. :)
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,497
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    Jan 21, 2010
    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?
     
  5. Gav1985

    Gav1985

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    Mar 12, 2013
    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.
     
  6. Gav1985

    Gav1985

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    Mar 12, 2013
    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
     
  7. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    I was talking about something much simpler :)

    [​IMG]
     
  8. Gav1985

    Gav1985

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    Mar 12, 2013
    Ahhhh, Genius. So the oscillation at Vout will vary according the capacitance of the humidity sensor?
     
  9. BobK

    BobK

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    Jan 5, 2010
    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
     
  10. Gav1985

    Gav1985

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    Mar 12, 2013
    Magic. Thanks for the help.

    Gav.
     
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