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Designing a schematic for capacitive humidity sensor

elektroboard
Prodigy 105 points
Other Parts Discussed in Thread: TLC555, HDC3020

Hi.

I want to design a schematic for capacitive humidity sensor. I saw schematic with sequare wave osillator (TLC555 etc.). And I found AD7745.

Is there converter from capacitive to frequance or from capacitive to voltage chip. How can I measure capacitive of capacitive humidity sensor?

Thanks.

  • 0
    TI__Guru* 91206 points

    Dear electroboard - 

    Thanks for the post and welcome to E2E!

    We make capacitive based %RH and temperature sensors, which have all the needed circuits integrated into them for measuring %RH. The capacitance is measured internally, and not available directly to the user, but the usable result is. 

    The parts in the portfolio are here, with the HDC302x devices being the newest. 

    https://www.ti.com/sensors/humidity-sensors/products.html 

  • 0 in reply to Josh Wyatt
    Prodigy 105 points

    Hi Josh.

    Thanks for reply.

    The sensor I need to use should be able to measure up to +190 celcius degrees.

  • 0 in reply to elektroboard
    TI__Guru* 91206 points

    Dear elektroboard - 

    OK - couple things - first, water boils at 100C°, so the %RH will be 0% above that. Second, you will have a hard time finding any semiconductor based devices which can withstand operating temp of 190°C (for example the 555 timer you are looking at has an operating max temp of 125°C and we only make one part which is rated for 200°C operation, as a temp sensor (LM95172). 

  • 0 in reply to Josh Wyatt
    Prodigy 105 points

    Hello. Some ready-made foods are dried at 150 degrees and humidity is measured at this temperature. The humidity sensor will be placed in a metal probe and the measurement will be made. therefore semiconductors will not be exposed to such high temperatures 

  • 0 in reply to elektroboard
    TI__Guru* 91206 points

    Dear elektroboard - 

    Thanks for sharing the application. I think drying food happens in the range of 140F (not C). (190C is 374F, which is a temperature you would cook at, not dehydrate) - in that case, then, you would be free to use a part like the HDC3020 or any of the other family members of that portfolio. Additionally, you would want to put a water vapor permeable cover on your probe, made from PTFE. 

    Like this, for example: 

  • 0 in reply to Josh Wyatt
    Prodigy 105 points

    Thank you for the contribution. I will review these products. I expect you to offer alternatives so that I can measure the value of the capacitive sensor as analog voltage or frequency. thanks 

  • 0 in reply to elektroboard
    TI__Guru* 91206 points

    Dear elektroboard - 

    You are very welcome - to set your expectations correctly, we do not offer any parts which provide a raw capacitance measurement value out from our %RH sensor. They are integrated, highly optimized and calibrated for a range of temperature and %RH operation. This is for ease of use (by you).