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humidistat question

Discussion in 'Electronic Design' started by gdon, Oct 18, 2003.

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  1. gdon

    gdon Guest

    I am a humidor builder looking to add active humidification to my larger
    models. These models range from around 6 cubic feet to as large as 18. In
    smaller models, I use a passive humidification system which uses a 50/50
    solution of propylene glycol/distilled water. This does not work well in
    large models because they dry out too fast.

    What I want to design is a small device to measure humidity and activate a
    small 40mm fan if the relative humidity drops below 66%, and turn off the
    fan when RH exceeds 70%. The accuracy of the device should be +/-2%. The
    fan will pass air over a wicking fiber which is attached to a distilled
    water reservoir.

    I am aware of the various humidity sensors available as indicated in other
    But the part that I need help with is the circuit design. I am a newbie!

    I was hoping that somebody could point me in the right direction? Any tips
    or links are very much appreciated.

  2. gdon wrote:
    The basic component that decides if one voltage is more or less
    positive than another is the comparator:

    They have two voltage inputs, and lots of gain, so that if the voltage
    connected to the + input is even a few thousandths of a volt more
    positive than the signal connected to the - input, the output goes
    positive (usually this means the output switch which connects to the
    negative supply rail turns off).

    See the data sheet for the very common dual unit, LM393:

    It has some circuit suggestions that you might be able to adapt as
    your fan control. The versions with hysterisis (clean decisive
    switching without chatter) on page 9, for instance.
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