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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
    threads:
    http://www.digikey.com/scripts/us/dksus.dll?Criteria?Ref=141335&Cat=32375562
    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.

    Greg
     
  2. gdon wrote:
    (snip)
    The basic component that decides if one voltage is more or less
    positive than another is the comparator:
    http://www.national.com/catalog/AnalogComparators.html

    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:
    http://cache.national.com/ds/LM/LM193.pdf

    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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