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Blow sensor?

Discussion in 'Electronic Basics' started by Adam Aglionby, Sep 27, 2004.

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  1. Any suggestions for a sensitive thermistor or otherwise to act as a blow
    sensor, dont really need a calibrated or accurate response, just detect the
    presence or otherwise of a short pulse of moving air, like someone blowing
    across something.

    Hot wire anemometer seems a bit fragile and kinda struggling to find a
    thermistor that dosen`t need a hurricane blowing across it to change.

    Any Suggestions ?

    Thanks
    Adam
     
  2. Fairly small fairly low resistance value NTC thermistor in a bridge
    should work.
    Here are some part numbers for some small 1k thermistors available at
    Digikey:
    ERT-D2FGL102S (5mm disc)
    KC003G (like small glass diode)
    KC003N (2.79 mm dia, I think)

    These need only a few milliwatts to raise their temperature a degree
    C. Applying something like 5 volts across them should heat them well
    above ambient and make them sensitive to air velocity. You can use a
    second one operated at much lower current to compensate for air
    temperature changes.
     
  3. John Fields

    John Fields Guest

    ---
    For a nice little low-mass thermistor that you can self heat:

    http://www.thermometrics.com/assets/images/b050710.pdf

    Then hook it up like this:



    +V---+-------------+------+------------+
    | | | |
    | | [R4] [R7]
    [R1] [R2] | |
    | | | +---[R6]--+
    | | | | |
    +----[C1]-----+------|--+---|+\ |
    | | | | >--+----0UT
    | | +------|-/
    [RT1] [R3] | U1
    | | [R5]
    | | |
    GND>-+-------------+------+-----------------GND


    Select R1 to self-heat RT1 by a few (?) degrees above ambient, then
    select R2R3 to make U1+ less positive than R4R5 makes U1-.

    Select C1 (and the voltage differential between U1+ and U1-) so that
    slow changes in ambient temperature or moderate airflow past RT1 won't
    cause U1+ to go more positive than U1-, but that a sharp puff of air
    will. When that happens, the resistance of RT1 will increase quickly,
    causing the voltage on the R1RT1C1 junction to rise quickly. That
    quickly rising slope will then propagate through the smaller reactance
    it sees through C1 than a more slowly rising slope would, pulling the
    voltage on U1+ more positive than the voltage on U1-, pulling the
    output of U1, a voltage comparator, momentarily high.
     
  4. John Fields

    John Fields Guest

    ^
    with less attenuation
     
  5. dh

    dh Guest

    I once used an elecret microphone as a simple blow detector in an
    electronic flute project. Rectified the output through a capacitor and
    sampled the voltage level with an A/D input of a microcontroller.
    Sensitivity is affected a lot by the method you use to direct the
    airflow into the microphone.
     
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