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minimum current/voltage for diode?

Discussion in 'Electronic Basics' started by Tristar500, Jul 13, 2005.

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  1. Tristar500

    Tristar500 Guest

    Been working on an idea for the Bike project. (a detector that will
    open or close a relay if the peddler quits peddling.

    I tried placing a strong magnet on the rim of the bike and can get a
    multi-meter (analog) to make a very small jump as the magnet passes by
    a coil I've got positioned close to the rim. Plan is to use the small
    amount of generated current to charge a capacitor that would then have
    enough current to pull down a relay (or transistor switch). A resister
    would be in place drawing current away from the capacitor so that the
    rider had to keep charging the capacitor faster then the resister was
    discharging it or it will complete/disconnect a circuit alerting the
    rider that he has quit peddling. Kind of a nag alarm really, (gosh, I
    hate to get nagged, but Hey!, I'm not the one who is going to have to
    ride this exercise bike)

    My thoughts are that this would produce an A/C pulse so naturally I
    grabbed a diode and put it inline with the coil to convert it to pulsed
    DC. For some reason I get no signal at all now that a diode is
    in-line. Makes no difference which way the diode is oriented or what
    direction the wheel is turning.

    Is the signal so low from the coil that the diode can't handle it? I
    don't know what sort of diode I was using, I just pulled a couple of
    different ones from a old board and gave it a try.

    Do I need a special diode? Do I need a full wave rectifier for this to
    work?

    Thanks in advance, Lawrence
     
  2. ehsjr

    ehsjr Guest

    It's not AC. The voltage is likely too low for the diode
    to conduct. Use a reed switch instead of a coil:

    Reed
    Switch
    Battery+ ---[SWITCH]------+----+--->TO YOUR CIRCUIT
    | |
    [CAP] [R1]
    | |
    Battery- -----------------+----+

    Each time the magnet goes by, the reed switch closes,
    and the capacitor charges to the battery voltage.

    Here's a complete circuit if you need it:


    +-------------[RELAY]------------+
    | |
    | c
    | /
    Battery+ ---+---[SWITCH]---+----+---[R2]---b NPN
    | | 470 \
    [CAP] [R1] e
    | | |
    Battery- ------------------+----+------------+

    R2 limits the current to the base of the transistor. The
    values of the CAP and R1 will depend primarily on the
    speed of the wheel (number of rotations per second), but
    also on the gain of the transistor and the resistance of
    the relay coil as well as its drop out voltage. Higher
    values of R1 and the CAP result in the relay staying
    energized longer. You'll need to experiment to find good
    values. If you make R1 variable you can require the rider
    to go at different minimum speeds to keep the relay
    energized.

    Ed
     
  3. mike

    mike Guest

    What great lengths we go to because we have no discipline.
    If I quit pedaling, I know that I quit pedaling. I don't need no damn
    nag to tell me that. If I lack the discipline to achieve my goals, I'm
    gonna need a lot more than a nag.

    Since it's a stationary bike, use a reed switch and a magnet or a led
    and detector shining thru a hole. Build an oscillator with frequency
    variable around the desired wheel rpm. Use a frequency detector to sense
    when the wheel is turning slower than the set frequency.
    When it's lower, use a car spark coil to apply 40KV pulses to the seat.
    PUt a towel under the bike, cause there's gonna be a wet spot.

    OR, we could just have a little discipline!!!
    mike

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  4. kell

    kell Guest

    Is this a consumer product you are designing that will go into
    production, or a one-off hobbyist project?
    If the former realize that a reed switch after many thousands of cycles
    may get magnetized and stick closed. Consider using a hall effect
    sensor.
     
  5. ehsjr

    ehsjr Guest

    It's for kids on an exrecise bike, as I understand it.
    But it (mag/reed switch) is a common commercial design.
    Supposedly, they are good for 10^6 to 10^9 operations:
    http://www.madisonco.com/reference/electrical.htm

    As a side note, I have over 700 miles on my bicycle odometer
    which works on that principle - reed sw/magnet. The wheel is
    26" - that's over 50 thousand operations, and still working.
    So I have to ride another ~ 13,300 miles to achieve the
    bottom end of reed switch failure life. I get tired just
    thinking about it! :)

    Ed
     
  6. John Fields

    John Fields Guest

     
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