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Discussion in 'Misc Electronics' started by Rich Delaney, Nov 7, 2004.

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  1. Rich Delaney

    Rich Delaney Guest

    Help! I'm stumped trying to figure this out.

    My sister has one of those perpetual motion
    thigamajigs. It's a pendulum, presumably
    ferromagnetic, about 10" long. It swings above
    a base which contains a battery and a coil.
    Simple enough, n'est-ce pas?

    Obviously, the coil current transfers a 'kick' of
    energy to the pendulum on each pass. Now here's
    the riddle: the pendulum's motion is inherently
    symmetric, which implies that there should be a
    cyclic transfer of energy back and forth between
    the B-field and the pendulum. I.e. on the downswing,
    it should gain kinetic energy, and then on the
    upswing, should lose energy to the field (or perhaps
    vice versa). Hence, no net transfer, and the
    mechanism should simply halt.

    But the dang thing goes on and on! There IS a
    one-way energy path. Anyone care to take a crack at this?
  2. Dave

    Dave Guest

    you already answered your own question... the base contains a battery, read
    this as 'source of energy', the 'kick' on each swing that makes up for
    losses due to mechanical friction and hysterisis heating or induced current
    heating of the metal parts.

  3. It's not a static magnetic field in the base. If it were, the toy
    wouldn't need a battery.

    It's all in the timing. There's probably one or two pickup coils in the
    base that will sense the swinging magnet's position.
  4. Mark Martin

    Mark Martin Guest

    Are you sure there isn't also a package of electronic circuitry in the
    system somewhere? Obviously the battery isn't merely running current
    constantly through the solenoid. Otherwise, as you note, there'd be no
    important difference between it & a pendulum swinging in the gravity
    field. Also, the battery would be drained awfully quickly.

    But there's a permanent magnet in the pendulum bob. When it swings it
    induces current in the coil. On one swing, the current runs one way, on
    the the backswing, the current runs the other way. In fact, it's a
    continuously alternating current, graphed by a sine wave. A circuit
    could be made to recognise when the bob is at some phase in its swing,
    and it'd then tell the battery to let a burst of stored energy into the
    coil, which then pumps the pendulum with just enough energy to
    suppliment its losses.

    -Mark Martin
  5. CWatters

    CWatters Guest

    I suspect all the base needs to do is apply a repulsive magnetic pulse at
    regular intervals (remember how pendulums work). No need to sync it to the
    swing of the pendulum - get the frequency and pulse width right and that
    will probably happen all on it's own.
  6. There is probably a transister involved as well. There is probably a lead
    from a sensor coil on the driver coil core that is connected to the base
    or gate of the transistor. There are thus probably three leads off the
    coil core: a ground lead, a sensor tap, and the main driver lead. The
    pendulum has a magnet in it. As it approaches the coil the flux change
    dB/dt in the coil core generates a voltage in the sensor coil that causes
    the transistor to on the driver coil. The driver coil is arranged so as
    to increase the flux in the coil core. This creates a positive feedback
    that sustains the driver coil current until the magnet starts to depart
    the vicinity of the core. At this time dB/dt reverses polarity and
    negative feedback shuts off the driver coil current, leaving the pendulum
    free to contiue on with the momentum it gained when attracted to the coil
    core as it approached. If there is no change in local magnetic field,
    i.e. no swinging pendulum, then the device is automatically in off mode.


    Horace Heffner
  7. Uncle Al

    Uncle Al Guest

    1) Parametric amplifier.
    2) A kid "pumping" on a swing.
    3) Remove the battery. Damp the pendulum to hang absolutely
    still. Replace the battery without disturbing the system -->
    nothing. Now, give the pendulum a little horizontal tap.
    4) Cheap compass: the field is pulsed and synchronized.
  8. Richard

    Richard Guest

    Yes the pendulum contains a magnet "N" side down (as per a compass ).
    The base actually shows a little "S" magnetism. I can see a large iron
    core electromagnet there. Up to a ~7º deflection the pendulum
    oscillates and damps down. Larger than that and it will "pump up" to
    full deflection. You can actually see the speed up when the bob
    approaches the base.
    I got this these for my kids. They won't let me take it apart.

  9. Mark Martin

    Mark Martin Guest

    In the name of all that's empirical, buy another one and tear its gutz
    out man!!!
    -Mark Martin
  10. Richard

    Richard Guest

    OK they're asleep now. I'm getting better at cracking stuff like this
    open. Two gentle taps with my rubber hammer.... OK there are just
    three basic parts to this thing.
    1. 9V battery
    2. 1 transistor? marked "LC845P" with the Motorola "M" logo on it.
    Anyone know this type? I only get a reference to it being obsolete.
    3. A large COAXIAL (inner and outer) coil. Tiny wire, measures .08mm
    The coil itself is 20.87mm Dia. X 23.97mm Long. The inner coil
    (different color) is about 10.65mm Dia.
    Connections are:
    B- inner coil
    Reading Xsistor L-R camfer up base up
    L other end of inner coil 1 lead of outer coil
    C B+
    R other end of outer coil

    That's it!
    I think Gregory had it right with the sense coil and one drive coil.


    Now I get to use that Plastix (Locktite) glue which BTW works really
    well. Maybe solvent just in case.
  11. Uncle Al

    Uncle Al Guest

    Parametric amplifier. It slightly pumps the pendulum against air
    resistance losses. LC845P may be a small IC rather than a transistor.
  12. No need. Look here:

    I built one, and it works just fine.

  13. Jim Kelley

    Jim Kelley Guest

    Probably just a transistor being used as a switch with current gain.
    Not enough components for a PID (parametric) amp. Though that would be
    the fancy way to go.

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