[Phys-L] Re: Foucault pendulum circuit

[Brian Whatcott <betwys1@SBCGLOBAL.NET>]

At 09:54 PM 11/13/2005, you wrote:
> Hi All:
>
> I've got the task of repairing our 36 year old Foucault pendulum
> display. A few years ago the campus electronic systems repair guys
> ripped out the (completely functional) vacuum tube circuit to replace it
> with a solid state circuit. They destroyed and discarded the old
> circuit. Then the guy retired and said we give up, it's all yours. Sigh.
> Thus, I no longer have any real clue as to the original sensing system
> or power requirements for the big electromagnet ring.
>
> All I really know is that the original sensors were a set of three
> lever style contact switches that touched the cable when it swung.
>
> I've started to design my own system using LEDs and photodiodes with a
> simple logic circuit to trigger a timer to drive a power MOSFET to turn
> on the magnet. When I built it on the bench it worked great. When I
> installed it and connected it to the actual electromagnet it no longer
> worked correctly (gets very unstable). I guessing that the larger amount
> of inductance in the coil is feeding back into my circuit. (It is a real
> effort to work with the actual system since it is up in a very tight
> spot in the attic rafters and is hard to access comfortably. With
> significant effort I've now yanked the ring out and have it on my bench
> to test with the next circuit. The rest of the pendulum is welded
> together in place and won't fit through the opening. It will have to be
> worked on in the rafters.)
>
> I'd rather not reinvent the wheel. I didn't have much luck with web
> searches for circuits. Does anyone have a circuit they can share?
>
> Thanks, John
>
>
> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
> John E. Sohl, Ph.D.


There was a useful summary of electric Foucault drives in SciAm
  in the 60s (??)   I take it that if the three contact switches were
  high on the pendulum, then they were arranged to form a triangle
  which would be in contact at either one or two wires until the
  pendulum arrived close to the center, at which point all contact
wires would open. This would be the stimulus for a monopulse
  through the ring coil (at the foot of the pendulum?).
  This method would provide two drive pulses per cycle.

A non contact method could replicate this with three LED beams
in a small triangle surrounding the rest point of the pendulum.
The beams' shapes would be such as to ensure at least one beam
  is interrupted by the pendulum, save in  its narrow central zone.

Simply driving the coil with a convenient  (and continuous)  alternating
current is said to supply the drive required for favorable configurations.

Brian Whatcott    Altus OK    Eureka!
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