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Help with sensors for a pendulum

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Fried Circuit

New Member
Hi. I'm looking for some sort of sensor set up to use with a Foucault pendulum project I'm working on, but I don't know where to begin. What I need is a sensor set up that will detect when the ball of the pendulum passes through a certain radius (coming and going) and that will also detect when the ball passes through the exact center of its swing. I also need to be able to connect the sensor system to a logic circuit that has already been designed and built. This circuit has two inputs. One input ("A") should trigger whenever the sensor detects the ball at the outer radius, and the other input ("B") should trigger when the ball passes through the exact center of its swing. Both inputs should "flip-flop," going from 0 to 1 or 1 to 0 each time they are triggered. Any sensor using magnetic fields won't work because it will interfere with other elements of the project. Lastly, it would be best if the sensor system is not very expensive. Thank you for your help and I will be happy to supply additional information if necessary if I am able.


New Member
While I do not know exactly what a Foucault pendulum looks like, if the 'balls' you describe have a shiny, reflective surface you could use an optical sensor to sense reflected light as the ball passes close by. This would be the cheapest method, optical sensors are available already pre aligned in a plastic package that simply mounts on a bracket. RS components sells one for about $12.- Australian (#307-913)
Then you could use inductive proximity sensors, these sense metal close by (within a few millimetres) but these are quite expensive.


New Member
Or you can build trembler... these can be difficult to construct properly. I'll try and do it with ascii, hopefully it doesn't fail me. (by the way, this one takes like $5 to make a dozen or two)

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Okay, so it's not a great work of art, but the center is a spring stretched out so that it's very trembly (moves at the slightest touch)

The case is a conductive metal, hollow cylinder you can pick up thin copper pipe for about $1 a foot, and need only an inch to contruct this.

In between the two on the very top, is a rubber washer, or other non coductive material that has a hole just small enough to fit the spring in, and leave a bit of room for some soldering.

Now solder the spring to positive, and the cylinder to negative (or reverse, not important)

Insert the spring in the cylinder.

Now mount it on the case and connect to the rest of your circuit. Make sure that when you mount it, the ball contacts the trembler, just gently, even a simple brushing will be all that it takes.

But use with caution, if you make the spring too sensitive, or the pendulum is too powerful, it may set off the trembler just moving.

Also very useful for alarm systems, these can be made sensitive enough to detect vibrations in the ground, especially if they're burried.


New Member
Pendulum sensors

The best I can think of is:-
An active light beam sensor underneath the pendulum
reflected light from the bob will give a positive detection of mid-swing
To detect extreme of swing is tricky as you need to cover 360 degrees of rotation,
are you using a shaped cone guide thing at the pendulum's anchor point?
could a metal ring be fixed around the pendulum (maybe part of the cone guide)?
as the pendulum reaches the exreme limit of its swing it touches the ring, this completes a circuit - simple conductors touching. You could argue that as the pendulum is at the extreme of its swing this interference is negligable ?

Why do you want this second sensor anyway?
Simply detect dead-centre with an opto-sensor or something and use this to energise the assistor magnet for a set time via a simple monostable timer circuit. The pendulum should be predictable enough for this ?



Active Member
Hi Fried Circuit,

As you probably know, for a Foucault type setup to work reasonably
well it has to be a reasonable size. Also it has to have the minimum
possible interference with its freedom of movement.

The suggestion from mechie to use a circular contact near the top of the string is sound and if your unit is purely for demonstration that
would be the easiest way, and the most straightforward, and simple to

But it does rely on the pendulum having enough swing energy to make
the contact at each swing, so unless there is some mechanism for
maintaining the swing, it would soon lose momentum and fail to

An adaptation of this could be a disc mounted near the top of the
string, obscuring a fixed circular opening beneath it.
If the circular opening beneath the disc were somewhat smaller than
the disc, then the disc would have to move sideways to allow some light to pass.

To differentiate a signal light, it is usual to impose a frequency
upon it and give its receiver sensitivity to the same frequency.
This means of course that the unit can be viewed in normal light.

Briefly, the sensor would respond to outgoing swing past the set
radius, and cease to respond when the swing went inside the set
radius. Essentially the same as mechie put forward, but using a
mask and a disc for light.
Thinking about it, it might be possible to do it without a mask
i spose it depends on the sensitivity of he receiver.

I hope my little drawing shows the concept, its only crude but
you should get the idea. I don't think sensing the centre point is
a problem.

Best of luck with it,
John :)



New Member
foo-cou ?

john1 said:
Briefly, the sensor would respond to outgoing swing past the set radius, and cease to respond when the swing went inside the set
So if the sensor/light source cannot be dead-centre will it detect equally for all 360 degrees of rotation?
Maybe a pool of lights almost big enough to surround the bob weight, all pointing up at the fulcrum/anchor point such that with the pendulum at rest there is a shadow from the bob just big enough to cover a similar circle of sensors arranged in a circle around the fulcrum.
With this arrangement the bob takes the place of your card disk and there is less directional sensitivity.

I still reckon a simple detector for dead-centre triggering a timer would be easiest and most predictable.


Active Member

I agree.
Simpler the better.

I don't see the point in sensing the extent of arc.
The point of a Foucault arrangement is to show the
rotation of the ground beneath our feet.

It could be that it is used in this arrangement as
some way of maintaining the swing, by operating some
system applying energy to the pendulum.

I would be interested to find if this is the case.

A typical home type set-up would have a string length
of about seven or eight feet, the point at the bottom
would typically travel about eighteen inches maybe a
bit more, over a sheet of paper with a circle marked
with maybe degrees, or compass points, or just hand
drawn lines.

The speed of the swing is usually very slow, in the
order of about three or four seconds per cycle.
(assuming normal ceiling height)

If this business is intended to make the pendulum keep
swinging, then i feel a simpler method would be much

If this business is not aimed at maintaining the swing
then i would be very interested in its purpose.

The method mentioned by mechie, using a short delay,
sounds about the simplest to me. It would involve
the use of an electro-magnet which you say you want
to avoid, but you would have to be quite innovative to
come up with an alternative.

It's been a few days now,
will we hear from Fried Circuit again ?

Regards, John :)

Fried Circuit

New Member
Hi, I'm still here. Thank you very much for your replies. I haven't been around much lately due to work. (The pendulum project is sort of a side project at the moment) I haven't had a chance to discuss sensor options with the rest of my group yet, but we are planning to have a meeting soon. I'll bring this information along with some other options I've found and see how things turn out. Thanks once again for your help, but I may be back soon if my team does not find a good solution.
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