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Topic: Making an eternal pendulum (Read 10075 times) previous topic - next topic


I'll look at clock pendulum constructions. Although in this case the forces are applied to the pendulum axis so it's completely different.

Sorry, I got lost: When you refer that neodymium magnet, are you addressing the issue of restricting the pendulum movement in order to try to keep it linear and parallel to the wall? Or are you talking about stimulating the pendulum?

The neodymium magnet is in the pendulum and will be pushed away by the electromagnetic field somewhere hidden away.

I hope all these magnets and electromagnets don't mess up the digital frame image :)


As long as you don't use a CRT monitor it will be ok.


Sep 23, 2007, 10:43 pm Last Edit: Sep 23, 2007, 10:46 pm by nununo Reason: 1
I see! Now I get it. So:

- I place a strong magnet in the bottom of the pendulum (behind the frame);
- place the electromagnet beneath it, in the center attached to the wall;
- place a hall effect sensor attached to the wall, also in the center

Like this:

The black rectangle is the electromagnet, that small red thing is the hall effect and the ball is the strong magnet.

So, when the pendulum reaches the center, the hall effect sensor senses it. After a short delay (giving time for the pendulum to move slightly away from the center) I activate the electromagnet which will kick the pendulum magnet. This works in both directions!

Is this what you mean? I like it. Very simple!

Or did I get something wrong?



Yes, exactly what I meant. Maybe you can use the electromagnet to measure the incoming pendulum. When you measure something you can switch on the electromagnet. This saves you a hall sensor.



How can I use the same electromagnet both as a sensor and actuator at the same time in Arduino?

Any advice on which electromagnet and magnet to buy? and where? (european websites prefered)

BTW, I just ordered the Arduino and a Proto Shield.



Sep 24, 2007, 08:53 am Last Edit: Sep 24, 2007, 08:55 am by bigengineer Reason: 1
How can I use the same electromagnet both as a sensor and actuator at the same time in Arduino?

Put it on an analogue input. You can switch from input to output and vice versa.

Any advice on which electromagnet and magnet to buy? and where? (european websites prefered)

Farnell, RS-components, conrad.

BTW, the distance between electromagnet and magnet can't be very big. Is your display going to be in between them?



BTW, the distance between electromagnet and magnet can't be very big. Is your display going to be in between them?

No! They will almost touch. The magnet will be attached to the back of the digital frame. The electromagnet will be attached to the wall and can be placed right bellow the magnet. Just like the picture I showed above.

Thanks again ;)


1) Electromagnets
I'm browsing for components on the stores you suggested (the others are fine but I didn't find anything in Conrad)

Is this what I need?

How do I know if I need 10mA, 35mA or 40mA? Since they're so cheap maybe I should buy one of each.

I also found these, which says its an Audio Transducer. Are they usable?

I really don't know which to choose.

2) Magnets
I've heard about these new extra powerful magnets. Are these the ones you're calling neodymium? Do you think I really need one of these? Or should I go for a normal one?

3) Hall effect
I don't know if I'll need one, but I'll order it together with the electromagnet just in case. Any generic hall effect will do? Or should I require some special specs? Will this one work?

I also found this one...

(This is exciting)



Sep 24, 2007, 10:17 am Last Edit: Sep 24, 2007, 10:21 am by bigengineer Reason: 1
What you selected is wrong, except for the hall sensor maybe. I think you are better off with making the electromagnet yourself. Get a large nail, put it in a drill and make a coil, experiment with it. I can't solve everything for you. Get a magnet from a loudspeaker and start playing.

Get your magnets here.




I got some wire and a large nail and tried to make my coil. Didn't work. I believe I know why. Being a portuguese living in Latvia doesn't make things easy in terms of communication. When I bought the wire I made it clear that it had to be enamel coated because it was meant for a coil. They say yes yes yes but I'm afraid they didn't know what they were saying.

So, I will buy some enamel coated wire online in Maplin and try again.

Can anyone please tell me which diameter should the wire have? Is thinner better? They sell from 0.125mm to 2.0mm. It will be powered by Arduino.

Here's the link:

Thanks in advance,


I think you want the 26 gauge (.45mm) or 30 gauge (.31mm). If I recall, the rules of thumb are:

1) More loops in the coil -> more magnetic strength
2) You need thicker coil wires if you are handling high currents (which this won't be)
3) The thinner the better to get maximum coil density, but when you get really small, the wire can kink, break, or stretch too easily making it hard to hand wire a coil.

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