# Wiring circuit for electromagnets to drive a pendulum

I’m wanting to set-up electromagnets, one on top of the other, to drive a pendulum. Using Core product DFRO797, a 5V 2W 15kg electro magnet or similar. The top magnet produces an induction current, from the moving magnet in the base of the pendulum, and this triggers the bottom, powered, magnet via a transistor. the poles of the permanent magnet in the pendulum and the powered electromagnet are opposite such that the magnet/pendulum gets a push as it swings past bottom dead centre.
My questions;
Is it possible to setup such a circuit?
What transistor should I use?
What power supply?
Really appreciate any help…

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Hi Dick,

Welcome back

Would it be possible to get a photo of your intended setup? (I think I have a rough understanding)

I came across this project that may be similar though it doesn’t list the nit-picky details such as inductance: https://www.instructables.com/Electromagnetic-Pendulum/

If you’re trying to make a continuously swinging pendulum then it’s absolutely possible, though timing will be frustrating. You might be able to use a comparator circuit or microcontroller to tune it.

In terms of the transistor being used at a minimum, you’ll need to be able to switch 5V, to 2W. if you want to use the full power of the coil, in the project above a 2N4401 was used

Instead of using an electromagnet to trigger the magnet, I’d go for a hall effect sensor or magnetometer (which requires a microcontroller but allows tuning).

The power supply is usually the last thing to be calculated out as you sum all of the components that you use.

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Hello Liam,
The Instructables project (video 1) you suggested is where this all started. I tried making the coils, first using 27AWG with 1500 turns,
the resistance was too low and then using 38AWG with 9800 turns resistance way too high!
I need to try something in between!

Then I came across another idea using a reed switch and thought this may be a better option.
Both these videos are pretty much what I want to achieve.The big difference is my pendulum is part of a kinetic metal sculpture,
yet to be completed.
But it will need more oomph especially to get going but it will have plenty of momentum, which I need because the pendulum,
via the shaft it is attached to, will move other parts.

I will try again using both circuits as suggested and hopefully get a good result.

But if not, I was wondering if it were possible to use two electro magnets, like video 1 you sent, but stack the magnets one
on top of the other with the transistor connecting them. The top magnet doing the pushing and the bottom magnet providing
the induction current/voltage to the transistor.
The magnets on order are 5V 2W 15kg holding capacity.

I don’t know what a hall effect sensor is but will check it out. The timing will be an issue for sure.
My understanding is the timing in video 1 was adjusted by changing the distance between the permanent magnet on the bottom of the pendulum
and the induction coil. Seems like it’s doable.

If you think my idea of stacking the electro magnets is a possibility I will start there. The electro magnets are available and cheep.
I have on idea how to calculate the spec for the transistor required to do this??
I have a few 2N4401transistors.

Thank you for your suggestions they are very much appreciated.

Regards
Dick

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