DIY magnetic levitation toys

What it is

A magnet cannot simply hover above another magnet and stay there. A theorem proved in 1842 guarantees that any static arrangement of magnets is unstable, so the floating object always flips or escapes. Stable levitation has to cheat that rule, and learning how it cheats is the whole fascination of the project.

DIY magnetic levitation uses the interaction between permanent magnets and electromagnets to suspend an object in mid-air with no visible support, one of the most visually stunning demonstrations of electromagnetic force a home builder can make. An object that appears to defy gravity is genuinely captivating, and the stability is achieved through rapid electronic feedback that adjusts the electromagnet's current to hold the object at a constant height. Because Earnshaw's theorem rules out static levitation, the two real routes are dynamic spin stabilisation, like a levitating spinning top, or active feedback control, where a sensor watches the object's position and constantly corrects.

A feedback levitator is built from a handful of parts: an electromagnet, a ferrite core wound with a few hundred turns of wire, a Hall effect sensor positioned just below where the object floats, a controller, and a power transistor to modulate the coil current. The control loop is the clever bit. The Hall sensor measures how close the magnet is; if it is too close, the current drops, and if too far, the current rises, and this loop runs over a thousand times a second to hold the object suspended at its equilibrium point.

The reason feedback is unavoidable is that the magnetic force is nonlinear, rising with the inverse square of distance, so a tiny change in position causes a large change in force. That inherent instability is what makes static levitation impossible and feedback essential, and the sensor's job is to linearise it so the controller can compensate.

The hard part is tuning. Start with proportional control alone, raising the gain until the object barely holds, then add derivative control to damp the oscillation, because pure proportional control just oscillates and never settles. Get it right and you have a spectacular, genuinely wondrous object on your desk.