This Newton’s Cradle Refuses to Quit Thanks to Timed Electromagnetic Kicks

Newton’s Cradles have long been staples on office desks, their five steel balls transferring motion in a satisfying chain of collisions. Most versions eventually lose their rhythm as energy dissipates through air resistance, sound, heat, and slight changes in the shape of the balls during impact. A recent project changes the outcome by adding a small, precisely timed push that replaces those losses. The result looks like the classic toy yet keeps swinging for hours on battery power alone.

The solution is based on some cleverness, with an inductive proximity sensor that is activated when one of the end balls begins to descend. The microcontroller then takes over and provides a short charge of current to the electromagnet located immediately below the ball’s path, just enough to give it a slight nudge and restore energy that would otherwise be lost. Timing is key in this situation; if you do it too soon or too late, the ball will either not get any help or will be yanked out of its natural swing. The builder began with two sensors for early experiments to obtain a strong grasp on the motion, but then reduced it to one sensor and some predictive math based on the swing rate, which appeared to function just as well with less hardware.

The power comes from a rechargeable 18650 battery cell, which receives a voltage increase via a boost converter to give the electromagnet a solid, strong push. A MOSFET does the switching, while a diode protects the circuit from voltage spikes when the coil turns off. When it’s time to charge the battery, simply plug it into a USB-C socket embedded inside the base. There is an issue with ball weight changes, which might cause undesired vibrations in the intermediate balls. So the builder installed some small permanent magnets to provide a modest downward drag, helping to suppress resonance and keep the energy flowing smoothly from end to end.

The frame started off as a normal cradle that you could buy off the shelf. The main custom work is fitting the electromagnet and sensor into the base without changing the outward appearance too much. As it turned out, the build ended up looking like a cute little warship, complete with raised accents that conceal the battery, circuit board, and wires, as well as the charging port, which is tucked away under a magnetic cover.

Assembly begins by arranging the mechanicals by suspending the steel balls from the frame with strings or cables, and then attaching the electromagnet just below one of the end balls. Then you continue on to the electronics, aligning the proximity sensor to detect the ball’s approach, routing the wires to the microcontroller board, and soldering the power components in place. The microcontroller’s firmware detects the ball and calculates the pulse length, and then it’s just a matter of fitting everything into the enclosure, attaching the stabilizing magnets, and giving it a test swing to ensure everything works properly.
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