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Magnet Mobil STEM Project
A working, open-source pulse motor that makes electromagnetism, timing, and mechanical design visible, buildable, and open to experimentation.
TL;DR
The Magnet Mobil is a small vehicle driven by a visible electromagnetic pulse motor: flywheel magnets operate a reed switch, the switch pulses a hand-wound electromagnet, and a rubber band transfers the rotation to a road wheel.
It works and can be recreated in the classroom or at home. The illustrated build guide includes the complete parts list, circuit diagram, printable parts, coil-winding method, assembly sequence, testing procedure, and troubleshooting advice.
The parametric OpenSCAD files and other design files are provided in an open-source spirit. Makers can modify the geometry, experiment with the magnetic and mechanical design, or wire the simple circuit without the PCB if the specified parts are unavailable.
The prototype demonstrably drives, making it a useful hands-on STEM project.
1. Motivation & Background
I owned a YPS Magnet Mobil in 1980, and I loved it. What stayed with me was not speed or complexity, but the opposite: it made a motor from remarkably few parts and let me see how everything worked. A large red flywheel, six magnets, an electromagnet, a reed switch, a battery, and a rubber belt were enough to move the little vehicle across a table. There was no sealed motor and no hidden gearbox. The mechanism itself was the attraction.
The Mk II now drives successfully, and it still exposes the same simple mechanism that made the original memorable. Rebuilding it became both a return to a childhood object and an opportunity to pass on this gadget to the next generation of tinkerers.
A visible pulse motor
The Magnet Mobil is a small battery-powered model vehicle built around a visible electromagnetic pulse motor. Permanent magnets are fitted around the rim of its flywheel. As each magnet approaches, a switch briefly connects the battery to an electromagnet and gives the flywheel another pull. The flywheel's inertia carries it between pulses, while a small pulley and rubber belt transfer the rotation to one of the road wheels. The magnets control and transmit the force; the battery remains the source of the vehicle's energy. There is a great animation further below - feel free to scroll down if you want to see it now.
The magnetic switch is a small glass reed switch, visible on the white control arm above the flywheel. Inside the capsule, two flexible ferromagnetic contacts are normally separated. A passing magnet draws them together and closes the circuit; they spring apart again as the magnet moves away. This creates a position-sensitive switch without a mechanical lever or an electronic controller. It is sometimes loosely called a reed relay, although technically a reed relay includes its own operating coil; the Magnet Mobil uses a standalone reed switch operated directly by the flywheel magnets.
That simplicity is why I wanted to recreate the model. Its operation can be watched, adjusted, and understood one pulse at a time. In the process it brings together magnetic fields, electric current, inductance, switching, stored energy, inertia, friction, mechanical transmission, measurement, CAD, 3D printing, PCB fabrication, and the patient process of testing a design that almost works.
What was YPS?
For an Australian reader, some context is useful: YPS was never a familiar fixture at the local newsagent. In German-speaking Europe, however, it became part of the childhood landscape for a generation.
YPS was a German comic magazine built around a wonderfully direct promise: every issue came with a Gimmick. The magazine and its extra were sealed together in a clear plastic bag, so the contraption on the cover was also the thing readers took home to build, grow, test, or play with.
These Gimmicks were more than novelty giveaways. Many were inexpensive, self-assembly STEM kits: simple microscopes and radios, model aircraft, solar balloons, optical instruments, mechanical devices, and other experiments that put a scientific principle into a child's hands. They were not precision laboratory equipment, but that was part of their value. The low-cost parts made observation, construction, and experimentation accessible without requiring a specialist kit or workshop.
Other issues included toys, practical-joke devices, survival gear, craft projects, and tiny brine shrimp marketed as Urzeitkrebse—“prehistoric crabs.” Comics and illustrated instructions gave each object a story, explained how to assemble or use it, and encouraged the reader to try something rather than merely read about it.
The Magnet Mobil appeared in 1980 as YPS Extra No. 5, early in the more ambitious YPS Extra line. The specialist YPS Fanpage catalogue identifies the issue and places the first fifteen Extras between early 1980 and September 1982. Half toy, half experiment, it was designed to make a technical idea memorable by putting it directly into the reader's hands.
The motor on display
The original mechanism placed its six-magnet flywheel at the centre of the design, with the coil, switch, battery, and belt arranged around it.
That visibility is what makes the design worth revisiting. A conventional DC motor would be smaller, faster, and more reliable—but also far less revealing. Here, the wheel is not merely connected to the motor. It is part of the motor.
2. Recreating the Magnet Mobil
My goal was to make a functional and optically faithful, but not identical replica. I wanted to preserve the original idea while using the tools now available to make the model reproducible, adjustable, and open to further experimentation.
The original kit was made from moulded parts, and the tight cost constraints of a mass-market magazine extra. The Mk II can take a different path. Affordable 3D printers now make complex custom parts practical, while small PCBs are easy to design and manufacture.
The current Magnet Mobil Mk II therefore keeps the operating principle but rebuilds the structure around open, parametric CAD:
Element | Magnet Mobil Mk II design |
Flywheel | 80 mm diameter, normally fitted with six magnets, expandable to 12 magnets |
Chassis | Support-free printed base |
Timing | Pivoting control arm with a reed-switch cradle |
Electromagnet | 18 mm printed bobbin with a 14 mm winding area; successful coil approximately 460 turns and 4 Ω |
Transmission | Two printed pulley cones, a rubber-band belt, and 25 mm road wheels |
Electronics | Fabricated PCB from supplied Gerbers |
The model is written in OpenSCAD, with printable STL exports included for builders who do not want to edit the geometry. The main dimensions are grouped near the top of scad/assembly.scad, so builders can modify the SCAD files and produce a larger or smaller Magnet Mobil. Changing the flywheel_diameter parameter resizes the rotor and automatically adjusts the bearing height, electromagnet position, chassis length, and control-arm length around it. This is parametric adaptation rather than uniform scaling: dimensions tied to real hardware, including the magnets, shafts, and electromagnet bobbin, remain fixed unless their own parameters are changed. Within those practical limits, a new size can therefore be explored without repositioning every surrounding part by hand.
The construction details now live in the dedicated build guide, with quantities and component specifications in the parts list. Keeping those instructions in one place makes them easier to revise as the prototype is measured and refined. What follows here is the reasoning behind the design and the changes that made the Mk II work.
Six magnets, configurable to twelve
The standard Mk II uses six magnets. This produces a balanced flywheel and recreates the pulse spacing of the original Magnet Mobil: one possible electromagnetic impulse every 60 degrees. By setting flywheel_extra_magnet_cradles = true; creates as 12-magnet version of the flywheel. Filling them doubles the number of possible impulses to one every 30 degrees.
More frequent impulses can make the torque delivery smoother and give the motor more opportunities to start from rest, but twelve magnets do not automatically produce twice the useful power. They also double the number of reed-switch and coil events at a given flywheel speed and can increase average battery current and magnetic cogging. The better configuration depends on the coil, air gap, timing, mechanical drag, and load.
The completed 4 Ω driving prototype in the images shows the six-magnet arrangement. Twelve magnets provide a useful comparison of pulse frequency, current demand, cogging, and load behaviour.
Recreating the electromagnet
The original YPS Extra did not ask the reader to design or wind an electromagnet. The coil and its ferromagnetic core were supplied together as an assembled part, ready to install in the white plastic holder. That removed a large source of variation: the wire gauge, turn count, winding resistance, core material, and core geometry had already been chosen by the manufacturer.
For the Mk II prototype, that component no longer exists as an off-the-shelf part. We have to reproduce its function from a printed bobbin, enamelled copper wire, and a suitable ferromagnetic core. This makes the electromagnet one of the most consequential parts of the rebuild. Two models printed from identical STLs can behave very differently if their home-wound coils or cores differ.
The practical target was therefore not a visually exact copy of the original coil, but a replacement whose wire diameter, winding resistance, estimated turns, and supply voltage could be documented.
Core material is a first-order design choice. Soft iron or suitable low-carbon steel is a better starting point than an unidentified bolt or hardened steel: the aim is high permeability with low coercivity, so the core strengthens the field while the coil is on but retains as little magnetism as practical after the pulse. Some stainless steels are only weakly magnetic, while alloy and heat treatment can make apparently similar steel rods behave differently. This is one reason two models printed from identical files can perform very differently.
The successful driving prototype uses a hand-wound coil made from approximately 0.3 mm enamelled copper wire. It is estimated at roughly 460 turns, measures approximately 4 Ω, and is powered by two AA cells in series at approximately 3 V. With this electromagnet, the vehicle drives on its 25 mm road wheels using a rubber band as the belt.
The circuit
The Magnet Mobil Mk II uses a deliberately simple circuit. The two-cell battery pack, power switch, electromagnet coil, and normally open reed switch form a single series path. The flyback diode is the only component connected in parallel: it sits across the coil, with its striped cathode on the positive-supply side.
The N.O. label means normally open. With no magnet nearby, the reed contacts are separated and no current flows through the coil; “normally” refers to this resting state without a magnetic field, not to the position of the power switch. As a flywheel magnet closes the reed switch, current flows through L1 and creates the electromagnetic pulse. When the reed opens again, the collapsing magnetic field tries to keep the coil current flowing. D1 then conducts briefly around the coil, limiting the reverse-voltage spike that would otherwise arc across the reed contacts. It does not limit the normal coil current.
The simplicity is part of the appeal, but it also means the reed-switch contact rating, the flyback diode, the coil resistance, and the supply configuration all matter directly. There is no transistor or MOSFET isolating the reed switch from the pulsed coil current.
3. Operation, Physics & Results
One Push at a Time
The Magnet Mobil is a pulse motor. Instead of driving the flywheel continuously, it gives the wheel a short magnetic pull (or push) at the right moment and then lets inertia carry it forward.
The cycle is simple:
A permanent magnet on the flywheel approaches the trigger position.
Its magnetic field closes a reed switch mounted on the adjustable arm.
The switching circuit briefly energises the electromagnet.
The resulting magnetic force accelerates the flywheel.
The magnet moves past the trigger, the switch opens, and the coil turns off.
The flywheel coasts until the next magnet begins the cycle again.
Click the Play button in the animation below. The highlighted magnet at the reed switch is the trigger; because the six magnets are evenly spaced, another magnet is aligned with the electromagnet and receives the pull.
Once the wheel is running, a small pulley on its shaft drives a rubber belt. The belt turns one road wheel and converts the flywheel's rotation into the slow movement of the vehicle.
The sequence is easy to describe. Making it work well is the interesting part. The pulse must arrive at the right angle, the coil must be strong enough, the flywheel must spin freely, and the belt must transfer power without adding too much drag. A few millimetres of switch movement can separate a motor that runs from one that merely twitches.
Why reed-switch position matters
The switch position controls two separate things. Its sideways offset determines whether the passing magnets close the contacts reliably at all, while the angle of the complete white control arm determines when the electromagnet is energised. A poorly placed capsule may respond to only some magnets; a correct capsule position with poor timing may produce nothing more than a twitch.
That visible relationship between position and timing is central to the model. Rotating the arm changes the point at which electrical energy becomes a magnetic impulse, making an otherwise abstract timing problem directly observable. The successful prototype uses an offset capsule and a control-arm angle chosen through testing. The measurement and adjustment procedure, including the initial angle, is documented in the build guide.
Why the Core and Air Gap Matter
A coil does not create useful force from current alone. Its turns and current provide the magnetomotive force that drives magnetic flux, but the route available to that flux determines how much field reaches the flywheel magnet. It is helpful to picture this route as a magnetic circuit. Its opposition to flux is called reluctance, the magnetic counterpart of electrical resistance.
Air carries magnetic flux poorly compared with a soft iron or low-carbon steel core. Placing a suitable core inside the coil therefore strengthens and concentrates the magnetic field at the end facing the flywheel. The material matters: two metal rods of the same size can produce noticeably different pulling forces.
The air gap between that core face and the passing magnets is usually the largest source of reluctance in the circuit—even though it is physically the shortest part. Reducing the gap can therefore increase the flux density and the attractive force substantially. In a simplified, unsaturated magnetic circuit, the field rises roughly as the gap becomes shorter, while magnetic pressure depends on the square of the field. The real Magnet Mobil is less tidy: flux spreads at the core face, the permanent magnet is part of the circuit, and the core eventually approaches magnetic saturation. The design goal is therefore the smallest mechanically safe gap, not zero clearance. Flywheel wobble, shaft play, print distortion, or a slightly proud magnet can otherwise turn a strong magnetic design into a source of friction or contact.
There are three useful limits to remember. First, once the core begins to saturate, extra coil current produces much less additional flux and mostly more heat. Second, a better core and smaller gap raise the coil's inductance. That can slow the rise of current during a very short pulse. Third, the permanent flywheel magnets attract the ferromagnetic core even when the coil is off. This creates a detent, or cogging, torque as each magnet approaches and leaves the core. A narrower gap strengthens that unpowered attraction as well as the useful powered pulse. Core material, gap, turn count, resistance, supply voltage, and switch timing therefore form one system rather than independent upgrades.
That interdependence makes the Magnet Mobil a fantastic opportunity for hands-on experimentation. Change one variable at a time—core material, air gap, turn count, supply voltage, or switch position—and observe how the motor's starting behaviour, speed, cogging, current, and temperature respond. The effects are immediate and visible, turning abstract ideas about magnetic circuits, inductance, and mechanical load into measurements that can be made at the workbench.
Driving the Magnet Mobil
Getting the flywheel to spin is only the first milestone. Driving the Magnet Mobil across a surface requires enough torque to overcome the belt load, bearing friction, wheel friction, and the inertia of the complete vehicle.
With approximately 460 turns, 4 Ω measured resistance, and two AA cells supplying approximately 3 V, the Magnet Mobil drives successfully. The demonstrated chassis retains the existing 25 mm road wheels and uses an ordinary rubber band between the flywheel pulley and the grooved driven wheel.
An open-ended STEM platform
The interactions that make the model demanding are also what make it interesting. Moving the angle of the control arm changes the timing. Changing the coil changes the magnetic field. Narrowing the air gap strengthens both the useful pulse and the unpowered cogging force. Tightening the belt may reduce slip while adding friction. Every adjustment has an effect that can be seen, heard, or measured.
For a classroom, those effects can become experiments: map speed against switch angle, compare magnet counts, measure coil current, test different core materials, or record how long the flywheel coasts with the belt connected and disconnected. The model makes the usual diagrams of fields, current, inertia, and friction tangible.
The most useful way to approach Magnet Mobil Mk II remains in the spirit of the original: not as a product to consume, but as a small machine that rewards curiosity.
I invite teachers, students, and hobbyists to recreate the Magnet Mobil and adapt it to the tools and components they have available. The CAD files, printable parts, circuit information, and build documentation are provided in an open-source spirit so that the design can be studied, changed, and improved. Not having every specified part need not stop a build; it can be an invitation to creative tinkering. The circuit, for example, is simple enough to wire by hand without a PCB, while the parametric OpenSCAD model can be adjusted for different dimensions or locally available hardware. Build one, measure what it does, and share what you learn.
