🌀 First Spin

What you're building

The Level 4 hello-world: a single loop with a battery, a slide switch, and a DC motor. Flip the switch and the motor spins; flip it back and it stops. One extra part sits across the motor — a 1N4001 diode — doing nothing visible during normal operation, and everything important the moment the switch opens.

If you tape a paper blade to the shaft, this project is a one-speed desk fan. If you drop the motor onto a small gearbox, it's the drivetrain of a robot. Level 4 is the first time your Ω box can move something in the real world.

Meet the motor

A DC motor is a coil of copper wire wound around a spindle, sitting inside a permanent magnet. Push current through the coil and the coil becomes an electromagnet; the fixed magnet pushes it around; commutator brushes flip the current every half-turn so it keeps going the same way. That's the entire idea — a magnet shoving another magnet in circles.

Two things follow from "coil of wire":

• A motor is (mostly) a resistor. The DC resistance of a small 3 V hobby motor is around 30 Ω. At 4.5 V, that's 4.5 ÷ 30 = 150 mA through the coil — well within what fresh AAs deliver, but fifteen times what an LED wants. Motors are hungry.
• A motor is also an inductor. A coil of wire stores energy in its magnetic field. That storage is what saves your circuit from itself — and, unmanaged, what breaks it. More on this below.

The little 3 V motor you have is happy anywhere between about 2 V and 5 V. Below ~1 V it won't turn at all (friction wins); above ~5 V the brushes wear fast and the case warms up. Our 4.5 V battery pack is right in the sweet spot.

Do the math

Straight Ohm's law, one loop, no branches:

• Battery: 4.5 V.
• Motor coil: ~30 Ω resistance.
• Current through the loop when the switch is closed: 4.5 V ÷ 30 Ω ≈ 150 mA.
• Power the motor eats: V × I = 4.5 × 0.15 ≈ 0.68 W. Most of that becomes spin (and a little sound and heat); the battery pack barely notices.

Compare to First Light: an LED at 4.5 V through a 220 Ω resistor pulled ~11 mA. A motor pulls fifteen times more. The AA cells were designed for this — they can sustain hundreds of mA for hours — but if you tried the same trick with a coin cell (CR2032), the button battery would sag under load and the motor would barely twitch. Big loads need big batteries.

The kickback story (why the diode is there)

Flip the switch off while the motor is spinning. The current through the coil wants to stop instantly — but the magnetic field stored in the coil has other ideas. As that field collapses, it forces the coil to keep pushing current somewhere, for a fraction of a millisecond, at whatever voltage it takes.

Without a safe path, the coil finds one across the open switch contacts: an arc, a click, a tiny spark. You can see it on switches with clear plastic bodies, and hear it as a snap. Repeated arcs pit the switch contacts and — worse, at Level 6+ when you replace the switch with a transistor — the voltage spike will blow the transistor open with one flick.

The 1N4001 diode sitting across the motor is the fix. It's a one-way street:

• When the motor is running normally: the diode's cathode (the banded end) is at the battery + side, its anode is at the switch side. That's reverse-biased — the diode is a wall, no current flows through it, and it's invisible to the circuit.
• The instant the switch opens: the coil's collapsing field yanks the switch-side node above the battery + rail. Now the diode's anode is higher than its cathode — it flips into forward-biased mode and hands the coil a safe loop: coil → diode → back into coil. The stored magnetic energy dumps as a tiny burst of current recirculating through the diode, which drops it as heat in the coil's own resistance. Gone in a millisecond. No spark, no click, no dead transistor.

This diode-across-the-coil trick is called a flyback diode or freewheel diode. Every relay, every motor driver, every solenoid you'll ever meet has one. Learn to spot it and you'll see it everywhere.

Build it

1. Power rails: battery + to a top row, battery − to a bottom row. Same as every project.
2. Slide switch: put it inline with the + rail so it can cut power to the motor. Wire from + rail → one leg of the switch → out the other leg into a fresh row (call it A).
3. Motor: one motor lead in row A (the switch's output), the other motor lead over to a row that connects to − rail. Motors have no polarity for direction — reversing the leads just spins it the other way.
4. Flyback diode across the motor: cathode (banded end) into row A (the same row as the motor's positive-side lead); anode into the − rail row (the same row as the motor's negative-side lead). The diode sits in parallel with the motor, banded end toward +.
5. Flip the slide switch. Motor spins. Flip it back. Motor stops quickly — no click, no spark. If you had no diode, you'd feel a faint snap at the switch on the way off; with the diode, everything is quiet.

The most common first-try mistake is putting the diode in backwards — banded end (cathode) toward the switch side instead of toward +. Wired that way, the diode is forward-biased the moment you close the switch and shorts the battery through itself. It'll get hot fast; you'll blow the diode and possibly the battery-pack fuse. If the motor doesn't spin and the diode is warm to the touch, disconnect and flip it around.

Try it with a paper fan blade

Cut a 6-cm paper circle, slit four blades from the edge in toward the centre, and twist each blade a few degrees. Push the centre onto the motor shaft. Turn it on. You have a working desk fan — the same design as a laptop CPU cooler, minus the housing.

That paper blade is doing something interesting: it turns the motor into a fluid pump. The current the motor pulls now depends on how much air the blade is pushing against — put your finger in front of it and the motor draws a little more current. This is your first "the load talks back to the source" moment, and it becomes central at Level 9 when a robot's wheels hit carpet vs. tile.

What you built into your toolbox

Every Level 4+ project that spins something inherits this pattern: motor + flyback diode across the motor. In Handheld Fan you swap the slide switch for a transistor driven by a button; in Sunrise Fan the transistor is driven by an LDR. The motor and diode never change.