💨 Handheld Fan

What you're building

First Spin had a switch you flipped by hand and the motor came on. That works — but a switch that has to carry 150 mA is a switch that has to be built for it. Here you swap the "switch" for a transistor, and the "flipping" for a tiny button current that the transistor amplifies into a big motor current.

Press and hold the button: fan spins. Let go: fan stops. The button itself carries less than 4 mA — a mosquito of a current — but through the transistor's β amplification, that mosquito commands ~140 mA of motor. Same physics as Panic Doorbell and Warm Hand Alarm; the difference here is the mouth: this time the transistor is driving a motor.

Once you own this, you own the pattern for every driven-motor project on the site: sensor decides, transistor amplifies, motor spins. Level 4 uses buttons and light sensors; Level 7 uses code from an Arduino; Level 9 uses two of them side by side to drive robot wheels.

Reuse from Level 3

You already learned the "brain" side at Level 3:

• The 2N2222 transistor turns on when its base sits above about 0.7 V relative to its emitter (the "wake-up line").
• A 1 kΩ base resistor between the trigger and the base politely limits the base current so the trigger source (button, sensor, chip) never has to deliver more than a couple of milliamps.
• When on hard, the transistor's collector-to-emitter link becomes a near-short — the classic V_CE(sat) ≈ 0.2 V drop.

Everything on the "brain" side is exactly what Panic Doorbell did with a button. Only the mouth side is new — a motor + flyback diode instead of a buzzer + LED.

Do the math

Button pressed:

• The button drops the battery's 4.5 V directly onto the top of the 1 kΩ base resistor. The base clamps at ~0.7 V, so the base resistor sees 4.5 − 0.7 = 3.8 V across itself.
• I_base = 3.8 V ÷ 1 kΩ ≈ 3.8 mA. That's the little current the finger and the button and the base resistor pass around together. The button barely notices.
• The transistor's β is around 200, so it could pass up to 200 × 3.8 mA = 760 mA through the collector — vastly more than the motor wants.
• The motor sees 4.5 V − V_CE(sat) = 4.3 V across its 30 Ω coil. I_motor = 4.3 ÷ 30 ≈ 143 mA. That's slightly less than First Spin's 150 mA because the transistor stole 0.2 V — but the paper fan blade won't notice.
• The transistor is deep in saturation (760 mA capable, 143 mA asked-for). It runs cool.

Button released:

• I_base drops to zero. The transistor snaps off.
• The motor's coil, still spinning, would try to keep pushing current — its magnetic field collapsing generates a voltage spike aimed at the transistor's collector.
• The 1N4001 flyback diode across the motor catches that spike, recirculates the coil current back into the coil, and the field dumps as heat in the coil's own resistance. The transistor's collector never sees more than ~0.7 V above the +4.5 V rail. It survives.

Without that diode, the flyback spike can reach tens of volts — the 2N2222 is rated for a maximum V_CEO of about 40 V, and one bad spike over that number opens the transistor's collector-emitter junction like a fuse. The diode is not optional.

Build it

If your First Spin is still on the board, you're inserting a transistor between the switch-side of the motor and the − rail, and moving the switch to the base side. It's a small remodel.

Fresh build:

1. Power rails: battery + to a top row, battery − to a bottom row.
2. Button lane (the trigger): battery + → one leg of the push button → out its other leg → 1 kΩ → into a fresh row (call it B). Row B is going to be the transistor's base.
3. Transistor: flat face toward you, legs are Emitter (left), Base (middle), Collector (right).
   • B in row B (from the button's 1 kΩ).
   • E to the − rail.
   • C into a fresh row A.
4. Motor: one lead to the + rail; the other lead into row A (the collector).
5. Flyback diode across the motor: cathode (banded end) in the + rail row, anode in row A. Exactly the same placement as First Spin, because the motor and diode haven't moved — only their partner on the low side has changed from a switch to a transistor.
6. Press and hold the button. The fan spins. Release: fan stops, quickly and cleanly, with no click.

The most common first-try mistakes:

• Button and base resistor swapped. If the 1 kΩ is on the battery-plus side and the button is between it and the base, the circuit still works — but any voltage spike on the button side hits the base directly. The base resistor should sit right next to the base, always.
• Motor and diode on the wrong side of the transistor. Motor + diode go between + and collector, not between collector and −. If yours is reversed, the transistor is on the "wrong side" of the load — technically called "low-side switching" and legitimate, but the flyback path needs to change too. Stick to high-side motor, low-side transistor for now.
• Diode backwards. Banded end must face +. Reversed, it shorts the battery through itself as soon as you turn on.

Try it with your paper fan blade

Push the paper fan onto the shaft (same one from First Spin). Press the button and — this is the fun part — you can feel the breeze on the back of your hand. It's small, but it's real air being moved, by a circuit you built.

Notice the release: unlike First Spin, there's no click even without the diode explicitly in view. That's because the transistor turns off far faster than a mechanical slide switch opens — and the diode still handles the spike quietly in the background. You've replaced a mechanical device with an electronic one that's faster, quieter, and controllable.

What "controllable" opens up

The button is just a placeholder. Anything that can put ~1 V on the base through a 1 kΩ will fire this circuit:

• A light sensor on a voltage divider — Sunrise Fan (next project).
• A temperature sensor on a voltage divider — hot rooms auto-cool.
• An Arduino pin — Level 7, and now every fan is programmable.
• A motion detector — the same fan turns on when someone walks past.

Same circuit downstream, different circuit upstream. That's the whole gift of the transistor.