🎚️ Dial-a-Spin

What you're building

Dial-a-Brightness let you fade an LED from bright to almost dark by twisting a knob. This project does the same thing to a fan: twist the knob one way and the motor spins full-speed; twist the other way and it slows down until it's just barely turning. No switch — as long as the battery is on, the fan is doing something. The knob decides what.

The parts you already own do all the work. The pot is a knob-shaped resistor. Slide it into the transistor's base line — the same place the 1 kΩ base resistor lived in Handheld Fan — and turning the knob starves or feeds the transistor's base current. That base current, times β, is how much current the transistor lets through to the motor.

The big idea: base current commands motor current

The 2N2222's rule from Level 3 was I_C ≤ β × I_B. Whichever is smaller — β × I_B or "how much the load wants" — wins. In every L4 project before this one, β × I_B was way bigger than the motor asked for; the transistor sat deep in saturation and the motor got its full 143 mA. Here the pot lets you crank β × I_B down until it's smaller than what the motor wants — and once you cross that line, the transistor stops being a switch and starts being an amplifier. It passes exactly what the base commanded, no more.

That's the two-personality trick of a transistor:

• Saturation (deep on): I_B big enough that β × I_B ≥ what the load wants. Transistor drops V_CE(sat) ≈ 0.2 V, load runs at full current. This is Handheld Fan at rest.
• Active / linear region: I_B smaller. β × I_B < what the load wants. Transistor passes exactly β × I_B, V_CE floats up to whatever it needs to. This is where the motor slows down.

Level 6's 555 timer keeps the transistor in saturation and pulses it on and off fast (PWM) — a totally different way to control fan speed. This project is the "hold it in the middle" way. Both are valid; both are used in the real world.

Do the math

Same battery, same motor coil, same 0.7 V transistor wake-up. Only the base current changes as the knob turns. With the pot in series with the 1 kΩ base safety, base current is

I_base = (4.5 − 0.7) V ÷ (1 kΩ + R_pot) = 3.8 V ÷ (1 kΩ + R_pot)

And the transistor's collector current cap is β × I_base ≈ 200 × I_base.

• Knob all the way down (pot ≈ 0 Ω): I_base = 3.8 mA. Cap = 760 mA. Way over the motor's 143 mA appetite → deep saturation → motor at 143 mA. Full spin.
• Knob at 25 % (pot ≈ 2.5 kΩ): I_base ≈ 1.1 mA. Cap ≈ 220 mA. Still over 143 mA → still saturated → motor at 143 mA. Full spin. The first quarter of the knob's travel does nothing visible because the transistor was already saturated with room to spare.
• Knob just past 40 % (pot ≈ 4.3 kΩ): I_base ≈ 0.72 mA. Cap ≈ 143 mA. Transistor teeters on the edge of saturation — same amount as the motor wants. Any further and it drops into active region.
• Knob at half (pot ≈ 5 kΩ): I_base ≈ 0.63 mA. Cap ≈ 126 mA < 143 mA → active region. Motor at 126 mA. V_CE floats up to (4.5 − 126 mA × 30 Ω) ≈ 0.7 V — no longer near V_CE(sat).
• Knob at 75 % (pot ≈ 7.5 kΩ): I_base ≈ 0.45 mA. Motor at ≈ 89 mA. V_motor ≈ 2.7 V. Fan noticeably slower.
• Knob at max (pot ≈ 10 kΩ): I_base ≈ 0.35 mA. Motor at ≈ 69 mA. V_motor ≈ 2.1 V — roughly half of full-speed voltage → about half the RPM. The fan barely moves air, but the blade still turns.

Notice that the first 40 % of knob travel does nothing you can see. That's the "wasted" range where the transistor was already saturated with plenty of headroom. If it bothered you, you could shrink the base safety resistor (say to 220 Ω) — but then, at the zero end of the knob, base current would be 17 mA. Fine for the 2N2222 (which handles up to ~200 mA base), but pointlessly wasteful.

Why the 1 kΩ safety is still there

Fair question — the pot already has resistance. Yes, but only most of the time. At one extreme of the knob's travel the pot is basically 0 Ω, and without the 1 kΩ safety the base current would be 4.5 V ÷ 0 Ω — which the 2N2222's base junction would turn into "as much as the battery can push", and that ends with a burnt transistor. The 1 kΩ safety resistor keeps I_base under ~4 mA at the worst case — comfortable for the 2N2222 forever.

Same idea as the 220 Ω "defensive part" in Dial-a-Brightness, sized here for the transistor's base rather than the LED.

The heat story

Here's the subtle part of the physics. When the transistor is saturated (knob at low end), it dissipates almost no heat: V_CE × I_C = 0.2 V × 143 mA ≈ 30 mW. That's the Handheld Fan number.

But at knob halfway (V_CE ≈ 0.7 V, I_C = 126 mA), heat = 88 mW — three times the saturated case. At knob = 75 %, V_CE ≈ 1.8 V, I_C = 89 mA → 160 mW. At knob max, V_CE = 2.4 V, I_C = 69 mA → ~170 mW.

The 2N2222 is rated for 625 mW of dissipation, so nothing's in danger — but you will feel the transistor warm up as you hold the knob mid-travel. That warmth is where the "wasted" motor power went. A saturated transistor is thermally cheap; a linear one runs hot. This is exactly why big motor controllers use PWM (fast on-off pulsing) instead of analog dimming: it keeps the transistor in saturation, out of the hot linear region.

Build it

If your Handheld Fan is still on the board, this is a two-part swap: remove the push button and put the pot in its place. Same base resistor. Same motor + diode. Same transistor.

Fresh build:

1. Power rails: battery + to a top row, battery − to a bottom row.
2. The knob lane: battery + → one outer leg of the pot.
3. The pot's middle leg (the wiper) → 1 kΩ safety → transistor Base.
4. The transistor: flat face toward you, legs left-to-right are Emitter, Base, Collector. Emitter → − rail.
5. Motor + flyback diode in parallel between the + rail and the transistor's collector. Cathode (banded end) of the diode on the + side. Same placement as First Spin and every L4 project.
6. Twist the knob. The fan speeds up and slows down.

Tiny reliability tip (from Dial-a-Brightness): bridge the pot's unused outer leg to the wiper with a short jumper. If the wiper ever loses contact for an instant (dust, an old pot), the base line still sees the full 10 kΩ instead of an open circuit — the fan slows to a crawl rather than glitching off. Real electronics ships with this trick.

What "in the middle" earns you

The interesting knob position is somewhere around 60 %. The transistor is out of saturation and the motor is running at roughly 90 mA at 2.7 V — audibly slower than full speed, still spinning healthily, and the transistor is dissipating about 160 mW. Rest a finger on the flat face of the 2N2222 and you'll feel it warm.

That warmth is a real thing about analog electronics: whenever you're between "fully on" and "fully off", you're spending power as heat. The pot doesn't feel this heat (the pot only sees the tiny base current); the transistor pays the bill.

What this pattern opens up

• Level 5 replaces the pot with an Arduino pin's PWM output — same base-side control, but pulsed instead of held mid-range, so the transistor stays in saturation and runs cool.
• Level 7 replaces the pot with a program. Turn the fan on and off, ramp its speed, tie its speed to a temperature reading.
• Level 9 wires two of these — one per wheel — to give a robot per-wheel speed control.

Every one of those is this circuit with the knob upgraded.