🆘 Panic Fan

What you're building

Handheld Fan let you spin the motor by pressing one button. This one gives you two buttons — one on each side of the workspace, or one on your desk and one across the room — and either one spins the same fan. Release both and the fan stops.

You've built this "either-one triggers it" pattern before, at Level 2, in Panic Doorbell. Same wiring trick: two buttons in parallel. The only thing that changed is the mouth on the far end of the transistor — it's a motor now instead of a buzzer + LED.

Reuse from Level 2 (the OR gate)

Two switches sitting side by side — both bridging the same two spots on the board — give the loop two ways to close. Only one needs to be closed for current to flow. Two closed doesn't help anything, but doesn't hurt either.

• Series (AND): + → [A] → [B] → next stage → −. Break either and the loop dies. That's The Vault, Two-Hand Bell.
• Parallel (OR, this project): + → [either A or B] → next stage → −. Either alone is enough. That's Panic Doorbell — and now this.

Reuse from Level 3 (the transistor)

• 1 kΩ base resistor between the OR-gate output and the base keeps the button current small (~3.8 mA) even at full press.
• 2N2222 turns on when its base sits above ~0.7 V.
• With β ≈ 200, a 3.8 mA base drive commands up to 760 mA through the collector — vastly more than the motor needs.

Reuse from Level 4 (the motor + diode)

• DC motor, ~30 Ω coil, ~143 mA at 4.3 V (transistor keeps V_CE at ~0.2 V).
• 1N4001 flyback diode across the motor, banded end (cathode) toward battery +. Reverse-biased and invisible while the fan runs; catches the coil's kick the moment you release the button.

Do the math

Either button pressed:

• The OR-gate output lifts to ~4.5 V (minus a millivolt across the closed button — negligible).
• Base resistor sees 4.5 − 0.7 = 3.8 V, so I_base ≈ 3.8 mA.
• β × I_base = 760 mA capable — deep saturation.
• Motor lane: (4.5 − V_CE(sat)) ÷ 30 Ω = (4.5 − 0.2) ÷ 30 = ≈ 143 mA.
• Battery total: motor + base + a whisker of leakage ≈ 147 mA.

Both buttons pressed at the same time:

• Same 147 mA. The second closed button is a redundant path — current only flows through one of the two parallel closed switches (whichever has slightly lower contact resistance) or a little through each; either way, the motor draws the same 143 mA because that's what the motor asked for. Adding more parallel paths through a resistor-free rail doesn't add current.

Nobody pressing:

• No path from the + rail to the base. I_base = 0. Transistor off.
• Motor idle, battery draws microamps of leakage. You can leave this armed for weeks and barely dent an AA.

Compare to Panic Doorbell, where either button rings a buzzer: same brain, thirstier mouth. And compare to Handheld Fan: same mouth, wider (OR-gated) brain.

Build it

1. Power rails: battery + to a top row, battery − to a bottom row.
2. Two buttons in parallel: the top row branches out to two nearby columns. Each column has a push button that drops from the + rail to a merge row (call it row M). Pressing either connects the + rail to row M.
3. Base link: row M → 1 kΩ → transistor Base leg.
4. The transistor: flat face toward you, legs left-to-right are Emitter, Base, Collector. Emitter → − rail.
5. Motor lane: motor between the + rail and the Collector row.
6. Flyback diode across the motor: cathode (banded end) in the + rail row, anode in the collector row. Same placement as First Spin.
7. Press button A. Fan spins. Release: stops. Press button B. Same thing. Press both. Same. Release both: quiet.

Both buttons wire from the same + rail to the same merge row. The mistake to avoid is running each button to a different merge row — that turns the two buttons into two separate fan circuits sharing nothing, and one of the two won't have a base link.

Where two buttons help

• A shared "cooldown" trigger for a hot enclosure. Router in a cupboard? One button by your desk to spin the cupboard's cooling fan when it feels warm; a second button inside the cupboard for the person servicing it.
• Two-station fan for a shared room. Sibling A's side of the room has a button; sibling B's has another; either can turn on the middle fan.
• Redundancy. In the real world, safety-critical fans (an overheating server rack, a wood stove blower) get two independent "spin now" triggers so if one wire breaks, the other still works.

Anywhere a spinner needs to be triggerable from more than one place, OR is the pattern.

What this opens up

Add more buttons? Just wire another push button in parallel with the first two. Three buttons, five buttons — the transistor doesn't care. Every one is another way to close the same trigger loop. At Level 9 you'll wire two sensors in parallel this same way to fire a robot's motor when either sees a line.