🔐 Vault Chime

What you're building

Handhold Chain grew up and found its voice. Three slide switches sit in a line, exactly like at Level 1, except now there's a buzzer plus a red LED hanging off the far end instead of just an LED. All three switches have to be on at the same moment: then the LED lights AND the buzzer beeps together. Open any one — even for a half-second — and both outputs die at once.

This is the vault-door pattern: the alarm only arms when every lock confirms it's in place.

The big idea: series-AND still works, no matter the output

Up at Level 1 you learned that switches in a line make an AND gate — every single one has to be closed before the loop can carry current. The Vault showed it with two slide switches. Handhold Chain did it with three. The lesson there was: a closed slide switch is basically a wire, so chain length doesn't dim or slow anything.

Now you're going to prove the same rule still holds when the output is a buzzer in parallel with the LED instead of just the LED alone. The buzzer is louder than any part you've used before — a real 30 mA current path, almost three times what the LED is sipping — so there's a tempting but wrong guess that "with more current flowing, the switches might get warm" or "the dim-chain effect might kick in". Neither happens. Series switches don't care how big the load behind them is, only whether every single one is closed.

• Any one switch open: loop broken, 0 mA. Buzzer silent, LED dark.
• All three closed: both lanes come alive. Buzzer and LED each do their own math, and the battery supplies the sum.

Do the math (all three closed)

With every slide switch closed, the merge rail reaches the full 4.5 V and both lanes conduct in parallel.

• Buzzer lane. The active buzzer looks like a ~150 Ω load (its datasheet rating of ~30 mA at 4.5 V implies 150 Ω). Ohm's law: I_buzz = 4.5 V ÷ 150 Ω = 30 mA. Full-volume beep.
• LED lane. Battery 4.5 V − red LED Vf 2.0 V = 2.5 V left for the 220 Ω. I_LED = 2.5 ÷ 220 ≈ 11 mA. First-Light brightness.
• Battery total: 30 + 11 = 41 mA. Same total as Arm & Ring, just with the arm pattern widened to three gates instead of one.

Notice what doesn't happen when you widen the chain from two switches to three: the current is unchanged. A closed slide switch has a few milliohms across its contacts, nothing next to the hundreds of ohms doing the real work. You could add seven more switches in series and the current would still be 41 mA — as long as every single one of them is closed.

Do the math (any switch open)

The series rule: one open link anywhere means no current anywhere. No matter which switch is open, the merge rail can't reach the battery's plus side. The buzzer sees 0 V across itself and the LED sees 0 V across its 220 Ω.

• I_buzz = 0 mA (buzzer silent)
• I_LED = 0 mA (LED dark)
• Battery total = 0 mA (nothing anywhere)

That's the point of a vault door: a partial agreement is no agreement. One guard's keypad can't open the room, even if the other two did their part.

Where "all-three-must-agree" shows up

• Bank vault alarms really do use three or four parallel keys that must all turn within a few seconds of each other. Our slide switches are the toy version — real vaults use mechanical tumblers and electrical interlocks in series, same AND gate.
• Missile silo launches in cold-war movies use two or three turnkeys at once. Not just drama — a real series-AND would stop one person acting alone.
• Chemical reactors and industrial presses interlock pressure switches, temperature switches, and operator "hold-to-run" buttons in series so a single sensor failure can't start the dangerous machine.
• Elevator doors chain every floor's "door closed" contact in series: the car won't move until all floors agree they're sealed.

Every one is this circuit at heart. The only differences are the number of switches in the chain and the size of the thing hanging off the end.