🛟 Escape Hatch

What you're building

The Vault had two locks in a line and no way past them. This time the same lock is still there — slide A and slide B, both must be closed for the LED to light — but now there's a red emergency button wired around the whole pair. Press it and the LED lights the instant, no matter which locks are open.

Real fire doors work exactly like this. Two keyed deadbolts hold them shut in normal use, and a big push-bar bypasses both when someone slaps it on the way out. You're building the electrical version.

The trick this project is really teaching is mixing AND with OR in one circuit. That's a compound logic gate — the first one you own that does more than one Boolean thing at once.

The big idea: AND-then-OR

The Vault was pure AND: slide A closed AND slide B closed → LED lit. Everything else → dark.

Two-Door Signal was pure OR: front switch OR back switch → LED lit. Only both open → dark.

Escape Hatch is (A AND B) OR C. It reads as one sentence: "both locks turned, or the emergency button pressed." That "or" is the key move: adding a parallel path around the two series switches means the loop can complete two different ways. Either way lands on the same 220 Ω + LED lane.

Truth table (0 = open, 1 = closed/pressed; last column = LED):

Slide A	Slide B	Button	LED
0	0	0	🌑 dark
1	0	0	🌑 dark
0	1	0	🌑 dark
1	1	0	🔴 lit
0	0	1	🔴 lit
1	0	1	🔴 lit
0	1	1	🔴 lit
1	1	1	🔴 lit

Half the rows are dark, half are lit. The dark ones all share one feature: button up, and at least one slide open. The moment the button goes down, or both slides close, the loop finds a way through.

Do the math (both slides closed, button up — normal open)

This is just First Light with two closed switches in the way. Closed switches carry current for free — they don't slow it down. Loop:

• Battery push: 4.5 V
• LED forward drop: 2.0 V
• Voltage left for the 220 Ω: 4.5 − 2.0 = 2.5 V
• Current: I = V ÷ R = 2.5 ÷ 220 ≈ 0.011 A = 11 mA

Same 11 mA as the very first project you built. Adding switches to a series loop never changes the current, only whether the current is allowed to happen at all.

Do the math (both slides open, button pressed — emergency bypass)

The main path is now broken twice over, but the emergency button lays a whole new wire around it. That wire replaces slide A + slide B with one close-to-zero-Ohm bridge, and everything downstream (220 Ω, LED) sees the same 4.5 V. Same math:

• Voltage left for the 220 Ω: 2.5 V
• Current: 11 mA

The current doesn't know or care whether it took the two-slide path or the emergency-button path — the two paths end at the same node.

Do the math (both slides open AND button up — everything off)

Both parallel paths are broken. No loop → 0 mA. LED dark. This is where the vault sits when nobody is around.

What if BOTH paths are on at once?

Say all three switches are closed. Then two paths (slides-in-series and button-bypass) both connect the same two nodes — the same places on the +rail and just before the 220 Ω. Parallel identical zero-resistance paths still add up to zero-ish resistance. The current finds them equally happy. Nothing lights any brighter; the LED still runs at its usual 11 mA. Adding a parallel wire around an already-closed loop is a no-op — the current can only be limited once by the same 220 Ω downstream.

Where "AND with an emergency OR" shows up

• Fire-escape doors with two deadbolts and a push-bar bypass.
• Panic strips on gym or theatre exits — the door has all its regular latches, plus a horizontal bar that pushes them all open at once.
• Elevator door override keys — the elevator's normal safety interlocks must all agree before it moves, and a firefighter's key bypasses all of them.
• Two-key nuclear launch panels with a "presidential authorization button" — a plot device, but built from the same AND-OR pattern.

Any time a system has strict day-to-day rules and a controlled way to bypass them, this circuit is what's under the hood.