🎮 Quiz Buzzer

What you're building

A two-team quiz buzzer. Each player gets one big push button. Press yours and your LED lights and your buzzer beeps — nothing on the other side moves. Press theirs and the same thing happens on their half. Nothing shared between the two teams except the battery.

This isn't a "first-to-buzz-wins" latch (that needs a transistor — you meet those at Level 3). It's the honest version: press-and-hold sounds your alarm; let go and it stops. Perfect for family trivia night, a classroom-style game, or two siblings racing to answer.

The big idea: two independent lanes sharing one battery

Every project so far mixed things: two switches feeding one output (Panic Doorbell), one switch feeding two outputs (Twin Bells), or a mix of both (Silent Alarm). This project does something new: it puts a full copy of a doorbell circuit on each side of the board and asks the same 3×AA pack to power both.

• Team A lane. Battery + rail → Team A button → Team A hot node → (drops into two parallel spurs: 220 Ω + red LED, and a buzzer) → − rail.
• Team B lane. Same thing, mirrored on the right: battery + rail → Team B button → Team B hot node → (220 Ω + green LED, and a buzzer) → − rail.

Because the two team hot nodes are only connected via a pressed button back up to the + rail, pressing Team A does nothing to Team B's hot node — it stays disconnected from the battery until someone presses Team B. That's the definition of lane independence.

Do the math (one team pressed — the "answer!" state)

Only Team A is answering. Team A's hot node is pinned to 4.5 V by the closed button; Team B's hot node is floating (button open).

• Team A LED lane. Battery push 4.5 V, red LED forward drop 2.0 V, voltage across the 220 Ω: 4.5 − 2.0 = 2.5 V, current I = 2.5 ÷ 220 ≈ 11 mA.
• Team A buzzer lane. The active buzzer looks like a ~150 Ω load at 4.5 V (from its datasheet spec: about 30 mA at 5 V). Current I = 4.5 ÷ 150 = 30 mA.
• Team A total: 11 + 30 = 41 mA.
• Team B: 0 mA on both lanes — the button is open, the hot node is disconnected, nothing to push current through.
• Battery total: 41 mA.

Green LED, if you asked, has almost the same math — its forward drop is about 2.1 V, so its lane current works out to (4.5 − 2.1) ÷ 220 ≈ 10.9 mA. Same eyeful of brightness as the red.

Do the math (both teams pressed — the "we tied!" state)

Both buttons closed. Both team hot nodes at 4.5 V. Both LED lanes at their steady ~11 mA. Both buzzers at their steady ~30 mA.

• Team A total: 41 mA (unchanged)
• Team B total: 41 mA (unchanged)
• Battery total: ~82 mA

Notice what did not happen: Team B's press did not dim Team A's LED or quiet Team A's buzzer. Each lane is a self-contained loop with the full 4.5 V across it. All the battery had to do was supply the sum.

Fresh AAs deliver hundreds of mA without breaking a sweat, so 82 mA is well inside the safe zone. If both teams press for a full hour solid — 82 mA × 1 h = 82 mAh — a fresh AA pack has ~2000 mAh, so you'd get about 24 hours of continuous both-teams-buzzing on one set of batteries. Real quiz nights end much sooner than that.

Why the two lanes really can't hear each other

Look at Team A's hot node. It can only reach the battery's + side through Team A's button. Nothing else. Team B's button, Team B's LED, Team B's buzzer — none of them touch Team A's hot node. They're on the other half of the board.

So when Team B presses, Team B's hot node jumps to 4.5 V, but Team A's hot node stays wherever it was (floating if the button is open, 4.5 V if it's already pressed). The two halves are electrically separate loops — glued together only by the shared + and − rails.

Every truly multi-user system in electronics builds on this: two input channels on a mixer, two USB-charging phones on the same wall adapter, two headphones on a splitter. Each side does its own thing; the shared power supply just has to be big enough for the sum.

Where "independent lanes on one power supply" shows up in the world

• USB hubs. One 5 V wall adapter feeds four phone chargers. Each phone charges at its own rate; unplugging one doesn't slow the others down — as long as the adapter can supply the total.
• Christmas lights, modern LED style. Each little string is independently driven from the same wall supply. If one string dies, the others keep glowing (unlike the ancient series-wired kind where one dead bulb killed the whole strand).
• Split power strips. Every outlet on a 5-outlet strip is in parallel across the wall's 120/240 V. Plugging in a lamp doesn't dim the toaster; both just add to the total the wall breaker sees.
• Two-team scoreboards. In every buzzer-in game show, the two teams run through completely mirrored electronics. The moderator's console is the only wiring the two sides share.

The rule that unifies them: any number of loops can share a power supply; each loop still does its own math. The supply has to be big enough for the sum, and that's the only coupling.

Build it

1. Power rails. Battery red (+) to a top row of the breadboard; battery black (−) to a bottom row.
2. Two team buttons on the top rail. Push button A on the left half of the + rail; push button B on the right half of the + rail. Each button's entry leg lands on +; each button's exit leg drops onto its own team hot row a little lower down.
3. Team A hot row. From button A's exit leg, run a jumper across two adjacent columns — this is Team A's local + node. Two spurs will hang off it.
4. Team A LED spur. From Team A's hot row: 220 Ω → red LED long leg → red LED short leg → − rail. Press button A. The red LED should light.
5. Team A buzzer spur. From Team A's hot row (a hole over from the LED spur): active buzzer + leg → buzzer − leg → − rail. Press button A. The buzzer should ring at the same time as the LED.
6. Team B mirror. Repeat steps 3–5 on the other side of the board, using the green LED instead of red. Now pressing button B fires green LED + buzzer B; nothing on Team A moves.
7. Try it. Press A alone — Team A rings. Press B alone — Team B rings. Press both together — both ring, no louder, no quieter, entirely independent.

What's really happening in the wires

Trace Team A's loop with your finger while pressing button A:

• + rail → button A (closed) → Team A hot row → 220 Ω → red LED long → red LED short → − rail → inside the battery back to +.

That's a complete loop, so 11 mA flows. Now do the same for the buzzer spur — same start and end points, different middle. That's also a complete loop, so another 30 mA flows. Both loops share the battery, but nothing else.

Now try to trace Team B's loop while only A is pressed. You'll get to Team B's hot row and find it disconnected from + (Team B's button is up). Loop broken → no current → both Team B outputs dark and silent. That's the whole story.

If you drew this as two side-by-side ladder rungs, it would look like:

+ ──┬── BTN_A ── TEAM_A ─┬─ 220Ω ─ REDLED ── −
    │                    └─ BUZZ_A ────────── −
    │
    └── BTN_B ── TEAM_B ─┬─ 220Ω ─ GRNLED ── −
                         └─ BUZZ_B ────────── −

Two rungs of a ladder, both grounded on the same rail. Add a third team and it's just a third rung with its own button, LED and buzzer. The pattern scales.