🌇 Dusk Chorus

What you're building

A dusk announcer. Sit it on the windowsill. All day it's silent and dark. The sun goes down — the LDR sees the fading light — and at the same instant a red lamp glows and an active buzzer starts chiming. One sensor, two mouths. Great for calling everyone in for dinner, closing the chicken coop, or just knowing when it's time to switch on the porch light.

Curfew Beeper proved that the Automatic Night Light brain drives a buzzer just as happily as it drives an LED. This project connects both outputs to the same brain at once, so one transistor commands two lanes in parallel.

The big idea: one brain, two mouths in parallel

You already know the "brain × mouth" grid from Curfew Beeper:

	LED output	Buzzer output
Dark triggers	Automatic Night Light	Curfew Beeper
Bright triggers	Sunrise Alarm	Cookie Jar Alarm

Dusk Chorus is a fifth cell: "dark triggers" driving both mouths at once. Fanning out two output lanes from a single transistor doesn't need any new physics — the two lanes are just wired in parallel from the transistor's collector, exactly the way Doorbell fans a single button into a buzzer lane and an LED lane.

Once you can do this, the grid isn't a grid anymore — it's a pantry. Every project after this becomes "pick a brain, pick a handful of mouths, wire them side by side, done."

The sensing half — Automatic Night Light, unchanged

The divider is 10 kΩ on top, LDR on the bottom — the same as Automatic Night Light, Curfew Beeper, Reading Light, and Vacation Nightlight. When the room is bright, the LDR shrinks to about 1 kΩ and the middle sits low; when the room goes dark, the LDR balloons past 100 kΩ and the middle climbs up past the transistor's wake-up line.

• Bright, LDR ≈ 1 kΩ: V_middle = 4.5 × (1 ÷ 11) ≈ 0.41 V. Well below the 0.7 V wake-up. Transistor off. Everything silent and dark.
• Dark, LDR ≈ 100 kΩ: V_middle tries to climb toward 4.5 × (100 ÷ 110) ≈ 4.1 V. The base clamps at 0.7 V and drinks the extra push as current. Transistor on, hard.

If any of that is new, read Automatic Night Light first. The same math, the same voltage divider, the same reasoning.

The output half — buzzer AND LED, side by side

Once the transistor is on hard, its collector-to-emitter link becomes a near-short (the classic V_CE(sat) ≈ 0.2 V drop). Both output lanes now hang off the top rail on one side, and off the collector on the other side. Each lane sees 4.5 − 0.2 = 4.3 V across itself and does its own Ohm's-law math:

• Buzzer lane: the active buzzer behaves like a ~150 Ω load at 4.3 V, so I_buzzer = 4.3 ÷ 150 ≈ 29 mA. Loud, same as Curfew Beeper. No resistor needed on this lane — the buzzer self-limits.
• LED lane: red LED "eats" 2.0 V, so 4.3 − 2.0 = 2.3 V sits across the 220 Ω. I_LED = 2.3 ÷ 220 ≈ 10.5 mA. Just a hair under the 11 mA of First Light (because the transistor stole 0.2 V), still comfortably bright.

The two lanes are independent. Adding the LED lane does not quiet the buzzer, and adding the buzzer lane does not dim the LED. Each lane sees a full 4.3 V push and each does its own math.

Do the math (all together)

Battery total when it fires: the transistor's collector has to supply the sum of the two lane currents.

• Buzzer lane: ~29 mA
• LED lane: ~10.5 mA
• Sum through the collector: ~39.5 mA

The transistor can handle it easily. With β ≈ 200 and a base current of about 0.3 mA through the 1 kΩ base resistor, its maximum collector current at that base drive is 60 mA — so 39 mA is well inside the transistor's happy zone (it stays saturated).

Battery total when it's quiet: microamps of leakage. You can leave Dusk Chorus on the windowsill for weeks and barely scratch the AAs.

Build it

If your Curfew Beeper is already breadboarded, this is a 2-minute upgrade — you're adding a lane, not rebuilding the circuit.

Fresh build:

1. Power rails: battery + to a top row, battery − to a bottom row.
2. The sensing half (LDR on bottom): battery + → 10 kΩ → middle row → LDR → battery −.
3. The base link: middle row → 1 kΩ → transistor Base (middle leg, flat face toward you).
4. The transistor: Emitter (left leg) → − rail.
5. The buzzer lane: battery + → buzzer + leg; buzzer − leg → transistor Collector (right leg).
6. The LED lane in parallel with the buzzer — this is the new step:
   • From battery + rail: → 220 Ω → LED long leg.
   • LED short leg → same transistor Collector as the buzzer's negative leg. Both lanes share the collector row.
7. Point it at a lamp: silent, dark. Cup your hands over the LDR: after a short in-between chirp, beep + red glow at the same time.

The key wiring step is that both lanes come out of the collector row. If you accidentally wire the LED to the emitter row instead of the collector, it will light whenever the battery is connected (bypassing the transistor entirely) — a common first-try mistake. Trace the LED lane: + rail → 220 Ω → LED → collector → transistor's guts → − rail. That's the loop.

The mushy transition zone — now audible AND visible

At dusk, as the LDR slowly climbs from 1 kΩ to 100 kΩ, the transistor doesn't snap on. It fades in. You'll hear the buzzer scratch and chatter for a heartbeat before it commits to a steady tone, and you'll see the LED brighten from off through a faint orange glow into full red. Both mouths cross the analog region together, because they share the same brain.

At Level 6 the 555-timer chip turns that mush into a crisp click. For now, watching the two outputs cross the wake-up line together is a great way to see that the transistor is not a light switch — it's an amplifier, and in the middle it's an analog one.

Deploy your dusk announcer

• West-facing windowsill if you want a sunset alarm; east if you want a dawn alarm (which would need the LDR moved to the top of the divider, converting the brain into Sunrise Alarm + Cookie Jar Alarm — try it as a rewire challenge).
• Daytime test: turn off the room lights and pull the curtains. If the alarm chimes and the LED lights, the trigger is set correctly. Then leave it running and watch the real sunset.
• If the buzzer chatters on-and-off for more than a couple of minutes at dusk, that's the analog mush zone taking its time through the transition. It'll settle into steady chiming as the sky darkens further. If it never settles, the LDR is probably in an ambiguous half-shadow — move it toward a brighter spot or a shadier one, not on the boundary.

Where "one brain, many mouths" leads next

At Level 4 the mouth family grows to include a motor (with a protection diode). Once you own that, Dusk Chorus becomes a one-line rewrite: the same LDR + transistor now drives buzzer + LED + a fan — or a paper-barrier motor to close a coop door. At Level 9 that same brain drives the wheels of a light-avoiding robot. Every "sense the world → do many things at once" project in the whole site is this pattern.