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Level 3 · Sense · project 13 of 17 · 30 min · ●●●○○

Dawn Chorus

Cookie Jar Alarm with a red LED added in parallel with the buzzer — one light sensor drives a chime and a lamp the moment the sun hits.

new here? Start with First Light — one battery, one resistor, one LED. It teaches the loop, LED direction, and why the 220 Ω resistor matters.

You'll make a sunrise chime and lamp
You'll learn LDR drives buzzer + LED
Watch for LED lane needs its own resistor — the buzzer lane does not

What you need

🔧 Try before wiring →

you'll learn: Voltage dividers · Transistor switching · Series & parallel · Ohm's law · LED polarity

Circuit map

The circuit you'll build, running on the Workbench right now.

simulating…

If it doesn't work

The boring ways this circuit actually breaks — check these first before you unwire everything.

  • Only the LED lights when the sun hits. The buzzer's + isn't wired to the shared collector rail. Both outputs must fork off the same jumper coming out of the transistor's collector.
  • Only the buzzer sounds. Flip the LED (long leg toward +) and confirm the 220 Ω is in the LED lane. The buzzer runs raw.
  • Both silent even in bright light. 2N2222 pins E-B-C (flat face toward you). Then confirm the LDR is on TOP of the divider (same as Cookie Jar Alarm) with the 10 kΩ on the bottom, and the 220 Ω base resistor is between the divider middle and the base.
  • Both on all the time, even in the dark. Divider flipped — LDR must sit on top. Swap it with the 10 kΩ. (Or you built Dusk Chorus wiring by mistake.)
  • Buzzer weak, LED barely visible even in bright sun. You used a 1 kΩ base resistor instead of 220 Ω. One kΩ was fine for Cookie Jar Alarm's single mouth; two mouths need more base drive — swap it for a 220 Ω.
  • Buzzer sounds but LED is very dim. You wired the LED in series with the buzzer instead of parallel. Each output gets its own lane; they share only the collector rail and −.
  • LED and buzzer both flicker on-and-off at dawn. You're in the analog fade-in zone — the LDR is passing slowly through 5–20 kΩ. Wait for full daylight, or use a brighter test light on the sensor to see it commit.

What you're building

Dusk Chorus for the other end of the day. Aim the little light sensor at a window and, the moment the first strong light of morning hits, a red LED and a buzzer fire together. One sensor, two outputs, no code. Great as a sunrise alarm clock, a "the sun is on your desk" cue, or a plant-cue that says "the shelf is bright enough".

Cookie Jar Alarm already showed that the Sunrise Alarm brain drives a buzzer as happily as it drives an LED. This project adds both outputs onto the same transistor at once — the same "one brain, two mouths" trick you learned in Dusk Chorus, applied to the rising edge of the light sensor.

The brain × mouth grid, completed

The full grid from Warm Hand Chorus had one empty cell. This project fills it:

LED output Buzzer output Both in parallel
Light sensor (dark) Night Light Curfew Beeper Dusk Chorus
Light sensor (light) Sunrise Alarm Cookie Jar Alarm this project
Warm sensor Warm Hand Alarm Fever Beeper Warm Hand Chorus

Two brains × three mouths, plus a temperature row. Every cell is a project on this site. Once you can see the grid you can walk to any cell and predict the circuit before you look at it.

The sensing half — from Cookie Jar Alarm, one tweak

The divider is LDR on top, 10 kΩ on the bottom — identical to Sunrise Alarm and Cookie Jar Alarm. LDR is roughly 1 kΩ in bright light and ≈ 100 kΩ in the dark.

  • Night / dark (LDR ≈ 100 kΩ): V_mid = 4.5 × (10 ÷ 110) ≈ 0.41 V. Well below the 0.7 V wake-up. Transistor off. Silent and dark.
  • Sunrise / bright (LDR ≈ 1 kΩ): V_mid tries to reach 4.5 × (10 ÷ 11) ≈ 4.1 V. The base clamps at 0.7 V and drinks the extra push as current. Transistor on, hard.

The one change from Cookie Jar Alarm: the base resistor is 220 Ω, not 1 kΩ. Two mouths on one transistor draw more collector current than one mouth does, and the transistor can only pull as much collector current as its base drive × β allows. Cut the base resistor by ~5× and you multiply the available drive by ~5×, which is plenty of headroom for both lanes together. The "why" section below unpacks this — it's the most important idea in the project.

If the divider math is new, read Cookie Jar Alarm first. Same divider, same reasoning, one smaller resistor.

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 hang off the top rail on one side and off the collector on the other. 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 Cookie Jar Alarm. 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 (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.

Why the base resistor got smaller

Every 2N2222 has a hidden speed limit called β (beta) — roughly 200 for this part. It says: the collector can only carry β times as much current as the base does. Base gets 0.1 mA → collector can carry at most 20 mA. Base gets 1 mA → collector can carry 200 mA. Push harder on the base and the collector opens up.

Cookie Jar Alarm's base resistor was 1 kΩ, giving the transistor about 1.7 mA of base current in bright light. That's enough for β·I_b ≈ 340 mA — plenty for a single 29 mA buzzer.

Add an LED lane and the collector demand jumps to ~40 mA (29 mA buzzer + 10.5 mA LED). Still comfortably inside 340 mA, so why shrink the base resistor?

Because the LDR itself is a bottleneck. At bright light the LDR is ~1 kΩ, and that 1 kΩ plus the 1 kΩ base resistor together dominate the divider — the middle node can only supply so much current before V_mid sags away from the 0.7 V base-clamp voltage. A 220 Ω base resistor makes the base a lower-impedance load, so the transistor sees more base drive and settles deeper into saturation.

Net effect: swap 1 kΩ → 220 Ω on the base and you get the classic V_CE(sat) ≈ 0.2 V signature with both mouths at full strength. Keep the 1 kΩ and both mouths still fire — but a hair weaker and a hair slower to commit. The exact same swap Warm Hand Chorus makes for the same reason.

Do the math (all together)

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

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

The 2N2222 handles it comfortably: β·I_b is much larger than the 40 mA asked for, so the transistor is fully saturated, V_CE(sat) ≈ 0.2 V, both mouths at full strength.

Battery total in the dark: the divider draws a trickle down the LDR (100 kΩ) and the 10 kΩ — about 40 µA. That's a hundredth of what the alarm draws when it fires. Leave Dawn Chorus clipped to a window all night and barely scratch the AAs.

Build it

If your Cookie Jar Alarm is already breadboarded, this is a two-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 top): battery + → LDR → middle row → 10 kΩ → battery −. Mount the LDR so its face points up at the window, not down at the breadboard.
  3. The base link: middle row → 220 Ω (the smaller base resistor — see "Why the base resistor got smaller" above) → 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. Cover the LDR with your finger (or lift it into a dark drawer): silence and dark. Move it into the sun, or shine a flashlight on it: beep + red glow together.

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, 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.

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, Dawn Chorus becomes a one-line rewrite: the same LDR + transistor now drives buzzer + LED

  • a fan — the sunrise cue that also stirs the air on your desk. Sun Beacon already does the LED + motor half of that idea. Adding the buzzer makes it a three-mouth chorus. Every "sense the world → do many things at once" project on the site is this pattern.

🧠 Your challenge

No single right answer. That's the point.

  • Green dawn lamp. Swap the red LED for a green one. Redo the LED lane math with the green LED's higher forward drop (2.1 V instead of 2.0 V). What's the new LED current? (Answer: 2.2 ÷ 220 ≈ 10 mA — a half-milliamp lower. The buzzer lane is unchanged, because the two lanes are independent.)
  • Add a second LED — a "getting light" trailer. Fan a third lane off the collector: another 220 Ω + LED (yellow, say). Now the chorus is buzzer + red + yellow whenever the sun hits. What's the new battery total when it fires? (≈ 29 mA buzzer + 10.5 mA red + 10.4 mA yellow ≈ 50 mA. Still inside what the transistor can drive at this base current.)
  • Sunset chorus. Swap the LDR and the 10 kΩ — LDR on the bottom, 10 kΩ on top. Predict what happens before you power it up. Does the alarm now fire at sunrise or at sunset? (Answer: at sunset. That's Dusk Chorus — the mirror of this project across the dark/light row of the grid.)
  • Less jumpy. Right now a passing cloud can wake the alarm for a moment. Add a 1 kΩ in series with the LDR on top of the divider. What does the divider math say the middle voltage does at bright daylight (LDR ~1 kΩ)? Does the chorus still fire? (Middle sits at 4.5 × 10 ÷ (1+1+10) = 3.75 V — still well past 0.7 V, alarm still fires. But a passing cloud that raises the LDR to ~5 kΩ now shifts V_mid down closer to the wake-up line, so the trigger is less twitchy.)
  • Add an off switch. Put your slide switch in series with the + rail before the divider. Now you can silence the chorus manually the moment you're awake. Battery draw when the slide switch is open? (Zero — the whole circuit is disconnected.)

For grown-ups: safety notes

  • Battery-safe throughout — 4.5 V is safe to touch at any brightness. Even a fully-firing chorus pulls only about 40 mA, comfortably inside what fresh AAs deliver.
  • Active buzzers are loud at close range. If you're using this as a sunrise alarm on a bedside table, aim the buzzer away from the pillow, not at it. A 2 kHz tone six inches from an ear at 6 a.m. is a very rude way to wake up.
  • Don't tape the LDR to a window in direct summer sunlight for hours. The LDR itself is fine, but a black breadboard and jumper wires can get warm in strong sun — reposition to shade if the parts feel hot. This is a general "electronics + direct sun" rule, not a fault of the LDR.
  • If the LED lights but the buzzer stays silent, one of two things: the buzzer's + leg is on the wrong side (buzzers are polarised — the plus mark on top faces the battery +), or the buzzer lane goes to the emitter row instead of the collector row (in which case the transistor isn't in charge of it). Trace the buzzer lane end to end.
  • If the buzzer chimes but the LED never lights, check LED polarity (long leg toward +, short leg toward the collector). Reversed LEDs don't hurt anything at 4.5 V but they also don't light.
  • If the transistor is warm to the touch, disconnect the battery and re-check E-B-C leg order. Backwards transistors are always the first suspect.
  • The mushy fade-in chatter as the sun slowly rises is not a fault — it's the transistor crossing the wake-up line slowly as the LDR drops through 20 kΩ, 5 kΩ, 2 kΩ. If you want a sharper trigger, swap in a smaller divider bottom (the 1 kΩ in your kit). Level 6 will give you a proper "snap to on" chip for this.
  • Never wire this to mains. Every part in this project is low-voltage; wall power would destroy the transistor, the LED, and the buzzer instantly, in that order.

Checked against

Keep building

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