🎺 Warm Hand Chorus

What you're building

Dusk Chorus for temperature. Cup a warm palm over the thermistor and, at the same instant, a red LED glows and a buzzer chimes. One sensor, two outputs, no code. Great as a fever alarm, a coffee-ready chime, or a "the sun is hitting my desk" warning taped to a window.

Fever Beeper proved that the Warm Hand Alarm brain drives a buzzer as happily as it drives an LED. This project adds both outputs onto the same transistor at once — the exact "one brain, two mouths" move you learned in Dusk Chorus, applied to the temperature brain.

The brain × mouth grid, temperature edition

You already know the grid from Dusk Chorus:

	LED output	Buzzer output	Both in parallel
Light sensor	Automatic Night Light	Curfew Beeper	Dusk Chorus
Warm sensor	Warm Hand Alarm	Fever Beeper	this project

The pattern is the same in both rows. Once you can see it, every sense-then-act project on the site is a swap of one cell for another.

The sensing half — from Fever Beeper, one tweak

The divider is thermistor on top, 1 kΩ on the bottom — the same as Warm Hand Alarm and Fever Beeper. Rough thermistor values from the NTC datasheet curve: 10 kΩ at 25 °C, ≈ 3.6 kΩ at 40 °C (warm hand), ≈ 1.5 kΩ at 60 °C (a hot mug).

• Room (thermistor ≈ 10 kΩ): V_mid = 4.5 × (1 ÷ 11) ≈ 0.41 V. Below the 0.7 V wake-up. Transistor off. Everything silent and dark.
• Warm hand (thermistor ≈ 3.6 kΩ): V_mid tries to reach 4.5 × (1 ÷ 4.6) ≈ 0.98 V. The base clamps at 0.7 V and drinks the extra push as current. Transistor on, hard.
• Hot mug (thermistor ≈ 1.5 kΩ): V_mid tries to reach 4.5 × (1 ÷ 2.5) ≈ 1.8 V. Way past the wake-up. Transistor fully saturated — no louder past the warm-hand state; a saturated transistor is a saturated transistor.

The one change from Fever Beeper: 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 Fever Beeper 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 Fever 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 (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.

Fever Beeper's base resistor was 1 kΩ, which gave the transistor about 0.17 mA of base current at a warm-hand V_mid ≈ 0.87 V. That's enough for β·I_b ≈ 34 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). Suddenly 34 mA of β-limited drive isn't enough. The transistor would drop out of saturation, V_CE would rise, and both mouths would come out weaker — buzzer around 21 mA, LED around 6 mA.

Fix: shrink the base resistor from 1 kΩ to 220 Ω. Same divider, same wake-up story, but the base now gets ~5× as much current — about 0.8 mA at warm-hand V_mid. β·I_b jumps to ~160 mA, and the 40 mA demand from both lanes together is easy work.

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 with the 220 Ω base resistor: β·I_b ≈ 160 mA of available Ic, and only 40 mA is asked for. Fully saturated, V_CE(sat) ≈ 0.2 V, both mouths at full strength.

Battery total when it's cool: microamps of leakage. You can leave Warm Hand Chorus clipped to a mug for a whole afternoon and barely scratch the AAs.

Build it

If your Fever Beeper 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 (thermistor on top): battery + → thermistor → middle row → 1 kΩ → battery −. Mount the thermistor so its bead sits out in the open air — not tucked under other parts.
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. Power up in a normal room. Silent and dark. Cup your palm over the thermistor for ten seconds: beep + red glow together as the bead warms.

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.

The mushy fade-in — now audible AND visible

As the thermistor warms from 10 kΩ down through 3.6 kΩ, V_mid climbs from 0.41 V past the 0.7 V wake-up line. The transistor doesn't snap on — it fades in. You'll hear the buzzer 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.

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, Warm Hand Chorus becomes a one-line rewrite: the same thermistor + transistor now drives buzzer + LED + a fan — the base of a real "overheating warning" gadget for a laptop vent. At Level 9 that same brain drives the wheels of a heat-avoiding robot. Every "sense the world → do many things at once" project in the whole site is this pattern.