🤒 Fever Beeper

What you're building

The Warm Hand Alarm, but louder. Palm the sensor for five seconds and a buzzer starts beeping. Take your hand away and it quiets back down as the thermistor cools. Same trick as Cookie Jar Alarm — but the sensor is heat, not light.

Point it at yourself and you have a rough proxy for the "am I running hot?" check every sick-day thermometer does. Tape it to a laptop vent and you have an overheating warning. The same sensing pattern works in many places.

The big idea (again): swap the mouth, keep the brain

Look at this project and Warm Hand Alarm side by side. The sensing half — divider + 1 kΩ base resistor + transistor — is identical. What changed is the output half: the LED lane became a buzzer lane, just like Cookie Jar Alarm did to Sunrise Alarm.

That's four projects now built from the same two "brains" and two "mouths":

• Warm Hand brain (thermistor) + LED = Warm Hand Alarm
• Warm Hand brain (thermistor) + buzzer = this project
• Cookie Jar brain (LDR, bright-triggered) + buzzer = Cookie Jar Alarm
• Sunrise brain (LDR, bright-triggered) + LED = Sunrise Alarm

Once you can pull a project apart into "sensing half" and "output half", you can spot the remix. Keep that pattern in your head for the rest of Level 3 and everything after.

Do the sensing math (from Warm Hand Alarm — reused here)

Same divider, same numbers, same 0.7 V transistor wake-up line:

V_mid = 4.5 V × R_bottom ÷ (R_top + R_bottom)

The thermistor is on top, the 1 kΩ is on the bottom (small on purpose — a bigger bottom would leave V_mid already past 0.7 V at room temperature and the buzzer would sing all day). 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 wake-up line. Buzzer silent. ✓
• Warm hand (thermistor ≈ 3.6 kΩ): V_mid = 4.5 × (1 ÷ 4.6) = 0.98 V. Above the wake-up line. Base clamps to 0.7 V, base current flows, transistor saturates.
• Hot mug (thermistor ≈ 1.5 kΩ): V_mid = 4.5 × (1 ÷ 2.5) = 1.8 V. Way past the wake-up line — transistor already fully on. No more "louder" past the warm-hand state; a saturated transistor is a saturated transistor.

Do the output math (the new part)

The active buzzer, from its datasheet check in Door Buzzer, behaves like a ~150 Ω load at 4.5 V. When the transistor saturates, it drops about 0.2 V from collector to emitter (V_CE(sat)) — the buzzer sees 4.5 − 0.2 = 4.3 V. So:

• Buzzer current when alarming: I = V ÷ R = 4.3 ÷ 150 = 28 mA. That's the buzzer's normal-loud tone, same one you heard in Cookie Jar Alarm.
• Buzzer current at rest (room temperature): transistor off → buzzer sees essentially 0 V → I ≈ 0 mA. Fresh AAs last for weeks in this state.
• Total battery current when alarming: ~28 mA — a lot less than a lamp + buzzer combo would draw, and comfortably inside what a battery pack handles for hours.

Notice there's no 220 Ω on the output lane. The buzzer, like in Door Buzzer and Cookie Jar Alarm, limits its own current — no speed bump needed. You used the "know what each part needs" rule from Door Buzzer twice before. You will use it again.

Build it

If your Warm Hand Alarm is still on the breadboard, keep the sensing half exactly as it is — just pull the 220 Ω + LED out and drop a buzzer in their place. Same base, same collector, new mouth.

Fresh build:

1. 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.
2. The base link: middle row → 1 kΩ → transistor Base (the middle leg, flat face toward you).
3. The output half: battery + → buzzer + leg; buzzer − leg → transistor Collector (right leg). Emitter (left leg) → battery −.
4. Power it up in a normal room. Silent. Now cup your palm over the thermistor for five to ten seconds. Listen — the buzzer fades in as the bead warms and the base voltage crosses the 0.7 V line. Take your hand away and it fades back off over a minute as the bead cools.

The fade-in / fade-out is the interesting part. Digital sensors snap on and off instantly. Analog sensors — the ones you'll use through Level 6 — glide through their in-between states, and the transistor follows. That's why an analog alarm feels less jumpy than a digital one, and it's why old-school stove and iron thermostats used exactly this pattern.

Debug like an engineer

Same trick as Warm Hand Alarm: test the halves separately. Pull the buzzer out and clip a spare LED (with a 220 Ω!) between the transistor's collector and battery −. Cup the sensor: the LED should fade in. If it does, the sensing half is doing its job and any "still no beep" trouble lives in the output half — check the buzzer's polarity first (+ leg toward battery), then the transistor leg order (E-B-C, flat face toward you).

Where "sensing brain + output mouth" leads

Two more mouths are coming: a motor at Level 4 (thermistor + motor = a fan that spins up when the room is warm) and a servo at Level 5 (thermistor + servo = a needle that swings across a scale — a real analog thermometer). Every one of those is the sensing half you already trust, wired to a new load. Level 6 adds a chip that latches the alarm on. Level 7 replaces the sensing half with code. The same pattern still holds as the projects get harder.