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the ladder · rung 3 of 10

Level 3 · Sense

Circuits that react to the world around them.

Up to now you've flipped switches. Here, parts sense the world: a light sensor drops its resistance when the sun sets, a transistor amplifies that tiny change into a bright LED, and a night light turns itself on.

New in the box

with everything through L3, your box now holds 17 component types.

Ideas this level teaches

  • Voltage dividers — Two resistors split a voltage — the trick behind almost every sensor.
  • Transistor switching — A tiny current at the base opens a gate for a much bigger one.

Projects at Level 3

Automatic Night Light A light that switches itself on when the room gets dark — no hands, no code. 35 min · ●●●○○ Sunrise Alarm The night light — flipped. Swap two parts and the LED wakes up with the sun instead. 25 min · ●●●○○ Cookie Jar Alarm The Sunrise Alarm's brain wired to a buzzer instead of a lamp — quiet in the dark jar, screaming when the lid comes off. 25 min · ●●●○○ Trip Wire Alarm A thin wire across a doorway holds the alarm silent. Break the wire and the buzzer screams — until you connect it back. 25 min · ●●●○○ Warm Hand Alarm Cup your hand over the sensor and the LED lights up — a thermometer that turns into a switch. 30 min · ●●●○○ Reading Light The Automatic Night Light with a knob on top — turn it to dial in how dark the room has to get before the lamp fires. Your first tunable threshold. 30 min · ●●●○○ Vacation Nightlight The Automatic Night Light with a manual override — flip the slide switch off and the lamp stays dark even in a pitch-black room. 25 min · ●●●○○ Study Lamp The Automatic Night Light with a force-on override — flip the slide switch and the lamp stays lit even in daylight, ignoring the sensor. 25 min · ●●●○○ Curfew Beeper The Automatic Night Light's brain with a buzzer where the LED used to sit — beeps the moment the room goes dark, so you never miss sunset. 30 min · ●●●○○ Fever Beeper The Warm Hand Alarm's brain wired to a buzzer instead of a lamp — silent in a normal room, beeping the second a warm palm covers the sensor. 25 min · ●●●○○ Dusk Chorus The Curfew Beeper with a red LED wired in parallel with the buzzer — one dusk sensor drives a chime and a lamp at the same time. 30 min · ●●●○○ Warm Hand Chorus Fever Beeper with a red LED added in parallel with the buzzer — one warm-hand sensor drives a chime and a lamp at the same time. 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. 30 min · ●●●○○ Trip-Wire Chorus Trip Wire Alarm with a red LED added in parallel with the buzzer — one thin wire holds the whole chorus silent; snap it and both mouths fire. 30 min · ●●●○○ Night Doorbell A doorbell that only works at night. The ring button fires buzzer and LED together — but only once the LDR sees darkness. In daylight the button does nothing. 30 min · ●●●○○ Alley Lamp Automatic Night Light with a push-button override. Dark room OR button press fires the lamp — a wired-OR built straight onto the transistor's base. 30 min · ●●●○○ The Upside-Down Transistor A PNP transistor switches the + side of a lamp and turns on when you pull its base DOWN. Same button, same LED, whole circuit flipped. 30 min · ●●●○○

What you can build so far

Every project at Levels 1–3 draws on the components you now own — earlier projects never stop being useful. With the full L3 box you can tackle any of these 60 projects:

🔦 First Light · 📡 Morse Code Messenger · 🎈 Twin Reds · 🌿 Two Greens · 🌙 Night Pin · 🚦 Tabletop Traffic Light · 🚲 Bike Torch · 🚦 Signal Pair · 🕯️ Dim Candle · 🔴 Three Sisters · 🥀 One for All · 📶 Step Up · 🪜 Longer Loop · 🔦 Bright and Dim · 🌈 Rainbow Loop · 🔆 Reserve Beacon · 🚫 Short Cut · 🔐 The Vault · 🚪 Do Not Disturb Sign · 🤝 Handhold Chain · 🚪 Two-Door Signal · 🚨 Signal Light · 🛟 Escape Hatch · 🧪 The Brightness Lab · 🔔 Door Buzzer · 📣 Pocket Beeper · 🎛️ Dial-a-Brightness · 📻 Dial-a-Buzz · 🔧 Continuity Tester · 🚨 Panic Doorbell · 🔈 Whisper Alarm · 🛎️ Two-Tone Doorbell · 🚨 Arm & Ring · 🔔 Twin Bells · 🖐️ Two-Hand Bell · 🔕 Silent Alarm · 🎮 Quiz Buzzer · 🔊 Loud/Polite Doorbell · 🎵 Duet Doorbell · 🔐 Vault Chime · 🤫 Hidden Mute · 🟢 Power Pilot · 🎚️ Shared Knob · 🌙 Automatic Night Light · 🌅 Sunrise Alarm · 🍪 Cookie Jar Alarm · 🪤 Trip Wire Alarm · 🌡️ Warm Hand Alarm · 📖 Reading Light · 🏝️ Vacation Nightlight · 📚 Study Lamp · 🌆 Curfew Beeper · 🤒 Fever Beeper · 🌇 Dusk Chorus · 🎺 Warm Hand Chorus · 🌅 Dawn Chorus · 🚨 Trip-Wire Chorus · 🌙 Night Doorbell · 🚨 Alley Lamp · 🙃 The Upside-Down Transistor