🌙 Automatic Night Light

What you're building

A light with no switch at all. Cover the sensor (or wait for sunset) and the LED fades on by itself. This is your first circuit that decides something on its own — and it does it with two tricks that run the entire world of sensors.

Trick 1: the voltage divider

Stack two resistors between + and −, and the point between them sits at a voltage decided by the ratio of the two:

middle voltage = 4.5 V × (bottom ÷ (top + bottom))

Now the sneaky part: your LDR is a resistor that changes with light — about 1 kΩ in bright light, over 100 kΩ in darkness. Put the fixed 10 kΩ on top and the LDR on the bottom, and the middle point moves:

• Bright room: LDR ≈ 1 kΩ → middle ≈ 4.5 × (1 ÷ 11) ≈ 0.4 V
• Dark room: LDR ≈ 100 kΩ → middle tries to rise toward 4.5 × (100 ÷ 110) ≈ 4.1 V

Light has become a voltage. Nearly every sensor you'll ever meet — game controller sticks, thermostat probes — is some version of this trick.

(One honest detail: once the transistor is connected, its base won't let the middle point actually reach 4.1 V — past about 0.7 V the base clamps it there and drinks the extra push as current instead. That's fine: the divider's whole job is just to shove the middle point past that 0.7 V line when it gets dark.)

Trick 2: the transistor

The divider's middle point can't power an LED by itself (the 10 kΩ chokes the current — you proved how little gets through 10 kΩ in the Brightness Lab). It needs a helper: the transistor, an electrically-flipped switch. Feed its base leg a small current and it lets a much larger current pour across from collector to emitter. Roughly 0.7 V at the base is the "wake-up" threshold: our 0.4 V bright-room voltage stays below it (LED off), our 4.1 V dark voltage sails past it (LED on).

Build it

1. The sensing half: battery + → 10 kΩ → middle row → LDR → battery −.
2. The deciding part: transistor flat face toward you; legs left to right are Emitter, Base, Collector. Middle row → 1 kΩ → Base. (The 1 kΩ keeps the base current small and polite — the transistor only needs a trickle.)
3. The output half: battery + → 220 Ω → LED (long leg first) → Collector. Then Emitter → battery −.
4. Lights on in the room: LED off. Cup your hands over the LDR: LED on.

Notice it doesn't snap — it fades as your hand closes over the sensor. Between full-off and full-on the transistor passes through an in-between region. (Level 6's chips will turn this mush into a crisp snap.)

Debug like an engineer

Not working? Don't rewire everything — test the halves separately. Wire a spare LED lane (220 Ω included!) from the divider's middle row to battery −: does covering the LDR change its glow? If yes, the sensing half works and your bug lives in the transistor half — check E-B-C order first; backwards transistors are the #1 classic. Splitting a problem in half is the fastest debugging tool ever invented.