🌅 Sunrise Alarm

What you're building

Take everything from the Automatic Night Light and swap exactly two parts — the 10 kΩ resistor and the LDR trade places in the divider. That one swap flips the whole circuit upside-down. Instead of the LED turning on when the room gets dark, it now turns on when the room gets bright. Great for a wake-up-with-the-sun light on your windowsill.

If the night light was Challenge #1 of that project, this is the answer — worked out and built.

Why swapping two parts inverts everything

The voltage divider is the same equation you already know:

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

A bigger resistance on the bottom lets the middle voltage rise. A bigger resistance on the top holds it down. Trade top and bottom and you trade "lets it rise" with "holds it down" — a total role reversal.

• Night light: 10 kΩ on top, LDR on bottom. Dark → LDR big → big bottom → middle pulled up toward 4.5 V → transistor on.
• Sunrise (this project): LDR on top, 10 kΩ on bottom. Bright → LDR small → the top hardly holds anything back → middle pulled up toward 4.5 V → transistor on.

The rule stays the same: "middle voltage rises → transistor turns on." The trigger condition flips. You can rearrange circuits to change what they detect, without changing what they do with the answer.

Do the math

Numbers make it concrete. LDR is roughly 1 kΩ in bright light and ≈ 100 kΩ in darkness (its official range on the datasheet — this matches your night-light figures).

• Bright morning: LDR ≈ 1 kΩ on top, 10 kΩ on bottom → middle tries to reach 4.5 × (10 ÷ 11) ≈ 4.1 V. Well past the 0.7 V wake-up line, so the transistor turns on hard and the LED lights.
• Night: LDR ≈ 100 kΩ on top, 10 kΩ on bottom → middle sits at 4.5 × (10 ÷ 110) ≈ 0.41 V. Below the 0.7 V line, so the transistor stays off and the LED is dark.

(An honest wrinkle for the bright case: the LDR at 1 kΩ is a low resistance for a divider, so it can shove in way more current than the base junction can drink. The base gulps what it needs at 0.7 V and the 1 kΩ base resistor mops up the rest — the middle ends up sitting well above the wake-up line, around 2.5 V in simulation, with the transistor saturated. All that matters for the LED is that we're comfortably past 0.7 V; the exact height doesn't change the answer.)

Build it

You already own this build — it's the Automatic Night Light with two parts swapped. If yours is still on the breadboard, just pull the 10 kΩ and the LDR and put each in the other's holes.

Fresh build:

1. The sensing half (INVERTED): battery + → LDR → middle row → 10 kΩ → battery −. Compare to the night light's "10 kΩ over LDR" — this is the same two parts, top and bottom traded.
2. The deciding part: transistor flat face toward you, legs are E-B-C. Middle row → 1 kΩ → Base.
3. The output half: battery + → 220 Ω → LED (long leg first) → Collector; Emitter → battery −.
4. Cover the LDR: LED off. Shine a phone flashlight on it: LED on.

The lesson the swap teaches

The two circuits share every other part — same transistor, same base resistor, same LED lane, same 4.5 V supply. Only the sensor half is different, and only inside the divider. This is how larger circuits get built. Once you know a good "decider" pattern (here, "divider → transistor → LED"), swapping the front end lets you detect a different thing: light, temperature, sound, or moisture. The back end stays exactly the same. At Level 8 you'll do the same swap with a soil moisture sensor.