Night Light

BRIEF The machine decides alone

Streetlights don't have a person flicking a switch at sunset. They feel the light fade and decide for themselves. Today you build exactly that: a light sensor, a threshold, and a lamp that comes on when your room gets dark. This is your first fully automatic machine, the first one that works while you are not even there.

Time: 45–60 minutes · Difficulty: 3/5 · You need: Lesson 04 done (analog readings and the Serial Monitor).

KIT What you need

• R3 board with USB cable
• Breadboard
• 1 photoresistor (the little disc with a squiggle on top, your kit has two)
• 1 × 10kΩ resistor (brown-black-orange stripes)
• 1 LED and its 220Ω resistor
• Jumper wires

BUILD Wire it

Unplug the USB. The photoresistor has no direction, either leg either way.

From	To	Why
R3 5V	Photoresistor leg 1	Top of the divider chain
Photoresistor leg 2	Breadboard row (the midpoint)	Where the two resistors meet
Same midpoint row	10kΩ resistor → R3 GND	Bottom of the chain
Same midpoint row	R3 pin A0	The board eavesdrops on the midpoint
R3 pin 9 (~)	220Ω resistor → LED long leg	The lamp (PWM pin, for the remix)
LED short leg	R3 GND	Loop complete

CODE Program it, in two stages

Stage 1: spy first. Never guess what a sensor sees, look.

// Stage 1: what does my room look like, in numbers?
void setup() {
  Serial.begin(9600);
}
void loop() {
  Serial.println(analogRead(A0));
  delay(200);
}

Open the Serial Monitor. Note the number in normal light. Now cup your hand over the sensor, note that number too. Write both down, really, on paper. Pick a threshold roughly in the middle.

Stage 2: the night light. Use YOUR numbers, not ours.

// GAMERIFT Beginner Kit, Lesson 06: Night Light
// Below the threshold, the lamp wakes up.

const int sensorPin = A0;
const int lampPin   = 9;
const int threshold = 400;   // <-- YOUR number from Stage 1 goes here

void setup() {
  pinMode(lampPin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int light = analogRead(sensorPin);
  Serial.println(light);

  if (light < threshold) {
    digitalWrite(lampPin, HIGH);   // dark: lamp on
  } else {
    digitalWrite(lampPin, LOW);    // bright: lamp off
  }
  delay(100);
}

1. Upload, then cup your hand over the sensor. Lamp on. Lift your hand. Lamp off.
2. Now switch the room light off tonight and watch your creation greet the dark by itself. That feeling? That is why people become engineers.

BREAK IT Now break it on purpose

1. Shine your phone torch at the sensor from across the room. How far away can it still push the lamp off? You are measuring your sensor's real-world range.
2. Set the threshold to 1000. The lamp is now on almost always, a badly calibrated sensor is worse than no sensor. Set it to 50 and it may never trigger. Feel how much the number matters.
3. Swap the photoresistor and the 10k resistor in the chain. Everything works, backwards, the reading now goes UP in darkness. Fix it in code instead of rewiring: change < to >. Sometimes software repairs hardware.

REMIX Make it yours

• The soft night light: instead of snapping on, use map() and analogWrite so the darker it gets, the brighter the lamp glows. Smooth, automatic, lovely.
• Wrong-way alarm: make it a "fridge alarm", an LED (or the buzzer, peek at Lesson 08) that panics when light appears, because someone opened the door.
• Sunrise detector: the second photoresistor in your kit is begging to be used. Two sensors, two windows, which side of the house gets the sun first?

LOG IT Show and tell

Film the moment the room light goes off and your lamp wakes up, that clip belongs in your lesson log. Stuck? Send a photo of your wiring via contact us.