Day 9: Light Sensor — Arduino Learns to See

Hi Anish! Yesterday we read a knob. Today we read light. A tiny component called an LDR acts like a light-powered knob — when the room is bright, it sends one value to Arduino; when the room is dark, it sends another. Arduino is about to “see.”

What you need today

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• Arduino Uno + USB cable
• Breadboard
• 1 LDR (photoresistor — looks like a round disc with a squiggly line on top)
• 1 × 10kΩ resistor (not the 220Ω we’ve been using for LEDs!)
• 1 LED
• 1 220Ω resistor (for the LED)
• Jumper wires

What is an LDR?

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LDR stands for Light-Dependent Resistor — also called a photoresistor. Its resistance changes based on how much light hits it:

• Bright light → low resistance (lets lots of current through)
• Dark → high resistance (blocks current)

By itself, an LDR doesn’t give you a voltage. To turn it into a voltage Arduino can read, we build a voltage divider — a fancy name for two resistors in a row. The LDR is one resistor. A fixed 10kΩ resistor is the other.

Don’t panic about the theory. The practical rule is: LDR on top, 10kΩ on the bottom, read the voltage from the middle.

The circuit

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graph TB
    V5["5V"] --> LDR1["LDR leg 1"]
    LDR1 --> MID["Junction
(middle point)"]
    MID --> A0["Pin A0"]
    MID --> R10k["10kΩ resistor"]
    R10k --> G1["GND"]
    PIN8["Pin 8"] --> R220["220Ω"] --> LEDp["LED +"] --> LEDn["LED −"] --> G2["GND"]

Wiring the LDR (as a voltage divider):

1. Plug the LDR into the breadboard — doesn’t matter which leg goes where (LDRs aren’t polarized).
2. Jumper from 5V to one leg of the LDR.
3. The other leg of the LDR goes into an empty column — let’s call it “the junction.”
4. In that same junction column, plug in the 10kΩ resistor, with its other end going to GND.
5. Jumper from the junction column to A0.

Now A0 is reading the voltage in the middle of the divider. In bright light, the LDR lets voltage through, so A0 reads a high number. In darkness, the LDR blocks it and A0 reads a low number. (Or it could be the other way around, depending on your LDR — we’ll check in code.)

Wiring the LED: pin 8 → 220Ω → LED → GND. Same as always.

Part 1: Read the light level

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Let’s start by just printing the LDR value and watching it change.

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void setup() {
  Serial.begin(9600);
}

void loop() {
  int light = analogRead(A0);
  Serial.println(light);
  delay(200);
}

Open the Serial Monitor. You should see numbers scrolling by. Wave your hand over the LDR — the number changes. Cover it with a finger to block the light — number drops (or rises, depending on your LDR wiring). Shine a flashlight on it — jumps the other way.

Write down roughly what value you see in a normal room, and what value you see when you cover the LDR. You’ll need those numbers for the next step.

For example, you might see:

• Normal room: around 600
• Covered (dark): around 200
• Flashlight: around 950

Your numbers will be different — every LDR and every room is unique. That’s why we measure first, then decide.

Part 2: Turn on the LED when it’s dark

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Now we use the reading to make a decision.

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void setup() {
  pinMode(8, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  int light = analogRead(A0);
  Serial.println(light);

  if (light < 400) {
    // It's dark — turn on the LED
    digitalWrite(8, HIGH);
  } else {
    // It's bright enough — LED off
    digitalWrite(8, LOW);
  }

  delay(100);
}

Upload. Cover the LDR with your hand or a book — the LED turns on! Uncover — LED off. Automatic light.

The threshold number

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The magic number here is 400 — the threshold. If the light value is below 400, we call it “dark.” Above, we call it “bright.” You need to pick this number based on your measurements. Too high and the LED will be on all the time. Too low and it will never turn on.

Use the Serial Monitor readings from Part 1 to pick something between your “normal” and “covered” values. If normal is 600 and covered is 200, try 400 or 450. Adjust until it feels right.

My LDR is backward!

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Some LDRs wired some ways read low in bright light and high in darkness — the opposite of what I described. If yours behaves like that, just flip the comparison: if (light > 600) instead of if (light < 400). Same idea, flipped.

The Serial Monitor is your friend — always print the raw value first so you know which way your sensor works.

Nothing new about the code, really

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Look at this code and notice what’s there:

• analogRead(A0) — Day 8
• Serial.println(...) — Day 8
• if/else — Day 3
• digitalWrite — Day 1

No new functions today. We’re just combining what you already know — reading an analog sensor (Day 8 idea) and making a decision from it (Day 3 idea). That’s how most Arduino projects actually work: you stack familiar tools in new ways.

The one new concept is the sensor itself — and the skill of measuring first, then writing the threshold.

Try this

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1. Print with a label. Change the Serial output to Serial.print("Light: "); Serial.println(light); so it’s more readable.

2. Three zones. Print "Dark", "Normal", or "Very bright" based on the reading. Use if / else if / else:

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   if (light < 300) { Serial.println("Dark"); } else if (light < 700) { Serial.println("Normal"); } else { Serial.println("Very bright"); }

3. Smooth LED brightness. Use map() (from Day 8) to make the LED brighter when the room is darker. Wire the LED to a ~ pin (like 9). Hint: int b = map(light, 0, 1023, 255, 0); analogWrite(9, b);

4. Two-LED alarm. Red LED (pin 8) on when dark. Green LED (pin 9) on when bright. Both off in the normal range.

Meet else if

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Challenge 2 above introduced a new thing: else if. It lets you check multiple conditions in order:

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if (condition1) {
  // runs if condition1 is true
} else if (condition2) {
  // runs if condition1 was false AND condition2 is true
} else if (condition3) {
  // runs if both above were false AND condition3 is true
} else {
  // runs if none of the above were true
}

Think of it like a waterfall of questions. Arduino tries each one from top to bottom and stops at the first one that’s true. Useful for “categories” — like mapping a number into dark/normal/bright zones.

You can have as many else if as you want.

What you learned today

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• What an LDR (photoresistor) is and how it responds to light
• Why sensors like the LDR need a voltage divider to work with Arduino
• How to measure first (with Serial.println) and then pick a threshold
• else if — chain multiple conditions together
• That today’s code had no new functions — just old ideas in a new shape

What is next

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Day 10 — combine everything from this week into a Smart Night Light. The LED turns on automatically when the room gets dark, with a button to override it manually. Your first project that’s genuinely useful.

Great work, Anish. Your Arduino can now see.