Day 10: Project — Smart Night Light

Hi Anish! Your second real project. Today we combine the LDR from Day 9 with the button from Day 7 and an LED to build a Smart Night Light — one that turns itself on when the room gets dark, with a button that lets you override it.

This is the first project that’s genuinely useful. You could actually set this on a shelf in your room and it would work.

What you need today

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• Arduino Uno + USB cable
• Breadboard
• 1 LDR + 1 × 10kΩ resistor (for the voltage divider, like Day 9)
• 1 push button (INPUT_PULLUP style, like Day 7 — no extra resistor)
• 1 LED (ideally a bright white one for the night-light feel)
• 1 × 220Ω resistor (for the LED)
• Jumper wires

What we’re building

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The rules:

1. When the room is dark, the LED turns ON automatically.
2. When the room is bright, the LED stays OFF.
3. If you press the button, it toggles manual mode. In manual mode, the LED is always ON (no matter the light).
4. Press the button again → back to automatic mode.

So there are two modes: AUTO (follows the LDR) and MANUAL (always on). The button switches between them.

The circuit

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Everything from the last few days on one breadboard.

graph TB
    V5["5V"] --> LDR1["LDR"]
    LDR1 --> JUNC["junction"]
    JUNC --> A0["Pin A0"]
    JUNC --> R10k["10kΩ"] --> G1["GND"]
    PIN8["Pin 8"] --> R220["220Ω"] --> LEDp["LED +"] --> LEDn["LED −"] --> G2["GND"]
    PIN2["Pin 2
(INPUT_PULLUP)"] --> BTNa["Button"]
    BTNa -.->|pressed| BTNb["Button"] --> G3["GND"]

Summary:

• A0 — LDR with 10kΩ pull-down (Day 9 wiring)
• Pin 8 — LED with 220Ω (Day 2 wiring)
• Pin 2 — button to GND (Day 7 wiring, INPUT_PULLUP — no extra resistor)

The code

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// Modes
bool manualMode = false;   // false = AUTO, true = MANUAL (always on)

// Button state tracking
int lastButtonState = HIGH;

// Light threshold — adjust based on YOUR room
int darkThreshold = 400;

void setup() {
  pinMode(8, OUTPUT);
  pinMode(2, INPUT_PULLUP);
  Serial.begin(9600);
}

void loop() {
  // Check button for state change (press detection)
  int buttonState = digitalRead(2);
  if (lastButtonState == HIGH && buttonState == LOW) {
    manualMode = !manualMode;              // flip the mode
    Serial.print("Mode switched: ");
    Serial.println(manualMode ? "MANUAL" : "AUTO");
    delay(50);                             // debounce
  }
  lastButtonState = buttonState;

  // Decide LED state
  if (manualMode) {
    digitalWrite(8, HIGH);                 // manual — always on
  } else {
    int light = analogRead(A0);
    if (light < darkThreshold) {
      digitalWrite(8, HIGH);               // auto — dark, turn on
    } else {
      digitalWrite(8, LOW);                // auto — bright enough, off
    }
  }
}

Upload. Here’s what should happen:

• Cover the LDR with your hand → LED turns on.
• Uncover → LED turns off.
• Press the button once → LED stays on permanently, even in bright light. Serial Monitor says “Mode switched: MANUAL”.
• Press again → back to automatic. LED follows the LDR again. Serial says “Mode switched: AUTO”.

What’s new in this code?

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Almost everything here you’ve seen before, but there are a few new tricks.

bool — true or false

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bool manualMode = false;

bool (short for “boolean”) is a new kind of variable. Instead of holding a whole number like int, it holds either true or false. Just two values — like a switch.

We use bool when something has only two states. “Am I in manual mode?” Yes or no. “Is the door open?” Yes or no. “Has the button been pressed?” Yes or no.

In fact, true is basically another name for HIGH (or 1) and false is another name for LOW (or 0). But when you use bool / true / false, your code reads more like English, which is nicer.

!manualMode — the NOT operator

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manualMode = !manualMode;

The ! (exclamation mark) means NOT. It flips true to false and vice versa:

• If manualMode is false, !manualMode is true.
• If manualMode is true, !manualMode is false.

So manualMode = !manualMode; means “flip the mode to its opposite.” Perfect for toggling. This is the cleanest way to flip a bool — shorter than an if/else.

The ternary operator — condition ? A : B

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Serial.println(manualMode ? "MANUAL" : "AUTO");

This one looks weird. The ? and : together form a mini if-statement you can use inline. Read it as: “Is manualMode true? If yes, use "MANUAL". If no, use "AUTO".” It’s a shortcut for:

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if (manualMode) {
  Serial.println("MANUAL");
} else {
  Serial.println("AUTO");
}

Same thing, one line instead of five. Use it when you want to pick between two small values. Don’t use it for big multi-line logic — that’s what if/else is for.

Variables at the top of the file

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Look at how I set up the variables:

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bool manualMode = false;
int lastButtonState = HIGH;
int darkThreshold = 400;

These are all global variables (remember Day 4) — declared at the top, visible everywhere. darkThreshold = 400 is a particularly important one: it’s the magic number you’ll tweak for your room. Keeping it at the top makes it easy to find.

Good habit: always put your “tweakable numbers” (like thresholds, delays, pin numbers) at the very top of the file. Future-you can find them in 2 seconds.

Tracing the flow

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Let’s walk through one run of the loop, slowly:

1. buttonState = digitalRead(2) — check button. Let’s say it’s HIGH (not pressed).
2. Is lastButtonState == HIGH && buttonState == LOW? HIGH and HIGH — no, the second half is false. Skip the mode flip.
3. lastButtonState = buttonState; — remember current button state for next loop.
4. Is manualMode true? Let’s say no, it’s still false (auto mode).
5. Read light = analogRead(A0) — say it’s 350.
6. Is 350 < 400? Yes. digitalWrite(8, HIGH) — LED on.
7. End of loop. Start again from step 1.

Now imagine the button gets pressed. On some loop:

1. buttonState = LOW.
2. Is lastButtonState == HIGH && buttonState == LOW? Yes! (Last time it was HIGH, now it’s LOW — the moment of the press.) Enter the if block.
3. manualMode = !manualMode; — was false, now true.
4. Print “Mode switched: MANUAL”. delay(50) for debounce.
5. lastButtonState = LOW.
6. Is manualMode true? Yes. digitalWrite(8, HIGH) — LED on.
7. From now on, the code skips the LDR check entirely until the button is pressed again.

Try this

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1. Add a second LED on pin 9 as a mode indicator. When in AUTO mode, pin 9 LED is off. When in MANUAL mode, pin 9 LED is on. Now you can tell which mode you’re in just by looking.
2. Smooth dimming. Instead of digitalWrite(8, HIGH) in auto mode, use analogWrite(9, brightness) where brightness is mapped from the LDR — darker room = brighter LED. Move the LED to a ~ pin first.
3. Off mode. Add a third mode: OFF (LED always off, regardless of light). Cycle with each button press: AUTO → MANUAL → OFF → AUTO → … You’ll need an int instead of a bool, since you have 3 modes now.
4. Hysteresis. Notice how the LED flickers on and off if the light is right at the threshold? Add two thresholds: int onBelow = 350; and int offAbove = 450;. LED turns on when light drops below 350, and turns off when light rises above 450. That 100-unit gap stops the flicker.

Why today mattered

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Look at what you just built. It’s one program that:

• Reads a sensor
• Reads a button with state change detection
• Makes decisions based on modes
• Controls an LED
• Prints debug messages
• Has a user-tweakable configuration at the top

This is not “toy Arduino.” This is what real embedded software looks like. Real engineers write programs with this exact shape. Today you learned the skeleton of every interactive gadget you will ever build.

What you learned today

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• bool — a variable that holds true or false
• !x — the NOT operator, flips true/false
• Ternary x ? a : b — inline if-else expression
• How to structure an interactive sketch with modes
• Tweakable configuration at the top of the file
• How to combine inputs (LDR + button) with outputs (LED) into one coherent program
• Hysteresis (in the challenges) — using two thresholds to prevent flicker

What is next

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Day 11 — time to make some noise. You’ll wire up a buzzer, play simple tones, and code a short melody. Arduino goes from seeing to speaking.

Seriously great work, Anish. You just shipped a useful gadget.