Automatic Night Light

The finish line

What you will build

The LED turns on below a measured light threshold, stays off in bright conditions, and avoids rapid flicker near the boundary.

Before you build

Materials, tools, and safety

Reuse-material cost: $0-$3 with reused materials. Supervision: Adult guidance recommended for wiring and cutting.

Build it

Step-by-step instructions

1. Step 1

   Build the divider

   Connect photoresistor to 5 V, fixed resistor to ground, and midpoint to A0.

   Keep leads apart.

2. Step 2

   Add the indicator

   Connect pin 9 through 220 Ω to LED anode and cathode to ground.

   Verify polarity.

3. Step 3

   Read raw values

   Print A0 in bright room, under a paper shade, and in a dark box.

   Record ranges.

   Builder checkpoint: After read raw values, the first subassembly should stay aligned when handled gently.

4. Step 4

   Choose thresholds

   Set an on threshold between dark and bright readings and an off threshold farther toward bright.

   Keep on and off values distinct.

   Watch for: If this stage binds or drifts, inspect threshold chatter before adding more parts.

5. Step 5

   Program the state

   If dark crosses on threshold, light LED; if bright crosses off threshold, turn it off.

   Retain state between thresholds.

6. Step 6

   Shield the sensor

   Add a short paper hood so the LED does not shine directly on its own sensor.

   Keep the opening visible.

   Builder checkpoint: After shield the sensor, operate the build slowly and confirm that controlled LED illumination begins without binding.

7. Step 7

   Run transition trials

   Cover and uncover gradually ten times.

   Record flicker and switching values.

8. Step 8

   Tune placement

   Move the sensor or adjust thresholds for the intended room.

   Document the final readings.

   Builder checkpoint: At the final checkpoint, The LED turns on below a measured light threshold, stays off in bright conditions, and avoids rapid flicker near the boundary.

See the engineering

Why it works

Input

light-dependent sensor resistance

Output

controlled LED illumination

Motion

light level-to-electrical switching

Energy losses

ambient reflections, sensor tolerance, battery drop, threshold chatter

const int sensorPin=A0, ledPin=9;
const int darkOn=360, brightOff=440; bool lightOn=false;
void setup(){ pinMode(ledPin,OUTPUT); Serial.begin(9600); }
void loop(){
  long total=0; for(int i=0;i<10;i++){ total+=analogRead(sensorPin); delay(3); }
  int reading=total/10;
  if(!lightOn && reading<darkOn) lightOn=true;
  else if(lightOn && reading>brightOff) lightOn=false;
  analogWrite(ledPin,lightOn?180:0);
  Serial.println(reading); delay(40);
}

Make it behave

Test, troubleshoot, and tune

Keep experimenting

Try another version

Build together

Classroom and access options

Reflect on the design

1. How did threshold gap change the measured result?
2. Where did ambient reflections affect the build most strongly?
3. What evidence shows that light threshold control explains the motion?
4. Which change would improve controlled LED illumination without creating a new problem?