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Embedded project 01 · Intermediate · about 45 minutes

Automatic night light

An LED that fades up as the room gets dark. It is a small build, but it uses almost everything in the course: a voltage divider, a sensor, the ADC, a little code, PWM and an LED with its resistor. Simulate it here first, then build it for real.

Builds on: Embedded systems, Circuit lab 02, PWM, Circuit lab 01

What you need

PartValue used hereWhy
Arduino Uno or a compatible 5 V board5 V logic, 10-bit ADCReads the sensor and runs the program. Other boards work with small changes.
Light-dependent resistor (LDR)—The light sensor. Values vary a lot between parts, which is why you calibrate.
Resistor (R1)10 kΩTop of the divider.
LEDred, about 2 VThe light.
Resistor for the LED220 ΩLimits LED current to about 14 mA at full brightness.
Breadboard, jumper wires, USB cable—Power comes from the USB port.

The circuit

Night light schematic Left: 5 V through a 10 kΩ resistor to pin A0, then through the LDR to ground. Right: pin 9 through a 220 Ω resistor and an LED to ground.
FromTo
5V pinone end of the 10 kΩ resistor
other end of 10 kΩpin A0 and one leg of the LDR
other leg of LDRGND
pin 9220 Ω resistor, then LED long leg (anode)
LED short legGND

The LDR sits at the bottom of the divider, so a darker room gives a higher reading. Pin 9 is one of the Uno’s PWM pins (marked ~).

Simulate it

Move the light level and follow the signal through every stage. The numbers use exactly the same arithmetic as the sketch below, including Arduino’s whole-number map().

Bench · light → LEDFade mode
  1. Light—you set this
  2. LDR—illustrative model
  3. Pin A0—divider output
  4. analogRead—0–1023
  5. Duty—analogWrite value
  6. LED current—average
Program

The night light

The sketch

This updates as you change the program and its levels. Copy it into the Arduino IDE once you have calibrated the levels for your room.

Build and calibrate it

  1. Build the divider only. Wire 5V → 10 kΩ → A0, and A0 → LDR → GND. Leave the LED out for now.
  2. Check it with a multimeter. Measure between A0 and GND. Cover the LDR with your hand: the voltage should rise. Shine a light on it: the voltage should fall.
  3. Read real values. Upload the calibration sketch below, open the Serial Monitor at 9600 baud (the Arduino IDE’s, or this site’s browser serial monitor), and note the reading in normal room light and with the LDR covered. 
    void setup() {
  Serial.begin(9600);
}

void loop() {
  Serial.println(analogRead(A0));
  delay(250);
}
  4. Set your levels. Put BRIGHT_LEVEL a little above your room-light reading and DARK_LEVEL a little below your covered reading. Try them in the simulator above.
  5. Add the LED. Wire pin 9 → 220 Ω → LED anode (long leg), and the LED cathode (short leg) → GND.
  6. Upload the night-light sketch and test it by slowly covering the LDR. Point the LED away from the LDR so it does not light its own sensor.

If it does not work

What you seeLikely cause
Reading stuck near 1023The LDR is not connected to GND, or R1 and the LDR are swapped and the room is very bright.
Reading stuck near 0A0 is shorted to GND, or the 5V wire to R1 is missing.
Readings jump around wildlyA0 is not connected to the divider at all, so the input is floating.
LED never lightsLED fitted backwards, wrong pin in the code, or the levels are set so the reading never passes BRIGHT_LEVEL.
LED switches on and off repeatedly at duskIts light is reaching the LDR, or (in on/off mode) the ON and OFF levels are too close together.
Safety

Power this project from USB only. Do not connect it to mains lighting. If you later want to drive an LED strip, use a separate low-voltage supply, a logic-level MOSFET and a shared ground; see driving bigger loads.

About the simulation

The LDR model is illustrative (1 MΩ dark to 1 kΩ bright on a logarithmic scale), and the ADC is ideal. Real readings depend on your LDR, so always calibrate with the Serial Monitor. The LED current uses the Circuit lab 01 model: (5 V − 2 V) ÷ 220 Ω while the PWM output is high, averaged over the duty cycle. On an Uno, pin 9’s PWM runs at about 490 Hz, far too fast to see.

Take it further

Knowledge check