Video
Circuit Diagram
Source Code
1// ─────────────────────────────────────────
2// BlueGrays · bluegrays.com
3// Measuring Light Intensity with an LDR
4// ─────────────────────────────────────────
5// Reads the ambient light on A0 through a voltage divider made of an LDR
6// and a 10k resistor, and prints the 0-1023 value to the Serial Monitor
7// every 0.5 s. The value rises as the light gets brighter.
8//
9// Pins:
10// A0 -> Junction of the LDR and the 10k resistor
11// LDR other leg to 5V, 10k other leg to GND
12
13int value; // Light reading (0-1023)
14
15void setup() {
16 Serial.begin(9600); // Serial at 9600 baud
17}
18
19void loop() {
20 value = analogRead(A0); // Read the LDR voltage
21 Serial.print("Value: ");
22 Serial.println(value);
23 delay(500); // Wait 0.5 s
24}
25FAQ
How does LDR work?▾
As the amount of light falling on the LDR (light-dependent resistor) increases, its resistance decreases. When a voltage divider is created using a 10 kΩ resistor, changes in light are converted into changes in voltage and can be read as a value between 0 and 1023 using `analogRead`.
What should the readings be in light and dark conditions?▾
It depends on the connection type; generally, high values are observed in bright conditions and low values in dark conditions (or vice versa). Note the bright and dark values in its natural environment using the Serial Monitor; these will be used to select threshold values in future projects.
I'm seeing garbled characters in the Serial Monitor. Why is that?▾
The baud rate in the Serial Monitor must match the value specified in the `Serial.begin()` function in the code (e.g., 9600). If they differ, garbled characters will appear on the screen.
Is there a more sensitive light sensor than an LDR?▾
Yes, I2C light sensors like the BH1750 measure light directly in lux (lx) and are much more consistent than LDRs. An LDR is sufficient for simple night lights and darkness detection.