How to Use a Simple Arduino Light Sensor to Measure Sky Darkness Levels Over Time

Measuring how dark the night sky gets is a surprisingly fun DIY project that can help amateur astronomers, photographers, and anyone curious about light pollution. With just an Arduino, a cheap light‑dependent resistor (LDR), and a few accessories you can log sky brightness continuously, visualize trends, and even compare different locations.

What You'll Need

Tip: If you want higher accuracy you can swap the LDR for a photodiode (e.g., TEMT6000) or a dedicated sky‑brightness sensor like the TSL2591, but the LDR works perfectly for a "quick and dirty" approach.

Wiring the Circuit

1. Create a voltage divider:

   • Connect one leg of the LDR to 5 V.
   • Connect the other leg of the LDR to A0 (analog input).
   • From the same node (LDR--A0 junction) attach the 10 kΩ resistor to GND.

   This forms a classic voltage divider where the voltage at A0 rises as the sky gets brighter and falls as it gets darker.

2. Optional SD card module:

   • Connect CS → pin 10, SCK → pin 13, MOSI → pin 11, MISO → pin 12, VCC → 5 V, GND → GND.

3. Power everything : Plug the Arduino into your power source and verify that the LED on the board lights up.

Visual aid:

5V ── LDR ── A0 (https://www.amazon.com/s?k=Arduino&tag=organizationtip101-20)
               │
               └─ 10kΩ ── GND

Calibrating the Sensor

The raw ADC reading (0‑1023) does not directly correspond to a magnitude scale, but you can calibrate it with a simple two‑point method:

1. Open the Serial Monitor and note the values for both conditions.
2. Map these extremes to a sky brightness index (e.g., 0 = complete dark, 1 = bright twilight).

Use the map() function in Arduino to convert live readings to the index.

Arduino Sketch

/*  Sky Darkness Logger -- https://www.amazon.com/s?k=Arduino&tag=organizationtip101-20 + LDR
 *  https://www.amazon.com/s?k=records&tag=organizationtip101-20 https://www.amazon.com/s?k=ambient+light&tag=organizationtip101-20 every minute and (optionally) writes to an https://www.amazon.com/s?k=SD+card&tag=organizationtip101-20.
 */

#include <SPI.h>
#include <SD.h>

const int LDR_PIN    = A0;          // Analog input from https://www.amazon.com/s?k=voltage&tag=organizationtip101-20 https://www.amazon.com/s?k=divider&tag=organizationtip101-20
const int SD_CS_PIN  = 10;          // https://www.amazon.com/s?k=chip&tag=organizationtip101-20‑select for SD module
const unsigned long LOG_INTERVAL = 60000UL; // 1 minute in ms

File dataFile;
unsigned long lastLog = 0;

void setup() {
  Serial.begin(115200);
  pinMode(LDR_PIN, INPUT);

  // Initialize https://www.amazon.com/s?k=SD+card&tag=organizationtip101-20 (if present)
  if (SD.begin(SD_CS_PIN)) {
    Serial.println("https://www.amazon.com/s?k=SD+card&tag=organizationtip101-20 ready.");
    dataFile = SD.open("skylog.https://www.amazon.com/s?k=CSV&tag=organizationtip101-20", FILE_WRITE);
    if (dataFile) {
      dataFile.println("timestamp,adc,https://www.amazon.com/s?k=Brightness&tag=organizationtip101-20");
      dataFile.flush();
    }
  } else {
    Serial.println("SD init failed -- logging to Serial only.");
  }
}

void loop() {
  unsigned long now = millis();

  // Take a reading every LOG_INTERVAL
  if (now - lastLog >= LOG_INTERVAL) {
    lastLog = now;

    int https://www.amazon.com/s?k=RAW&tag=organizationtip101-20 = analogRead(LDR_PIN);          // 0‑1023
    https://www.amazon.com/s?k=Float&tag=organizationtip101-20 https://www.amazon.com/s?k=Brightness&tag=organizationtip101-20 = map(https://www.amazon.com/s?k=RAW&tag=organizationtip101-20, 0, 1023, 0, 100); // 0‑100% dark‑to‑bright
    https://www.amazon.com/s?k=Float&tag=organizationtip101-20 darkLevel   = 100.0 - https://www.amazon.com/s?k=Brightness&tag=organizationtip101-20; // 0 = bright, 100 = dark

    // Timestamp in seconds since start (or you can add RTC for real date‑time)
    https://www.amazon.com/s?k=Float&tag=organizationtip101-20 t = now / 1000.0;

    // Send to Serial Plotter
    Serial.print(t);
    Serial.print(",");
    Serial.print(https://www.amazon.com/s?k=RAW&tag=organizationtip101-20);
    Serial.print(",");
    Serial.println(darkLevel);

    // Log to SD if available
    if (dataFile) {
      dataFile.print(t, 2);
      dataFile.print(',');
      dataFile.print(https://www.amazon.com/s?k=RAW&tag=organizationtip101-20);
      dataFile.print(',');
      dataFile.println(darkLevel, 1);
      dataFile.flush();
    }
  }
}

Key points of the sketch

• analogRead() fetches the voltage at A0 every minute.
• map() converts the raw 0‑1023 range into a 0‑100 % darkness scale (customizable).
• Serial Plotter shows a live graph of darkness vs. time.
• SD logging creates skylog.csv, which you can open later in Excel or Python for deeper analysis.

If you have an RTC (real‑time clock) module, replace the millis()‑based timestamp with a proper date‑time stamp for easier cross‑site comparison.

Deploying the Sensor Outdoors

1. Enclosure -- Place the breadboard and Arduino inside a weather‑proof box. Drill a small, clear polycarbonate window facing upward.
2. Mounting -- Secure the box on a pole, roof, or tripod, ensuring the sensor isn't shaded by nearby objects (trees, walls).
3. Power -- For long‑term operation, use a 9 V battery with a DC‑DC step‑down to 5 V, or connect a small solar panel + charge controller.
4. Data retrieval -- If you don't have an SD card, you can wire the Arduino to a Wi‑Fi module (ESP8266/ESP32) and push data to a cloud endpoint, but that adds complexity beyond the "simple" scope.

Analyzing the Data

Once you have a CSV file, you can explore:

• Daily cycles -- Plot darkness vs. hour to see sunset/sunrise curves.
• Seasonal trends -- Compare winter nights (longer, darker) to summer evenings.
• Light‑pollution spikes -- Identify sudden brightening when nearby lights turn on/off.

A quick Python snippet for visualizing a month of data:

import https://www.amazon.com/s?k=Pandas&tag=organizationtip101-20 as pd
import https://www.amazon.com/s?k=Matplotlib&tag=organizationtip101-20.pyplot as plt

df = pd.read_csv('skylog.https://www.amazon.com/s?k=CSV&tag=organizationtip101-20')
df['datetime'] = pd.to_timedelta(df['timestamp'], unit='s')
df.set_index('datetime', inplace=True)

plt.figure(figsize=(12,4))
plt.plot(df['darkness'], https://www.amazon.com/s?k=Label&tag=organizationtip101-20='Sky Darkness (%)')
plt.xlabel('Time')
plt.ylabel('Darkness %')
plt.title('Sky Darkness Over Time')
plt.legend()
plt.show()

Tips & Tricks

Wrapping Up

Building a simple Arduino light sensor for sky darkness measurement is an excellent way to get hands‑on with electronics, data logging, and environmental monitoring. With minimal hardware you can:

• Track night‑time light levels automatically.
• Build a personal database of sky conditions for astrophotography planning.
• Contribute data to community light‑pollution maps.

Whether you're a hobbyist astronomer or just love tinkering, the project scales easily---from a single backyard sensor to a network of stations covering an entire town. Happy measuring, and may your nights stay dark!