Overview

The AS3935 (manufactured by ams OSRAM) is a programmable Franklin lightning sensor IC designed to detect approaching thunderstorm activity. Mounted on a breakout module equipped with a specially tuned $500\text{ kHz}$ LC antenna coil, it analyzes radio frequency (RF) electromagnetic emissions generated by electrical discharges in clouds.

Capable of estimating storm front distances from $1\text{ km}$ to $40\text{ km}$ across 14 discrete distance steps, the AS3935 distinguishes between true lightning strikes (both cloud-to-ground and intra-cloud strikes) and man-made electrical noise (such as motor commutators, fluorescent light starters, and relays). Communicating over $I^2C$ or SPI, it is deployed in personal storm warning devices, golf course weather stations, and ESPHome smart home nodes.

Quick reference

Pinout

Breakout module header:

Specifications

Operating Principle & Internal Antenna Tuning

1. RF Sensing: Thunderstorm electrical arcs produce strong VLF/LF electromagnetic bursts. The onboard $500\text{ kHz}$ LC inductor coil picks up the magnetic component of these waves.
2. Internal Antenna Tuning (Register 0x08): Manufacturing tolerances alter LC coil inductance. The AS3935 features internal switchable tuning capacitors ($0\text{ to }120\text{ pF}$ in $8\text{ pF}$ steps) to trim the antenna resonance accurately to $500\text{ kHz} \pm 3.5\%$.
3. Interrupt Handling: When an RF burst occurs, IRQ goes High. The host MCU reads Register 0x03 (Interrupt Status):
   • 0x01: Noise level too high (Man-made disturber detected).
   • 0x04: Disturber detected.
   • 0x08: Lightning strike detected! Read Register 0x07 for estimated distance in km.

Wiring

Example (SparkFun AS3935 Library)

#include <Wire.h>
#include "SparkFun_AS3935_I2C.h"

#define AS3935_ADDR 0x03
#define IRQ_PIN 2

SparkFun_AS3935_I2C lightning(AS3935_ADDR);
volatile bool lightningInt = false;

void lightningISR() {
  lightningInt = true;
}

void setup() {
  Serial.begin(115200);
  Wire.begin();

  pinMode(IRQ_PIN, INPUT);
  attachInterrupt(digitalPinToInterrupt(IRQ_PIN), lightningISR, RISING);

  if (!lightning.begin()) {
    Serial.println("AS3935 Lightning Sensor not found!");
    while (1);
  }

  // Set environment: INDOOR or OUTDOOR
  lightning.setIndoorOutdoor(INDOOR);
  
  // Tune internal antenna capacitors to 500 kHz (e.g. 96 pF)
  lightning.tuneCapacitor(0x0C); 

  Serial.println("AS3935 Lightning Detector active and listening...");
}

void loop() {
  if (lightningInt) {
    lightningInt = false;
    
    // Read interrupt source
    int intVal = lightning.readInterruptReg();

    if (intVal == LIGHTNING) {
      byte distance = lightning.distanceToStorm();
      Serial.print("LIGHTNING STRIKE DETECTED! Estimated Distance: ");
      Serial.print(distance); Serial.println(" km");
    } else if (intVal == DISTURBER) {
      Serial.println("Electrical noise / disturber detected.");
    } else if (intVal == NOISE) {
      Serial.println("Noise floor too high.");
    }
  }
}

Common mistakes

• Leaving SI or CS floating in $I^2C$ mode: Both SI and CS must be tied High to 3.3V for $I^2C$ communication.
• Placing near switching power supplies or Wi-Fi antennas: High-frequency switching regulators, Wi-Fi routers, and laptop screens generate strong $500\text{ kHz}$ magnetic noise, causing constant DISTURBER interrupts. Keep the sensor at least $1\text{ meter}$ away from switching electronics.

Notes

• AS3935 Indoor vs Outdoor Mode: Setting INDOOR increases low-noise gain; setting OUTDOOR lowers gain to prevent false triggers under open-sky noise.