ADXL345 3-Axis Accelerometer Tutorial

1. Introduction

The ADXL345 is a small, thin, ultra-low power 3-axis accelerometer made by Analog Devices. It measures static acceleration (like gravity for tilt sensing) and dynamic acceleration (from motion, shock, or vibration).

What makes it special is its built-in motion detection features: single tap, double tap, free-fall detection, and activity/inactivity monitoring—all handled by the sensor itself!

📦 Key Features

High resolution: 13-bit, up to ±16g range
Ultra-low power: 23µA in measurement mode
Built-in tap and free-fall detection
Dual I2C and SPI interface

2. Technical Specifications

Parameter	Specification
Operating Voltage	2.0V – 3.6V (module: 3.3V – 5V)
Interface	I2C (up to 400kHz) / SPI (up to 5MHz)
Measurement Range	±2g, ±4g, ±8g, ±16g (selectable)
Resolution	13-bit (full resolution mode)
I2C Address	0x53 (SDO/ALT low) or 0x1D (SDO/ALT high)
Output Data Rate	0.1Hz to 3200Hz
Current (Measurement)	23 µA typical

💡 ADXL345 vs MPU6050

ADXL345 is accelerometer-only (no gyroscope) but has better built-in event detection (tap, free-fall). MPU6050 adds a gyroscope but lacks these hardware features. Choose based on your needs!

3. Wiring Guide

ADXL345 Pin	Description	ESP32	Arduino Uno
VCC	Power (3.3V-5V)	3.3V	5V
GND	Ground	GND	GND
SCL	I2C Clock	GPIO 22	A5
SDA	I2C Data	GPIO 21	A4
CS	SPI Chip Select	VCC (for I2C)	VCC (for I2C)
SDO	Address Select	GND (for 0x53)	GND (for 0x53)

Figure 1: ADXL345 I2C wiring with ESP32

⚠️ Important for I2C Mode

Connect the CS pin to VCC to enable I2C mode. Leaving it floating may cause communication issues.

4. Project: Acceleration & Tilt Monitor

Required Library

Install Adafruit ADXL345 and Adafruit Unified Sensor via Arduino Library Manager.

Basic Acceleration Reading

#include <Wire.h>
#include <Adafruit_Sensor.h>
#include <Adafruit_ADXL345_U.h>

Adafruit_ADXL345_Unified accel = Adafruit_ADXL345_Unified(12345);

void setup() {
    Serial.begin(115200);
    Serial.println("ADXL345 Accelerometer");

    if (!accel.begin()) {
        Serial.println("ERROR: ADXL345 not found!");
        while (1);
    }

    // Set measurement range
    accel.setRange(ADXL345_RANGE_16_G);

    Serial.println("ADXL345 Ready!");
}

void loop() {
    sensors_event_t event;
    accel.getEvent(&event);

    Serial.print("X: "); Serial.print(event.acceleration.x, 2);
    Serial.print(" Y: "); Serial.print(event.acceleration.y, 2);
    Serial.print(" Z: "); Serial.print(event.acceleration.z, 2);
    Serial.println(" m/s^2");

    delay(100);
}

Tilt Angle Calculation

void loop() {
    sensors_event_t event;
    accel.getEvent(&event);

    // Calculate tilt angles
    float pitch = atan2(event.acceleration.y, event.acceleration.z) * 180.0 / PI;
    float roll = atan2(-event.acceleration.x, 
                  sqrt(event.acceleration.y * event.acceleration.y + 
                       event.acceleration.z * event.acceleration.z)) * 180.0 / PI;

    Serial.print("Pitch: "); Serial.print(pitch, 1);
    Serial.print("° | Roll: "); Serial.print(roll, 1);
    Serial.println("°");

    delay(100);
}

5. Built-in Event Detection

The ADXL345 can detect these events in hardware:

Feature	Description
Single Tap	Detects a quick tap on any axis
Double Tap	Detects two taps in quick succession
Free-Fall	Detects when acceleration on all axes is near zero
Activity	Detects movement above a threshold
Inactivity	Detects lack of movement for a set time

6. Applications

Tilt sensing: Level indicators, inclinometers
Vibration monitoring: Machine health, earthquake detection
Drop detection: Package handling, device protection
Gesture control: Shake-to-activate features
Pedometers: Step counting in wearables

7. Troubleshooting

Sensor not detected

Check CS pin is tied to VCC for I2C mode
Verify SDO pin for correct address (GND=0x53, VCC=0x1D)

Z-axis always reads ~9.8

This is correct! Gravity causes ~1g on Z when horizontal

Readings are offset

Use calibration registers or apply software offset correction

8. Summary

3-axis accelerometer with ±2g to ±16g range
Ultra-low power (23µA)
Built-in tap, double-tap, free-fall detection
I2C address: 0x53 (default) or 0x1D