HTMAA 2025 Eghosa Ohenhen

Assignment: Measure Something with a Sensor

For this week's assignment, I designed and tested an input device to measure water bottle weight using an ADXL343 accelerometer. The goal was to add a sensor to a microcontroller board I designed and successfully read its data.

Sensor Selection: ADXL343 Accelerometer

I chose the ADXL343 3-axis accelerometer because it can detect both motion and orientation, which is useful for determining when a water bottle is stable enough to take accurate weight measurements.

Measurement range: \u00b12g, \u00b14g, \u00b18g, or \u00b116g
Resolution: 10-bit or 13-bit (up to 4mg/LSB)
Interface: I2C or SPI
Power: ~140\u00b5A at 100Hz output rate

Board Design

I designed a breakout board for the ADXL343 that connects to an ESP32-S3 microcontroller via I2C:

ADXL343 accelerometer schematic — Schematic showing ADXL343 connected to ESP32 via I2C with pull-up resistors

Key Design Features

I2C communication with 4.7k\u03a9 pull-up resistors on SDA/SCL
0.1\u00b5F decoupling capacitor for stable power
Breakout pins for 3.3V, GND, SDA, SCL, INT1, INT2
Compact layout for mounting on water bottle base

Fabricated ADXL343 board — Milled and assembled ADXL343 accelerometer board

Sensing Module Part 2: Accelerometer

Programming the ADXL343

I programmed the ESP32 to read acceleration data from the ADXL343 using the Adafruit library:

#include <Adafruit_ADXL343.h>

Adafruit_ADXL343 accel = Adafruit_ADXL343(12345);

void setup() {
  Serial.begin(115200);
  
  if(!accel.begin()) {
    Serial.println(\"No ADXL343 detected!\");
    while(1);
  }
  
  // Set range to +/-4g for water bottle movements
  accel.setRange(ADXL343_RANGE_4_G);
  
  // Set data rate to 100Hz
  accel.setDataRate(ADXL343_DATARATE_100_HZ);
}

void loop() {
  sensors_event_t event;
  accel.getEvent(&event);
  
  Serial.print(\"X: \"); Serial.print(event.acceleration.x);
  Serial.print(\" Y: \"); Serial.print(event.acceleration.y);
  Serial.print(\" Z: \"); Serial.println(event.acceleration.z);
  
  delay(100);
}

Testing & Results

I tested the accelerometer by tilting and moving the sensor board in different orientations:

\u2705 Flat/stable detection: Z-axis reads ~9.8 m/s\u00b2 (1g)
\u2705 Tilt detection: X and Y axes show angle changes
\u2705 Motion detection: Rapid changes indicate movement
\u2705 Noise level: ~\u00b10.1 m/s\u00b2 when stationary

\u2728 Use Case: Stable Measurement Detection

\n For the hydration monitor, the accelerometer determines when the water bottle is:\n

Upright: Z-axis dominant, X/Y near zero
Stationary: Low variance in readings over 1 second
Ready for measurement: Both conditions met for 2+ seconds

\n Only when these conditions are satisfied does the system take a valid weight reading from the load cell, preventing false readings from movement or tilted positions.\n

Group Assignment: Probing Analog & Digital Signals

\n For the group assignment, we used an oscilloscope to probe the I2C signals between the ESP32 and ADXL343:\n

SCL (clock): 100kHz square wave during data transmission
SDA (data): Digital pulses representing I2C address and data bytes
Voltage levels: Confirmed 3.3V logic levels with clean transitions
Pull-up function: Verified 4.7k\u03a9 resistors create proper idle high state

\n We also measured the analog voltage on the ADXL343's power pin to ensure stable 3.3V supply during operation.\n

Connection to Final Project

\n While the accelerometer was designed for this week's assignment, I ultimately used a load cell with HX711 amplifier for the final project instead. The load cell provides more direct weight measurement without needing complex tilt compensation.\n

\n However, the skills from this week\u2014I2C communication, sensor calibration, and signal processing\u2014directly transferred to working with the HX711 and other sensors in my hydration monitoring system.\n

ADXL343 Datasheet

Week 8

Input Devices