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Measurement Node

Build and configure a power measurement node using ESP32/ESP8266 and INA219.

Overview

A measurement node is a hardware device that reads voltage, current, and power from a DC circuit and sends measurements to BuckPow over WiFi.

The supported configuration uses:

  • MCU: ESP32 or ESP8266
  • Sensor: INA219 current/power sensor
  • Communication: WiFi + HTTP POST

Supported Hardware

Component Recommended Alternative
MCU ESP32 DevKit ESP8266 (NodeMCU, Wemos D1)
Sensor INA219 breakout board —
Display SSD1306 128x32 OLED (optional) —
Power Supply USB or regulated 3.3V–5V —

INA219 Sensor

The INA219 is a I2C current/power monitor that measures:

Measurement Range Resolution
Bus Voltage 0–26V 4 mV
Shunt Voltage ±320 mV 10 μV
Current ±3.2A (with 0.1Ω shunt) 0.1 mA
Power Calculated from voltage × current —

Calibration

BuckPow firmware uses the setCalibration_32V_2A() preset:

  • Bus voltage range: 0–26V
  • Current range: 0–3.2A
  • Shunt resistor: 0.1Ω

Wiring

ESP32 + INA219

INA219          ESP32
────────        ────────
VCC       →     3.3V
GND       →     GND
SCL       →     GPIO 22
SDA       →     GPIO 21

ESP8266 + INA219

INA219          ESP8266
────────        ───────
VCC       →     3.3V
GND       →     GND
SCL       →     D1 (GPIO 5)
SDA       →     D2 (GPIO 4)

Shared I2C Bus (with OLED)

Both INA219 and SSD1306 OLED share the same I2C bus:

Device I2C Address
INA219 0x40 (default)
SSD1306 OLED 0x3C (default)

Connect both devices to the same SDA/SCL pins. No additional wiring needed.

Firmware Variants

Variant Display Interval Features
buckpow_ina219 None (Serial) 1 second Basic measurement node
buckpow_ina219_oled SSD1306 128x32 5 seconds Local display + energy accumulation

Base Variant (buckpow_ina219)

The simplest configuration. Reads measurements and sends them to the API. Debug output via Serial Monitor.

OLED Variant (buckpow_ina219_oled)

Adds a local display showing:

V :   5.12 V
I : 241.0 mA
P : 1234.0 mW
E :   0.34 Wh 00:15
  • Voltage: Bus voltage in volts
  • Current: Current draw in milliamps
  • Power: Power consumption in milliwatts or watts (auto-scaled)
  • Energy: Cumulative energy in milliwatt-hours or watt-hours (auto-scaled) + uptime

Energy is accumulated locally: energyWh += power_mW × interval_ms / 3600000000

Measurement Range

Parameter Unit Typical Range
Bus Voltage V 0–26V
Shunt Voltage mV 0–320 mV
Current mA 0–3200 mA
Power mW 0–80000 mW

Low-power devices

For devices drawing less than 10 mA, consider using a lower calibration or a different shunt resistor for better accuracy.

Data Format

Each measurement is sent as a JSON POST request:

{
  "device_id": "esp32-ina219-01",
  "firmware_version": "1.2.0",
  "bus_voltage": 5.12,
  "shunt_voltage": 82,
  "current": 241,
  "power": 1234
}
Field Type Unit Description
device_id string — Unique device identifier
firmware_version string — Firmware version string
bus_voltage float V Bus voltage from INA219
shunt_voltage float mV Shunt voltage from INA219
current float mA Current draw from INA219
power float mW Power consumption from INA219

Server-side conversion

BuckPow converts mA → A and mW → W when storing measurements.

WiFi Behavior

Connection

  • Connects to 2.4 GHz WiFi network
  • Timeout: 30 seconds
  • Retries every 30 seconds if connection fails

Auto-Reconnect

If WiFi disconnects during operation:

  1. The device continues running
  2. Every 30 seconds, it attempts to reconnect
  3. Once reconnected, measurements resume sending

API Error Backoff

If the API returns an error or is unreachable:

  1. The device waits 10 seconds before retrying
  2. After 10 seconds, it attempts the next measurement
  3. This prevents flooding a down server

Firmware Configuration

Edit these constants at the top of the sketch:

// WiFi
const char* WIFI_SSID     = "your-ssid";
const char* WIFI_PASSWORD = "your-password";

// BuckPow API
const char* API_BASE   = "http://192.168.100.16:8000";
const char* API_PATH   = "/api/v1/measurements";
const char* NODE_ID   = "esp32-ina219-01";
const char* API_KEY    = "";
const bool  USE_HTTPS  = false;

// Timing
const unsigned long INTERVAL_MS = 1000;
Setting Description
WIFI_SSID Your WiFi network name
WIFI_PASSWORD Your WiFi password
API_BASE BuckPow server URL (no trailing slash)
API_PATH API endpoint path
NODE_ID Unique node identifier (auto-registers if new)
API_KEY Device API key (empty = no auth)
USE_HTTPS true for HTTPS (uses setInsecure() — no cert verification)
INTERVAL_MS Milliseconds between measurements

Uploading Firmware

Prerequisites

  1. Install Arduino IDE or PlatformIO
  2. Install board support via Board Manager:
    • ESP32: esp32 by Espressif
    • ESP8266: esp8266 by ESP8266 Community

  3. Install required libraries via Library Manager:

    Base variant:

    Library Author
    Adafruit INA219 Adafruit
    ArduinoJson Benoit Blanchon

    OLED variant (additional):

    Library Author
    Adafruit SSD1306 Adafruit
    Adafruit GFX Library Adafruit

Upload Steps

  1. Open the sketch (.ino file) in Arduino IDE
  2. Edit WiFi and API configuration constants
  3. Select board:
    • ESP32: Tools > Board > ESP32 Dev Module
    • ESP8266: Tools > Board > Generic ESP8266 Module
  4. Select port: Tools > Port
  5. Click Upload
  6. Open Serial Monitor at 115200 baud to verify

Verify

After upload, the Serial Monitor should show:

BuckPow INA219 Firmware v1.2.0
Host: http://192.168.100.16:8000
Proto: HTTP
Key:   abcd****ef01
Connecting to WiFi.... connected
IP: 192.168.100.42
INA219 detected
OK id=esp32-ina219-01
OK id=esp32-ina219-01

Using the API Key

  1. Create a device in the BuckPow dashboard (or let it auto-register)
  2. Copy the API key from the device detail page
  3. Paste it into the API_KEY constant in the sketch
  4. Upload the firmware
const char* API_KEY = "a1b2c3d4e5f67890abcdef1234567890abcdef1234567890abcdef1234567890";

HTTPS Support

Set USE_HTTPS = true for encrypted communication:

const char* API_BASE  = "https://your-domain.com";
const bool  USE_HTTPS = true;

Certificate verification

The firmware uses setInsecure() which skips SSL certificate verification. This is suitable for development. For production, consider implementing certificate pinning.

Troubleshooting

INA219 not found

ERROR: INA219 not found. Check wiring.
  • Verify SDA/SCL connections
  • Check that INA219 is powered (3.3V or 5V)
  • Confirm I2C address (default 0x40)
  • Run an I2C scanner sketch to verify

WiFi timeout

WiFi timeout!
Running offline.
  • Check SSID and password
  • Ensure 2.4 GHz network (ESP8266 doesn't support 5 GHz)
  • Move the device closer to the router

HTTP errors

HTTP 401
  • Verify API_KEY matches the device in BuckPow
  • Check that DEVICE_AUTH_ENABLED is true on the server
HTTP 400
  • Verify the measurement payload format
  • Check that device_id matches expectations

API unreachable

API unreachable
  • Verify API_BASE URL is correct
  • Ensure the BuckPow server is running
  • Check network connectivity (ping the server)
  • Verify firewall rules allow port 8000

OLED not displaying

ERROR: SSD1306 OLED not found. Check wiring.
  • Verify SDA/SCL connections (shared I2C bus with INA219)
  • Check OLED I2C address (default 0x3C)
  • Ensure 3.3V power supply