DIY Solar Power Meter

A compact handheld solar irradiance meter built using the Seeed XIAO ESP32-C3, INA226, LSM303AGR, and either a Waveshare 1.54 inch 240x240 ST7789 TFT LCD or a 128x64 OLED display.

This project is designed for measuring solar irradiance in the field and also displaying tilt, azimuth, and battery level in a compact handheld device.

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Open Green Energy

This project is developed by Open Green Energy.

Full Project Documentation

Step by step build guide is available here:

https://www.instructables.com/DIY-Solar-Power-Meter/

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3D Printed Enclosure

Download the 3D printable enclosure files here:

https://www.printables.com/model/1603265-diy-solar-power-meter

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🌐 Website
https://opengreenenergy.com

▶ YouTube Channel
https://www.youtube.com/c/opengreenenergy

📷 Instagram
https://instagram.com/opengreenenergy

❤️ Support Open Green Energy
If you like this project and want to support development of more open hardware tools:

https://www.patreon.com/opengreenenergy

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Project Variants

This repository contains two firmware versions of the same project.

1. LCD Version

This version uses a Waveshare 1.54 inch 240x240 ST7789 TFT LCD and offers a more graphical user interface.

Location:
firmware/LCD_Version/

Main features:

• 4 page UI
• Professional graphical display
• Boot screen with logo
• Weather style irradiance icon
• Home, Min/Max, Tilt/Azimuth/Isc, and Compass pages
• EEPROM based calibration

2. OLED Version

This version uses a 128x64 OLED display and provides a simpler, low power interface.

Location:
firmware/OLED_Version/

Main features:

• Compact monochrome UI
• 4 pages
• Battery display
• Irradiance, tilt, azimuth, and compass page
• Full calibration with EEPROM storage
• Display suitable for minimal hardware builds

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Features

• Measures solar irradiance in W/m2
• Measures tilt angle using accelerometer
• Measures azimuth / heading using magnetometer
• Battery voltage and battery percentage display
• Single button navigation
• Long press calibration
• EEPROM based calibration storage
• Two firmware variants for different displays
• Portable field measurement tool

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Hardware Used

• MCU: Seeed XIAO ESP32-C3
• Current Sensor: INA226
• Accelerometer / Magnetometer: LSM303AGR
• Display Option 1: Waveshare 1.54 inch LCD, 240x240, ST7789 SPI
• Display Option 2: 128x64 OLED display

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Working Principle

The meter estimates solar irradiance by measuring the short circuit current of a small reference solar panel through a shunt based current sensing arrangement using INA226.

The measured Isc value is corrected using a stored dark offset and then converted into irradiance using a calibration factor:

Irradiance = Corrected Isc × Calibration Constant

In the LCD firmware:

• Calibration constant = 15.05 W/m2 per mA

In the OLED firmware, the calibration constant can be adjusted in code based on the reference panel used.

Tilt and azimuth are measured using accelerometer and magnetometer data, then filtered for stable display.

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Pin Connections

Function	XIAO ESP32-C3 Pin
Battery Sense	D0
MOSFET Gate	D1
Push Button	D2

LCD Version Additional Pins

Function	XIAO ESP32-C3 Pin
LCD Reset	D3
LCD CS	D6
LCD DC	D7
LCD SCK	D8
LCD Backlight	D9
LCD MOSI	D10

OLED / Sensor I2C Bus

Function	Connection
SDA	I2C SDA
SCL	I2C SCL

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LCD Wiring

LCD Pin	XIAO ESP32-C3
VCC	3V3
GND	GND
DIN	D10
CLK	D8
CS	D6
DC	D7
RST	D3
BL	D9

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INA226 Wiring

INA226 Pin	Connection
VCC	3V3
GND	GND
SDA	SDA
SCL	SCL
ADDR	0x44

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Battery Divider

Battery voltage is measured through a resistor divider:

• Top resistor = 200k
• Bottom resistor = 100k

Connection:

VBAT -> 200k -> ADC pin D0 -> 100k -> GND

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Button Operation

LCD Version

• Short Press → Next page
• Long Press for 2 seconds → Calibration wizard

OLED Version

• Short Press → Next page
• Long Press for 2 seconds → Full calibration

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UI Pages

LCD Version Pages

1. Home Page

Shows:

• Irradiance
• Tilt
• Azimuth
• Battery level

2. Min / Max Page

Shows:

• Live irradiance
• Minimum irradiance
• Maximum irradiance

3. Tilt / Azimuth / Isc Page

Shows:

• Tilt angle
• Azimuth angle
• Isc current

4. Orientation Page

Shows:

• Compass style direction display
• Azimuth in degrees
• N / E / S / W direction

OLED Version Pages

1. Home Page

Shows:

• Battery percentage
• Irradiance
• Tilt

2. Min / Max Page

Shows:

• Live irradiance
• Minimum irradiance
• Maximum irradiance

3. Tilt / Azimuth Page

Shows:

• Tilt angle
• Azimuth angle

4. Compass Page

Shows:

• Compass circle
• Needle direction
• Azimuth value

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Calibration Steps

Both firmware versions support calibration and store values in EEPROM.

Step 1: Isc Offset

Cover the reference panel and save the dark current offset.

Step 2: Tilt Zero

Keep the device level and save the zero tilt position.

Step 3: Tilt Reference

Keep the device vertical and save the tilt reference position.

Step 4: Compass Calibration

Rotate the device slowly so the magnetometer offset can be calculated.

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Libraries Required

Install these libraries from Arduino Library Manager:

• Adafruit GFX Library
• Adafruit SSD1306
• Adafruit ST7735 and ST7789 Library
• Adafruit LIS2MDL
• Adafruit LSM303 Accelerometer
• Adafruit Unified Sensor
• EEPROM
• Wire
• SPI
• INA226.h

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Firmware Structure

DIY-Solar-Power-Meter
│
├── firmware
│   ├── LCD_Version
│   │   ├── solar_meter_lcd_v1.1.ino
│   │   └── oge_logo.h
│   │
│   ├── OLED_Version
│   │   └── solar_meter_oled_v1.0.ino
│   │
│   └── README.md
│
├── images
│
├── README.md
│
└── LICENSE