golpm

Go and TinyGo Laser Power Meter firmware and application

Overview

golpm is a complete solution for measuring laser/light power using a calorimetric approach. The system consists of firmware running on a Seeed XIAO SAMD21 microcontroller and a desktop application built with Fyne GUI framework.

Hardware Description

The laser power meter uses a calorimetric design with two thermally isolated 2g copper plates:

• Absorber Plate: Covered with black soot and folded into a half-cube to form a black cavity that absorbs as much radiation as possible. This plate includes:

  • NTC (Negative Temperature Coefficient) thermistor for temperature measurement
  • Three calibration resistors (~2kΩ, ~500Ω, and ~200Ω) that can be individually controlled

• Reference Plate: Also contains an NTC thermistor for differential temperature measurement

Measurement Principle

Two NTCs are connected in a bridge configuration with a differential amplifier comparing the two readings. The Seeed XIAO SAMD21's 12-bit ADC measures:

• The differential voltage from the NTC bridge (temperature difference)
• The voltage across the calibration resistors (for precise power calculations via voltage divider)

Calibration Heaters

Three resistors attached to the absorber plate allow calibration at different power levels:

• Heater 1: ~2kΩ
• Heater 2: ~500Ω
• Heater 3: ~200Ω

These can be turned on/off individually to provide known power inputs for calibration at approximately:

• ~10mW
• ~50mW
• ~100mW
• ~150mW
• ~160mW

Project Structure

golpm/
├── firmware/          # TinyGo firmware for Seeed XIAO SAMD21
│   ├── main.go       # Main firmware code
│   └── pins.go       # Pin definitions and constants
├── lpm/              # Fyne desktop application
│   ├── EPIC_01_BASIC_GUI.md
│   ├── EPIC_02_SERIAL_CONNECTION.md
│   ├── EPIC_03_HEATER_CONTROL.md
│   ├── EPIC_04_SAMPLE_CONVERSION.md
│   ├── EPIC_05_MEASUREMENT_AND_FITTING.md
│   └── EPIC_06_GRAPH_WIDGET.md
└── README.md         # This file

Firmware

The firmware runs on Seeed XIAO SAMD21 and:

• Reads ADC values from the NTC bridge differential amplifier
• Reads voltage across calibration resistors via voltage divider
• Controls three heater resistors via GPIO pins
• Outputs serial data in format: unix_micros,reading,voltage,heater1,heater2,heater3
• Accepts heater control commands: three-digit string (0 or 1 for each heater) followed by newline, e.g., "000\n" (all off) or "111\n" (all on)

Desktop Application

The Fyne-based desktop application provides:

• Real-time Data Acquisition: Reads measurements from the MCU via serial port
• Graphical Display: Shows real-time temperature readings and calculated slope
• Thermal Model Fitting: Models the heating as a stack of copper→glue→NTC with proper lag accounting
• Power Calculation: Calculates absorbed laser power from temperature slope using calibration data
• Calibration System: Interactive calibration process with 6 points (zero, ~10mW, ~50mW, ~100mW, ~150mW, ~160mW)
• Heater Control: Manual control of individual heaters
• Configuration Management: YAML-based configuration file with serial port, resistor values, measurement window, and calibration data

Features

• Real-time temperature measurement and display
• Automatic detection of heating and cooling phases
• Thermal model fitting for accurate slope calculation
• Multi-point calibration system
• Configurable heater power calculations
• Modular, SOLID-principle-based architecture

Development Status

See the epic files in the lpm/ directory for detailed implementation plans.