ISCInstrumentation & Signal Conditioning

LOMI Thermal Control Modification for Enhanced Composting

Reverse-engineered a commercial LOMI composter to drop its operating temperature from ~100 °C to a biologically useful ~50 °C — not by touching the firmware, but by soldering a precision parallel resistor across the NTC thermistor (CN5) to bias the analog feedback loop. Characterized empirically with a DS1922T iButton; a 19.3 kΩ resistor settled at ~49.6 °C.

Hardware & Architecture

Commercial LOMI main control board using an NTC thermistor to regulate the heating element.
Precision resistor soldered in parallel across the NTC terminals (connector CN5) to lower the equivalent resistance seen by the ADC.
DS1922T iButton data logger placed inside the chamber for independent thermal profiling.

Key Highlights

Parallel resistance R_eq = (R_NTC · R_p) / (R_NTC + R_p) prematurely reaches the controller's cutoff threshold, capping physical heat output.
Swept 12.2–19.3 kΩ: 12.2 kΩ → ~30 °C, 14.7 kΩ underdamped (overshoot past 60 °C), 15.5 kΩ → ~46 °C, 19.3 kΩ → stable ~49.6 °C.
Python parses DS1922T CSV exports and plots comparative thermal profiles against the 50 °C target.

Images

LOMI main control board. The parallel precision resistor modification was performed at the CN5 NTC header, interfacing directly with the ADC circuitry feeding the ESP32 module.

Empirical thermal profiles demonstrating the system's modified response to varying parallel resistance values (12.2 kΩ to 15.5 kΩ) plotted against the 50 °C biological composting target.

Schematic detailing the voltage divider network manipulation, illustrating how the inserted parallel resistance artificially scales the NTC voltage drop seen by the microcontroller.

Code

lomi_thermal_profiles.pyiButton Parsing & Thermal Visualizationpython

import pandas as pd
import matplotlib.pyplot as plt
import glob
import os

# --- Configuration ---
# Directory containing iButton CSV exports
DATA_DIR = "./lomi_thermal_logs/"
TARGET_TEMP_C = 50.0


def load_and_clean_ibutton_data(filepath):
    """
    Parses a DS1922T iButton CSV file.
    Assumes standard iButton format: 'Date/Time' and 'Value' (Temperature).
    """
    try:
        # Skip metadata rows typically found in iButton exports
        df = pd.read_csv(filepath, skiprows=14, usecols=["Date/Time", "Value"])
        df.rename(columns={"Date/Time": "Timestamp", "Value": "Temperature_C"}, inplace=True)
        # Convert timestamp to datetime and calculate elapsed minutes for normalization
        df['Timestamp'] = pd.to_datetime(df['Timestamp'])
        start_time = df['Timestamp'].iloc[0]
        df['Elapsed_Minutes'] = (df['Timestamp'] - start_time).dt.total_seconds() / 60.0
        return df
    except Exception as e:
        print(f"[ERROR] Failed to process {filepath}: {e}")
        return None


def plot_thermal_profiles():
    """Generates a comparative plot of all tested resistor configurations."""
    plt.figure(figsize=(12, 7))
    # Define the configurations to plot and their corresponding colors
    # (Matches the visual output from lomitrail.png)
    configs = {
        "12.2k Ohm": {"file": "LOMI_12.2K.csv", "color": "#1f77b4"},
        "13.3k Ohm": {"file": "LOMI_13.3K.csv", "color": "#f2a900"},
        "14.7k Ohm": {"file": "LOMI_14.7K.csv", "color": "#009e73"},
        "15.5k Ohm": {"file": "LOMI_15.5K.csv", "color": "#d55e00"},
        "16.9k Ohm": {"file": "LOMI(16.9K)_021726.csv", "color": "#8c564b"},
    }

    for label, meta in configs.items():
        filepath = os.path.join(DATA_DIR, meta["file"])
        if os.path.exists(filepath):
            df = load_and_clean_ibutton_data(filepath)
            if df is not None:
                plt.plot(df['Elapsed_Minutes'], df['Temperature_C'],
                         label=label, color=meta["color"], linewidth=2)
        else:
            print(f"[WARNING] File not found: {filepath}")

    # Plot Target Line
    plt.axhline(y=TARGET_TEMP_C, color='r', linestyle='--', linewidth=2, label=f'Target {TARGET_TEMP_C}C')

    # Formatting
    plt.title("Comparison of LOMI Temperature Profiles via NTC Manipulation", fontsize=14, fontweight='bold')
    plt.xlabel("Time (minutes)", fontsize=12)
    plt.ylabel("Internal Temperature (C)", fontsize=12)
    plt.grid(True, linestyle='--', alpha=0.6)
    plt.legend(loc="upper right", fontsize=10)
    plt.tight_layout()
    # Save output for engineering documentation
    plt.savefig("LOMI_Thermal_Characterization.png", dpi=300)
    print("[SYSTEM] Thermal characterization plot saved successfully.")
    plt.show()


if __name__ == "__main__":
    plot_thermal_profiles()