Community Fridge Probe Firmware

A temperature and door monitoring system for community fridges, running on the Raspberry Pi Pico (RP2040).

Features

• Temperature monitoring via analog sensor (TMP36 or thermistor)
• Door state detection via magnetic reed switch with debouncing
• Rolling average calculation for stable readings
• Visual status indication via LED patterns
• Serial telemetry output over USB

Hardware Requirements

Component	Connection	Notes
Raspberry Pi Pico	-	RP2040 microcontroller
TMP36 temperature sensor	GPIO26 (ADC0)	Or any analog temp sensor
Magnetic reed switch	GPIO15	Uses internal pull-up
Status LED	GPIO25	On-board Pico LED

Wiring Diagram

                    Raspberry Pi Pico
                   ┌─────────────────┐
                   │                 │
     TMP36         │  GPIO26 (ADC0) ◄──── Temp sensor output
     ┌───┐         │                 │
     │ 1 ├─────────│  3V3           ──── Temp sensor VCC
     │ 2 ├─────────│  (see above)    │
     │ 3 ├─────────│  GND           ──── Temp sensor GND
     └───┘         │                 │
                   │  GPIO15        ◄──── Reed switch terminal 1
                   │  GND           ──── Reed switch terminal 2
                   │                 │
                   │  GPIO25        ──── On-board LED (built-in)
                   │                 │
                   │  USB           ──── Serial output to host
                   └─────────────────┘

TMP36 Pinout (flat side facing you):

• Pin 1 (left): VCC (3.3V)
• Pin 2 (middle): Signal output → GPIO26
• Pin 3 (right): GND

Reed Switch: Either terminal to GPIO15, other to GND. No polarity.

Status LED Patterns

Status	LED Pattern	Meaning
OK	Solid on	Temperature normal, door closed
DOOR_OPEN	Slow blink (1s on/off)	Fridge door is open
TOO_WARM	Fast blink (200ms on/off)	Temperature exceeds threshold
ERROR	Triple flash	Sensor error or disconnected

Serial Output Format

Telemetry is printed every 5 seconds (configurable):

t=4.3C, avg=4.1C, door=closed, status=OK
t=4.5C, avg=4.2C, door=open, status=DOOR_OPEN
t=8.1C, avg=7.3C, door=closed, status=TOO_WARM

Building the Firmware

Prerequisites

1. Pico SDK - The official Raspberry Pi Pico C/C++ SDK
2. ARM toolchain - arm-none-eabi-gcc
3. CMake - Build system (3.13+)
4. Make or Ninja - Build tool

macOS Setup

# Install toolchain via Homebrew
brew install cmake
brew install --cask gcc-arm-embedded

# Clone the Pico SDK
cd ~
git clone https://github.com/raspberrypi/pico-sdk.git
cd pico-sdk
git submodule update --init

# Set environment variable (add to ~/.zshrc for persistence)
export PICO_SDK_PATH=~/pico-sdk

Linux (Ubuntu/Debian) Setup

# Install toolchain
sudo apt update
sudo apt install cmake gcc-arm-none-eabi libnewlib-arm-none-eabi build-essential

# Clone the Pico SDK
cd ~
git clone https://github.com/raspberrypi/pico-sdk.git
cd pico-sdk
git submodule update --init

# Set environment variable (add to ~/.bashrc for persistence)
export PICO_SDK_PATH=~/pico-sdk

Build Commands

# From the project root directory
mkdir build
cd build
cmake ..
make -j4

# Or using Ninja (faster):
cmake -G Ninja ..
ninja

The output file will be build/fridge_probe.uf2.

Flashing the Pico

1. Hold the BOOTSEL button on the Pico
2. Connect the Pico to USB while holding BOOTSEL
3. Release BOOTSEL - The Pico appears as a USB mass storage device named RPI-RP2
4. Copy the UF2 file to the drive:

# macOS
cp build/fridge_probe.uf2 /Volumes/RPI-RP2/

# Linux
cp build/fridge_probe.uf2 /media/$USER/RPI-RP2/

The Pico will automatically reboot and start running the firmware.

Viewing Serial Output

macOS

# Find the device
ls /dev/tty.usbmodem*

# Connect (example - your device name may differ)
screen /dev/tty.usbmodem14201 115200
# Press Ctrl+A then K to exit screen

Linux

# Find the device
ls /dev/ttyACM*

# Connect
screen /dev/ttyACM0 115200
# Or use minicom:
minicom -D /dev/ttyACM0 -b 115200

Alternative: Python

import serial
ser = serial.Serial('/dev/ttyACM0', 115200)
while True:
    print(ser.readline().decode().strip())

Configuration

All configurable values are in include/config.h:

Parameter	Default	Description
SAMPLE_INTERVAL_MS	2000	Time between sensor reads
TELEMETRY_INTERVAL_MS	5000	Time between serial output
HISTORY_BUFFER_SIZE	32	Rolling average window size
TEMP_OK_MAX_C	7.0	Temperature alarm threshold
DEBOUNCE_SAMPLES	5	Door sensor debounce reads

Project Architecture

┌─────────────────────────────────────────────────────────────────┐
│                         main.c                                   │
│  - Initialize all modules                                        │
│  - Run cooperative main loop                                     │
│  - Call update functions every ~10ms                             │
└─────────────────────────────────────────────────────────────────┘
                              │
                              ▼
┌─────────────────────────────────────────────────────────────────┐
│                       app_logic.c                                │
│  - Owns application state (history, status)                      │
│  - Coordinates sensor reading                                    │
│  - Determines status priority                                    │
│  - Outputs serial telemetry                                      │
└─────────────────────────────────────────────────────────────────┘
           │                   │                    │
           ▼                   ▼                    ▼
┌──────────────────┐ ┌──────────────────┐ ┌──────────────────┐
│   sensors.c      │ │  door_sensor.c   │ │  led_status.c    │
│  - ADC init      │ │  - GPIO input    │ │  - GPIO output   │
│  - Read temp     │ │  - Debouncing    │ │  - Blink patterns│
│  - Convert °C    │ │  - Pull-up       │ │  - State machine │
└──────────────────┘ └──────────────────┘ └──────────────────┘
           │                   │                    │
           ▼                   ▼                    ▼
┌─────────────────────────────────────────────────────────────────┐
│                     Pico SDK / Hardware                          │
│  hardware_adc.h    hardware_gpio.h    pico/stdlib.h             │
└─────────────────────────────────────────────────────────────────┘

Module Responsibilities

Module	Responsibility
main.c	Entry point, init sequence, main loop
app_logic.c	Business logic, state management, telemetry
sensors.c	ADC configuration, temperature conversion
door_sensor.c	GPIO input, software debouncing
led_status.c	LED control, non-blocking blink patterns
config.h	All configurable constants

Extending the Firmware

Adding Wi-Fi (Pico W)

1. Create network.c / network.h
2. Add pico_cyw43_arch_lwip_poll to CMake libraries
3. Call network_init() from main
4. Call network_update(millis) from main loop
5. Send data in app_logic.c or via new module

Adding Calibration

1. Add calibration offset to config.h
2. Parse serial commands in main loop
3. Store calibration in flash (see Pico SDK flash API)

Adding SD Card Logging

1. Add sd_card.c module using SPI
2. Log readings periodically from app_logic.c
3. Implement circular file buffer to avoid filling card

Troubleshooting

No Serial Output

• Wait 2-3 seconds after plugging in (USB enumeration)
• Check correct serial port (ls /dev/tty*)
• Verify USB cable supports data (not charge-only)

Temperature Readings Wrong

• Check TMP36 wiring (VCC, signal, GND order)
• Verify 3.3V supply (not 5V - will damage ADC!)
• Check conversion formula for your sensor type

Door Always Shows Open/Closed

• Test reed switch with multimeter
• Verify magnet is close enough when door closed
• Check GPIO pin assignment in config.h

LED Not Working

• GPIO25 is the on-board LED - should work without wiring
• Check for bad status determination logic
• Add debug prints to see current status

License

MIT License - See LICENSE file

Version History

• v1.0 - Initial release (sensors + LED + serial)