CARDIKARST: From a modular Arduino prototype to a custom PCB data logger

Arduino CO2 logger

Reliable long-term cave monitoring requires electronics that are compact, mechanically stable, and resilient in humid environments. In the early phase of CARDIKARST instrumentation, our data-logging setup followed a modular “prototype” approach: sensors and peripheral modules were connected by cables to a microcontroller platform (Adafruit Feather M0 Adalogger). This architecture was ideal for rapid testing and iterative development, but it also revealed practical limitations once the system was deployed for long periods in real cave conditions.

We have now advanced to a custom, fully integrated PCB-based data logger, designed specifically for environmental monitoring in subterranean settings. The new design consolidates key functions onto a single printed circuit board, significantly reducing wiring complexity and increasing robustness.

Why move beyond the modular cable-connected setup?

In the prototype configuration, the CO2 sensor, RTC module and other peripherals were linked to the Arduino Feather board via cables. While this approach offers flexibility, it typically introduces several field-relevant disadvantages:

  • More potential failure points: every connector and cable is a mechanical and electrical risk (loosening, corrosion, intermittent contact).
  • Higher sensitivity to humidity and condensation: exposed contacts and cable interfaces are vulnerable in cave microclimate.
  • Lower mechanical stability: movement during installation or long deployments can stress connections.
  • Bulkier assembly: separate modules and cabling increase volume and complicate mounting and housing design.
  • More electrical noise exposure: longer leads and multiple interconnects can increase susceptibility to interference and unstable readings – particularly relevant for sensitive measurements such as CO2.

What changes with a custom PCB logger?

A PCB is more than a “board that holds components”: it defines the mechanical structure and the electrical architecture of the instrument. By integrating functions directly on the PCB and minimizing interconnects, the new logger provides practical advantages for long-term monitoring:

  • Mechanical robustness: components are fixed to the same board and connected by copper traces rather than wires, reducing movement-related failures.
  • Higher reliability in humid environments: fewer exposed contacts, fewer plug connections, and shorter conductive paths reduce the likelihood of corrosion and intermittent faults.
  • Compact, field-friendly geometry: a custom PCB allows the layout to be optimized for housing constraints, mounting points, and sensor placement.
  • Cleaner signal paths: shorter and more controlled electrical connections reduce unwanted noise and improve measurement stability.
  • Better system integration: power management, storage, sensing, and user interface are designed as one coherent system rather than assembled from independent modules.

New CARDIKARST CO2 datalogger: integrated components and capabilities

The custom PCB logger integrates the following elements (system feature list):

Core system
  • Microcontroller: ESP32-S3 Mini-1 (dual-core LX7 CPU, integrated Wi-Fi and Bluetooth 5.0 LE)
  • Memory & storage: MicroSD card support for continuous data logging
Power management
  • Input sources: USB (5 V) or single-cell Li-ion/LiPo battery (3.7 V nominal)
  • Power multiplexer: automatic source selection between USB and battery
  • Voltage regulation: onboard regulation for stable operation of MCU and sensors
Sensors
  • CO₂ sensor: COZIR NDIR sensor, 10,000 ppm range
  • Environmental sensor: SHT30 (air temperature and relative humidity)
  • Real-Time Clock: DS3231 with battery backup for precise timekeeping
User interface
  • Display: 0.96″ OLED, 128×64 (I²C) for live values and status
  • Rotary encoder: navigation, configuration, and manual control
Connectivity & expansion
  • I2C port: additional digital sensors or displays
  • UART port: serial communication / debugging
  • SPI port: high-speed peripheral expansion
Data logging
  • MicroSD slot: time-stamped storage of continuous sensor records
  • RTC integration: preserves time accuracy across power cycles
Additional design goals
  • compact, low-power architecture for portable and long-term deployments
  • optional onboard status indicators (e.g., power/logging/connectivity)
  • flexible firmware updates (USB or OTA)

Why this matters for CARDIKARST monitoring?

For cave microclimate and CO2 dynamics, instrument reliability is not a “nice-to-have” feature – it is a prerequisite for obtaining continuous, interpretable time series. The move from a modular cable-connected prototype (Feather M0 Adalogger + separate modules) to a custom PCB logger reduces practical deployment risk and directly supports the project’s goals: long-duration monitoring, stable sensor performance, and consistent data quality across sites and seasons.