Build-It-Yourself: Low-Cost Systems for Field Ecophysiology An Open Handbook for DIY Environmental Measurement Systems

Build-It-Yourself: Low-Cost Systems for Field Ecophysiology An Open Handbook for DIY Environmental Measurement Systems Authors: Mathias Hoffmann, Milan Shay Kretzschmar, Reena Macagga, Wael Al Hamwi, Adrian Dahlmann, Maren Dubbert, David Dubbert, John Marshall, Michael Asante, Geoffrey Sossa Contact: [email protected], [email protected] Date: December 2025 License: Creative Commons Attribution 4.0 (CC-BY 4.0) Version: 2.1 Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 2 Table of Contents Abbreviation 1. Introduction 2. Hardware Solutions 2.1 Environnemental Variables 2.1.1 NiMH Solar Trickle Charger 2.1.2 Photosynthetic Active Radiation (PAR) Sensor 2.1.3 Environmental Variable Explorer (EVE platform) 2.1.3.1 EVE-Offline Workflow (Weather station config.) 2.1.3.2 EVE-Online Workflow (Weather station config.) 2.2 Plant Responses 2.2.1 Handheld System to measure Spectral Plant Indices (e.g., NDVI, PRI) 2.2.2 Automatic System to measure Spectral Plant Indices (e.g., NDVI, PRI) 2.3 Biogeochemical Cycling 2.3.1 Manual System to measure CO2 and ET fluxes 2.3.2 Mesocosm System for Automatic CO2 and ET flux measurements 2.3.3 Water-stable isotope bag sampling system 3. Software Solutions 3.1 MonksHillLab Logger App 4. Real World Use & Regional Hubs 4.1 Philippine Hub – Reuse of Pineapple Residues 4.2 Northern Ghana Hub – Moist Savannah Dryland Rotation Trial 4.3 Central Benin Hub – Alternate Wetting and Drying Rice Trial Appendix A - Full code listings Appendix B - 3D-printing/PCB board files Appendix C – Other Software Appendix D – Supporting Data Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 3 Abbrevaiations: PTFE Polytetrafluoroethylene PAR Photosynthetic Active Radiation BWP34 Photodiode GND Grounding line 5V 5V power line A2 Analog signal line ET Evapotranspiration CO2 Carbon dioxide CH4 Methane NDVI Normalized Difference Vegetation index PRI Photochemical Reflectance Index NiMH Nickel-Metal Hydride mAh Milliampere hour Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 4 1. Introduction This handbook provides open-source, low-cost designs for field-based ecophysiological measurement systems in agricultural research. It is intended to support researchers, practitioners, and students in building and deploying affordable monitoring tools that can be adapted to a wide range of field conditions. To provide structure and clarity, the handbook is organized around three key domains of ecophysiological monitoring: (1) environmental variables (e.g., air temperature, humidity, radiation, soil moisture), (2) plant health and development (e.g., leaf temperature, chlorophyll status, canopy reflectance), and (3) biogeochemical cycles (e.g., evapotranspiration (ET), CO₂ and CH₄ fluxes, nutrient dynamics). These categories reflect increasing system complexity—from relatively simple measurements of ambient conditions to more integrated assessments of ecosystem processes. Each chapter follows the same practical structure—covering use cases, required materials, wiring diagrams, step-by-step assembly, calibration, and code—making it easy to adapt and implement these systems regardless of technical background. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 5 2. Hardware Solutions 2.1 Environmental Variables 2.1.1 NiMH Solar Trickle Charger 2.1.1.1 Purpose & Use Case: The solar trickle charger module presented here is designed to provide a simple, low-cost, and field-deployable solution for recharging AA NiMH batteries in remote areas. Its primary use case is to support small-scale electronic devices—such as environmental sensors or data loggers—used in off-grid research and monitoring setups. Especially relevant in the Global South and other regions where access to stable electricity is limited, this charger enables reliable, low-maintenance battery recharging using solar energy. By leveraging basic components like a small solar panel and a Schottky diode, the system offers a practical way to maintain battery-powered devices in the field without the need for complex charging infrastructure or frequent battery replacement. 2.1.1.2 Bill of Materials: A comprehensive overview of all components required to build the solar charger, needed quantities, recommended suppliers and approximate prices (in €) is provided in Table 2.1.1.2. Tab. 2.1.1.2: Components required for solar charger assembly; includes quantity, typical suppliers, and approximate prices (no links provided due to frequent changes). Component Amount Supplier Price (approx.) Shotky-Diode 1 Amazon, Reichelt, Conrad 0.10 € Common E-Series electronic resistor (e.g., 10 Ω) 1 Amazon, Reichelt, Conrad 0.10 € 6AA Battery holder 1 Amazon 3.00 € Perfboard/MonksHillLab PCB board (2x2cm) 1 Amazon 0.50 € Wires (black, red)/Screw Terminal Blocks - Amazon 0.50 € 6V, 2.5W Solar Panel 1 Amazon 10.00 € Sum: 14.20 € Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 6 2.1.1.3 Wiring Diagram: A schematic overview of the wiring layout, showing all necessary connections between electrical components, is provided in Figure 2.1.1.3. Fig. 2.1.1.3: wiring scheme for the PAR sensor, illustrating the electrical connections between the photodiode, resistor, and microcontroller. The schematic focuses solely on the wiring layout required for sensor assembly, not placement of additional, non-electronical components within the sensor case. 2.1.1.4 Assembly Instructions: The following section provides a step-by-step assembly guide for constructing the NiMH solar trickle charger, detailing the integration of all components: 1. Solder a 5cm black and red wire to + and – pole of the solar panel 2. Solder two 2x2 Screw terminal block (screw terminal 1 to 4) on the tiny perfboard/PCB board Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 7 3. Remove 0,5cm isolation from the red wire coming from the solar panel and fix it in screw terminal 1 4. Connect screw terminal 1 (Anode) and 2 (cathode) using the shotky-diode 5. Connect screw terminal 2 and 3 using the 10 Ohm resistor 6. Connect screw terminal 3 with + of the 4AA battery holder (using a red wire) 7. Remove 0,5cm isolation from the black wire coming from the solar panel and fix it in screw terminal 4 8. Connect screw terminal 4 with - of the 4AA battery holder (using a black wire) 9. Seal solder connections using e.g. transparent nail polish; 2.1.1.5 Calibration: Precise calibration is not required. To verify functionality, insert fully discharged NiMH batteries of the same type and capacity into the holder and expose the solar panel to direct sunlight for several hours. A gradual voltage increase indicates proper trickle charging. A current-limiting resistor (e.g., 10 Ω) is used to regulate charging current, based on Ohm’s Law (I = V / R). This ensures a safe, low current suitable for trickle charging, typically around 1/100th of the battery capacity per hour (e.g., 20 mA for 2000 mAh cells). The resistor value can be adjusted to suit panel output and battery size—higher resistance lowers the current to prevent overcharging in bright sunlight. 2.1.1.6 Code: No specific code is provided for this module, as it operates passively without a microcontroller. 2.1.1.7 References: None. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 8 2.1.2 Photosynthetic Active Radiation (PAR) Sensor 2.1.2.1 Purpose & Use Case: The PAR sensor module presented here can be used to determine photosynthetically active radiation (PAR), the range of solar radiation between 400 and 700 nm that is relevant for plant photosynthesis. While this chapter focuses on the construction and calibration of the sensor itself, it is designed to be integrated into larger monitoring setups that include power supply and data logging via a microcontroller. The sensor module thus serves as a component in more comprehensive systems—such as automatic climate stations or gas flux chambers described in later chapters—where continuous PAR measurements are essential for understanding environmental conditions and light-driven plant processes. 2.1.2.2 Bill of Materials: A comprehensive overview of all components required to build the sensor, needed quantities, recommended suppliers and approximate prices (in €) is provided in Table 2.1.2.2. Tab. 2.1.2.2: Components required for PAR sensor assembly; includes quantity, typical suppliers, and approximate prices (no links provided due to frequent changes). Component Amount Supplier Price (approx.) Photodiode BPW34 1 Amazon, Reichelt, Conrad 0.70 € Common E-Series electronic resistor (e.g., 2.2kΩ, 4.7kΩ or 10kΩ) 1 Amazon, Reichelt, Conrad 0.10 € Visible light bandpass filter (400 to 700nm; ⌀ 9.5mm) 1 Aliexpress 0. 50 € Polytetrafluoroethylene (PTFE)-diffusor disk (⌀ 10mm) 1 Amazon 0.10 € 3D-printed PAR sensor case 1 - 0.10 € 3-wire cable, shielded, suitable for outdoor use (e.g., ⌀4 mm, ~1 m length) 1 Amazon, Reichelt, Conrad 0.50 € Sum: 2.00 € Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 9 2.1.2.3 Wiring Diagram: A schematic overview of the wiring layout, showing all necessary connections between electrical components, is provided in Figure 2.1.2.3. Fig. 2.1.2.3: wiring scheme for the PAR sensor, illustrating the electrical connections between the photodiode, resistor, and microcontroller. The schematic focuses solely on the wiring layout required for sensor assembly, not placement of additional, non-electronical components within the sensor case. 2.1.2.4 Assembly Instructions: The following section provides a step-by-step assembly guide for constructing the PAR sensor, detailing the integration of all components: 10. 3D-print case (black) using UV-resistant material 11. Place the BPW34 photodiode into the designated slot in the 3D-printed case, ensuring correct orientation (cathode/anode); 12. Solder 5V (red) and signal (e.g., A2) wire (yellow) of the cable to anode (horizontal mark on leg) and cathode of the BPW34, respectively; 13. Connect electrical resistor (no specific orientation) by solder one leg to the signal (e.g., A2) wire (yellow) and the other to GND wire (black) of the cable; 14. Seal solder connections using e.g. transparent nail polish; Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 16 2.1.3.1.7 References: None. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 17 2.1.3.2 EVE-Online Workflow (EVE platform; Weather station config.) 2.1.3.2.1 Purpose & Use Case: EVE-Online (Environmental Variable Explorer) is configured as a compact, low-cost, cloudenabled weather station designed for near–real-time monitoring of environmental variables. The system records photosynthetically active radiation (PAR, 400–700 nm), relative humidity (RH), air temperature, and air pressure at user-defined intervals (e.g., 15–30 min) and transmits the data wirelessly to a web server via the integrated ESP32 Wi-Fi module. This setup enables continuous, site-specific observation of microclimatic conditions during cropping seasons, with immediate remote access to the data. Data are automatically uploaded to a self-hosted backend and can be visualized or downloaded through a web interface, eliminating the need for manual data retrieval in the field. This chapter focuses on the construction, configuration, and data transmission workflow of the EVE-Online system. The continuous and remotely accessible environmental data provided by this setup support timely interpretation of crop–environment interactions and facilitate data-driven, climatesmart management decisions. 2.1.3.2.2 Bill of Materials: A comprehensive overview of all components required to build the automatic weather station, needed quantities, recommended suppliers and approximate prices (in €) is provided in Table 2.1.3.2.2. Tab. 2.1.3.2.2: Components required for weather station assembly; includes quantity, typical suppliers, and approximate prices (no links provided due to frequent changes). Component Amount Supplier Price (approx.) PAR sensor 1 DIY (see handbook) 2. 00 € Microcontroller (e.g., ESP) 1 AZ-Delivery, Amazon 7.00 € TPL5110 Nano Power Timer 1 Antratek, BerryBase 8.30 € BMP280 (weatherproof) 1 Aliexpress 3.50 € SHT40 module (weather proof) 1 Aliexpress 4.70 € 3D-printed PAR sensor/SHT41 case (EVE) 1 - 0.50 € Weatherproof junction box (10.0x6.8x5.0 cm) 1 Amazon 3.00 € PG7 cable fitting 2 Hardware store 0.50 € Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 18 NiMh Battery (2300 mAh; rechargeable) (Power Option A) 1 Amazon, Reichelt 4.40 € Solar panel (20 W), charger (5V out) and battery (12V; 9 Ah) pack (Power Option B) 1 Amazon 44.00 € Dupont connector (male/female)/female header; colored wires; screw terminal blocks - Amazon 0.50 € 3-/4-wire cable, shielded, suitable for outdoor use (e.g., ⌀4 mm, ~10 cm length) 1 Amazon, Reichelt, Conrad 0.50 € Sum (Power Option A): 34.90 € Sum (Power Option B): 74.50 € 2.1.3.2.3 Wiring Diagram: A schematic overview of the wiring layout, showing all necessary connections between electrical components, is provided in Figure 2.1.3.2.3. Fig. 2.1.3.2.3: wiring scheme for the EVE-Online weather station configuration, illustrating the electrical connections between the TPL5110, ESP32 Microcontroller as well as connected sensors (BMP280, SHT40 and PAR sensor). The schematic focuses solely on the wiring layout required for weather station assembly, not placement of additional, non-electronical components within the outdoor case. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 19 2.1.3.2.4 Assembly Instructions: The following section provides a step-by-step assembly guide for constructing the PAR sensor, detailing the integration of all components: 1. 3D-print PAR sensor case (EVE; black) and SHT41 sensor case (white) using UVresistant material 2. Assemble PAR sensor as explained within this handbook 3. Drill holes (Ø 4 mm) at the midpoint of one vertical side (top) of the weatherproof junction box 4. Fit the 3-wire cable connected to the PAR sensor through one of these hole and glue the PAR sensor on top of it 5. Fit PG7 on top of the 3D-printed SHT40 case (smaller part), glue smaller part and larger part of 3D-printed SHT40 case on top of each other, fit SHT40 cable through it until sensor head is fully covered by case and tighten PG7 up around the 4-wire cable of SHT40; finally fit 4-wire cable of SHT40 through one of two Ø 10 mm holes drilled into the vertical side of the weatherproof junction box, opposite to installed PAR sensor; fix wire to box with PG7; fit BMP280 through the second hole and fix with 2nd PG7 6. Solder male header on top side of electronical components (ESP32 and TPL5110) 7. Fit electronical components into junction box (double sided tape) and connect sensors with wires by using Dupont connectors 8. Seal solder connections using e.g. transparent nail polish; 9. Upload program code to microcontroller and check if the sensors are delivering values and if TPL5110 shuts the system off/on as intended; 10. Implement Backend dashboard and test to data transfer 2.1.3.2.5 Calibration: The PAR sensor, which outputs analog voltage, requires calibration against a high-accuracy PAR sensor under natural sunlight to derive a conversion function to µmol m⁻² s⁻¹. Temperature, relative humidity, and air pressure sensors do not require direct calibration but can be validated by comparing readings with a reference-grade weather station. Appendix D (Supporting Data) contains a 48h data set of a low-cost PAR sensor directly Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 20 compared with a high-cost SKP215 PAR sensor (trueness; Campbell Scientific, UK) as well as a 36h test of 10 low-cost PAR sensors directly compared with a high-cost SKP215 PAR sensor (accuracy; Campbell Scientific, UK). In addition, Appendix D (Supporting Data) contains a 7 days field deployment test/proof of concept of EVE-Online, comparing air pressure, air temperature and RH readings with high-cost instruments. 2.1.3.2.6 Code: Please see Arduino IDE code example given in Appendix A to implement the online workflow of the automatic weather station configuration of EVE. The needed script library for the backend dashboard and SQL scheme can be found in Appendix C. 2.1.3.2.7 Backend infrastructure and dashboard setup: Operation of the EVE-Online weather station configuration does not require programming expertise, but it does require access to a basic web server environment supporting PHP and MySQL. This can be provided by institutional infrastructure (e.g., a university server), a lowcost/free of charge shared web hosting service, or a local installation using a standard web server stack (e.g., XAMPP). The backend consists of a lightweight database and web dashboard that manage users, measurement sites, sensor nodes, and incoming data streams. Initial setup involves creating an empty MySQL database and importing a predefined SQL schema (provided in the Appendix C). Executing this schema automatically generates all required tables for user management, site configuration, node registration, and sensor data storage. The dashboard code is supplied as a ready-to-use package (Appendix C) and deployed by copying the contents of the provided www directory into the web server’s document root (e.g., public_html or www). Configuration files allow users to specify database credentials and define a secure authentication token that links individual EVE nodes to the backend. Once configured, a single initialization script is executed to create an administrator account. After setup, users can access the web-based dashboard to register measurement sites, assign sensor nodes, monitor incoming data in near–real time, and download datasets for further analysis. This backend design deliberately avoids complex dependencies and proprietary services, providing a transparent, self-hosted data pipeline that remains fully under user control. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 21 2.1.3.2.8 References: None. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 22 2.2 Plant Responses 2.2.1 Handheld System to measure Spectral Plant Indices (e.g., NDVI, PRI) 2.2.1.1 Purpose & Use Case: The handheld spectral measurement system presented here enables low-cost, in-situ measurement of vegetation indices by capturing reflectance data across six distinct wavelengths in the visible and/or near-infrared (NIR) wavelengths. These spectral measurements allow the calculation of widely used indices such as the Normalized Difference Vegetation Index (NDVI) and the Photochemical Reflectance Index (PRI), providing insights into plant physiological or health status, canopy structure, and light-use efficiency. This chapter focuses on the construction, calibration, and application of the handheld system tio measure spectral plant indices, which is designed for flexible deployment in field experiments, precision agriculture, and ecological monitoring. While primarily intended as a standalone handheld system, the working principle of made measurements can also be integrated into automatic systems. 2.2.1.2 Bill of Materials: A comprehensive overview of all components required to build the handheld measurement system for spectral plant indices, needed quantities, recommended suppliers and approximate prices (in €) is provided in Table 2.2.1.2. Tab. 2.2.1.2: Components required for assembly of the handheld system to measure spectral plant indices; includes quantity, typical suppliers, and approximate prices (no links provided due to frequent changes). Component Amount Supplier Price (approx.) Microcontroller (e.g., UNO) 1 AZ-Delivery, Amazon, Reichelt, Conrad 9.00 € HC-05 HC-06 Bluetooth Wireless RFTransceiver-Modul RS232 serielle TTL 1 AZ-Delivery, Amazon, Reichelt, Conrad 9.00 € AS7262/AS7263 6-canal spectral sensor 2 Aliexpress, Antratek, BerryBase 60. 00 € TCA9548A I2C Multiplexer 1 AZ-Delivery, Amazon, Reichelt, Conrad 3. 00 € PTFE-diffusor disk (⌀ 40mm) 1 Amazon 0.50 € 3D-printed AS7363/AS7262 sensor case 2 - 1.00 € Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 23 3D-printed TCA9548A case 1 - 0.50 € Rocker switch (round) 1 Amazon 1.00 € HMF ODK500 Outdoor case (19x13x5.5mm) 1 Amazon 20.00 € 6AA Battery holder 1 Amazon 2.00 € NiMh Battery (2300 mAh; rechargeable) 6 Amazon, Reichelt, Conrad 14.00 € 9V battery clip 1 Amazon 0.50 € 4-wire USB A extension cable, shielded, suitable for outdoor use (e.g., ⌀4 mm, ~1 m length) 1 Amazon, Reichelt, Conrad 3.50 € Dupont connector (female)/luster clumps 4 Amazon 1.00 € Wires (black, red, yellow, green, blue, brown) - Amazon 0.50 € Aluminum, steel rods, wood or PVC pipe (4m) 1 Hardware store 20.00 € Sum: 145.5 € 2.2.1.3 Wiring Diagram: A schematic overview of the wiring layout, showing all necessary connections between electrical components, is provided in Figure 2.2.1.3. Fig. 2.2.1.3: wiring scheme for the handheld system to measure Spectral Plant Indices, illustrating the electrical connections between the spectral sensors, I2C multiplexer, Bluetooth module and microcontroller. The schematic focuses solely on the wiring layout required for system assembly, not placement of additional, non-electronical components within the sensor and/or battery case. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 24 2.2.1.4 Assembly Instructions: The following section provides a step-by-step assembly guide for constructing the handheld system to measure spectral plant indices, detailing the integration of all components: 1. 3D-print cases (black) using UV-resistant material; 2. Solder 5cm long wires to 3.3V (red), GND (black), SCL (green) and SDA (yellow) of one of the AS7262/3 and fix (glue or double sided tape) it to 3D-printed AS7262/3 case (upward case/smaller outer walls); fit loose ends through its back 3. Fix (double sided tape) TCA9548A I2C multiplexer to the back of the upward case 4. Fix (double sided tape) 6-Luster clamp to top of TCA9548A I2C multiplexer 5. Solder loose ends of the 5cm long wire for SCL (green) and SDA (yellow) to SC0 (green) and SD0 (yellow) of TCA9548A I2C multiplexer; 6. Fix loose ends of 5cm long wires for 3.3V (red) and GND (black) to 1 and 2 of the 6luster clamp 7. Solder 15cm (!) long 3.3V (red), GND (black), SCL (green) and SDA (yellow) to the second of the AS7262/3 and fix (glue or double sided tape) it to 3D-printed AS7262/3 case (downward case/taller outer walls); fit loose ends through its back 8. Solder 5cm long wires for 3.3V (red), GND (black), SCL (green) and SDA (yellow) to VIN (red), GND (black), SCL (green) and SDA (yellow) of TCA9548A and fix their loose ends to 6-luster clamp 1, 2, 3 and 4 9. Solder 5cm long wires for SCL (green) and SDA (yellow) to SC1 (green) and SD1 (yellow) of TCA9548A and fix their loose ends to luster clamp 5 and 6 10. Fix loose ends of 15cm long wires for 3.3V (red), GND (black), SCL (green) and SDA (yellow) to 1, 2 5 and 6 of the 6-luster clamp 11. Cut 4-wire USB A extension cable into two similar long parts and remove ca. 5cm isolation on loose ends; fit one loose end through the hole in the TCA9548A case and connect the red, black, green and yellow wire of the USB A cable to luster clamp 1, 2, 3 and 4 (in case of different colors in USB A cable allocate wire color) 12. Drill a 5mm hole into the top end of the HMF ODK500 Outdoor case and a 20mm hole into the cover flap positioned near the hinge to insert the second USB A cable and install the rocker switch 13. Solder the red, black, green and yellow wire of the USB A cable to 3.3V, GND, SCL and SDA of the microcontroller Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 25 14. Solder 9V battery clip red wire to one of the rocker switch connectors and solder a 10cm long red wire to its second contact; than solder black wire of 9V battery clip and loose end of 10cm red wire to GND and VIN of microcontroller 15. Fix microcontroller to the box using e.g. double sided tape and insert batteries into 6AA battery holder and place it into the box as well (fix if necessary) 16. Solder a 7cm long red, black, white and blue cable to 5V, GND, TX and RX of the microcontroller and crimp a female Dupont connector to the loose wire ends; 17. Connect Dupont connectors on 5V, GND, TX and RX wires from microcontroller to VCC, GND, RX and TX male pins at HC-05 Bluetooth module and place the module within the box (note: RX goes to TX and TX to RX) 18. Seal solder connections using e.g. transparent nail polish; 19. Place the PTFE diffusor over the upward directed AS7262/3 on top of the 3Dprinted case and connect cases together to form sensor head 20. Seal 3D-printed sensor head where necessary using e.g., silicon. 21. Construct a 1.8-meter high pole with a 1-meter long reinforced cantilever arm and fix sensor head on arm using e.g., cable tie (make sure sensors are directed upand downwards as intended); fix battery/microcontroller box on pole and connect USB A plug (battery/microcontroller box) with socket (sensor head) 22. Upload program code to microcontroller and check if the sensor is delivering e.g., NDVI values in expected range via Bluetooth connection for different surfaces/canopies using the MonksHillLab Logger App; 2.2.1.5 Calibration: Since the sensor provides direct NDVI measurements based on reflectance in selected spectral bands, measurement values does not need to be converted prior to use. However, calibration is essential to ensure accuracy and comparability across different setups and conditions. NDVI values can be easily calibrated against reference surfaces with known NDVI values to derive a correction function. A practical and low-cost approach involves using colored reference targets, such as cardboards with defined reflectance properties (e.g. white, black, green, yellow), as demonstrated by Macagga et al. (2025). Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 32 2.2.2.6 Code: Please see Arduino IDE code example given in Appendix A to implement the automatic lowcost DIY NDVI measurement system. 2.2.2.7 References: Macagga R, Sossa G, Ayaribil Y, Bayot R, Sanchez P, Augustin J, Bellingrath-Kimura SD, Hoffmann M (2025) A new, low-cost ground-based NDVI sensor for manual and automated crop monitoring. Smart Agricultural Technology 11, DOI: 10.1016/j.atech.2025.100892 Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 33 2.3 Biogeochemical Cycling 2.3.1 Manual System to measure CO2 and ET fluxes 2.3.1.1 Purpose & Use Case: The sensor unit and data logger presented here can be used to determine CO₂ and ET fluxes using the manual closed-chamber method—two key fluxes in the C and water cycles, respectively. While this chapter focuses on the construction, calibration, and deployment of the manual version of the system, the setup is designed with flexibility in mind and can be readily adapted for automated operation. This modularity allows it to serve as a foundational component in both short-term field campaigns and long-term environmental monitoring efforts. The data generated provide critical insights into plant–soil–atmosphere interactions, especially in relation to photosynthetic activity, respiration, and water use. In later chapters, integration into automated systems is explained to facilitate high-resolution, continuous flux measurements across spatial and temporal scales. 2.3.1.2 Bill of Materials: A comprehensive overview of all components required to build the manual system to measure CO2 and ET fluxes (excluding the closed-chamber), needed quantities, recommended suppliers and approximate prices (in €) is provided in Table 4.1.2. Tab. 2.3.1.2: Components required for data logger and sensor unit assembly; includes quantity, typical suppliers, and approximate prices (no links provided due to frequent changes). Component Amount Supplier Price (approx.) Microcontroller (e.g., UNO) 1 AZ-Delivery, Amazon 9.00 € BMP280 1 AZ-Delivery, Amazon 2.00 € K30 FR NDIR CO2 sensor 1 Driesen & Kern 80. 00 € Data logger shield (UNO) 1 AZ-Delivery, Amazon 6.00 € SHT41 module (weather proof) 1 Aliexpress 8.00 € HC-05 HC-06 Bluetooth Wireless RFTransceiver-Modul RS232 serielle TTL 1 AZDelivery, Amazon, Reichelt, Conrad 9.00 € SD-card (2 GB) 1 Amazon 5.00 € 0.96`` OLED-display I2C 1 Amazon, AZ-Delivery 5.00 € PAR sensor 1 DIY (see handbook) 3. 45 € Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 34 3D-printed K30 FR sensor case 1 - 1.00 € B&W Outdoor Case Typ 500 (yellow) 1 Profikoffer 30.00 € 6AA Battery holder 2 Amazon 4.00 € NiMh Battery (2300 mAh; rechargeable) 12 Amazon, Reichelt, Conrad 28.00 € 9V battery clip 2 Amazon 1.00 € Dupont connector (male/female)/luster clumps 24 Amazon 6.00 € Perfboard/MonksHillLab PCB board (5x7cm) 1 Amazon 2.00 € Wires (black, red, yellow, green, blue, brown, white)/ Screw terminal blocks - Amazon 5.00 € >10-wire cable, shielded, suitable for outdoor use (e.g., DSUB; color coded; ~1.5 m length) 1 Amazon 8.00 € Rocker switch (4 connections) 1 Amazon 5.00 € PG9 cable fitting 1 Hardware store 2.00 € Hard foam plate 1 Amazon 5.00 € Sum: 224.5 € 2.3.1.3 Wiring Diagram: A schematic overview of the wiring layout, showing all necessary connections between electrical components, is provided in Figure 2.3.1.3. Fig. 2.3.1.3: wiring scheme for the manual system to measure CO2 and ET fluxes, illustrating the electrical connections between the microcontroller (e.g., UNO), HC-05 Bluetooth module, data logger module, OLED display, BMP280 module, K30FR, PAR sensor and SHT41. The Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 35 schematic focuses solely on the wiring layout required for sensor assembly, not placement of components and additional, non-electronical components within the sensor case. 2.3.1.4 Assembly Instructions: The following section provides a step-by-step assembly guide for constructing the manual system to measure CO2 and ET fluxes, detailing the integration of all components: 1. Prepare B&W outdoor case by drilling a Ø20mm hole into the left and back wall to install rocker switch and PG9 cable fitting 2. Use hard foam plate and carpet knife to cut custom inserts (1x vertical; 2x horizontal) that keep the 6xAA battery holder in position and install on left side of B&W outdoor case type 500 3. Fit (double sided tape or screws) the perfboard/PCB board with its longer side aligned along the shorter dimension of the B&W outdoor case type 500 4. Cut ends of >10 wire cables (e.g., DSUB) and remove isolation; fit one end through the PG9 into the B&W outdoor case type 100 and connect wires to close end of the perfboard/PCB board using screw terminal blocks soldered to that end 5. Place 6xAA battery holder and connect red wires to rocker switch while connecting black wires to left side of the perfboard/PCB board using screw terminal blocks soldered to that side; connect both black wires; 6. Solder microcontroller to PCB/perfboard by connecting VIN (red; microcontroller) with screw terminal block one red with from rocker switch, GND (black) microcontroller) with screw terminal block with one wire (black) from one of the battery holder ( power supply to microcontroller); 7. Connect 2nd (red) wire from rocker switch with screw terminal block connected to >10 wire cable (red) and screw terminal block with 2nd (black) wire from 2nd battery holder to screw terminal block connected to >10 wire cable (black) ( power supply to K30FR) 8. Solder a green and yellow 10cm wire through the PCB/perfboard to SDA and SCL of the microcontroller; connect the other ends to the screw terminal blocks with the green and yellow wire from the >10 wire cable ( connection for SHT41) 9. Solder a brown and white 10cm wire through the PCB/perfboard to digital pin 12 and 13 of the microcontroller; connect the other ends to the screw terminal blocks with the brown and white wire from the >10 wire cable ( connection for K30 FR) Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 36 10. Solder a green and yellow 10cm wire through the PCB/perfboard to SDA and SCL of the microcontroller; connect the other ends to SDA and SCL of the BMP280 placed on the PCB/perford thus fixing it to the board; solder in addition a 10cm red and black wire to VIN and GND of the BMP280 and solder the loose ends to 5V and GND of the Microcontroller 11. Place data logger shield on top of microcontroller 12. Crimp a male and female Dupont connector to 4 10cm wires (black, red, green and yellow) and use these wires to connect the OLED display with the microcontroller through connecting VCC (red; OLED) to 5V (red; microcontroller), GND (black; OLED) to GND (black; microcontroller), SDA (green; OLED) to SDA (green; microcontroller) and SCL (yellow; OLED) to SCL (yellow; microcontroller) 13. Fit (double sided tape) OLED display to hard foam plate custom insert 14. Crimp a male and female Dupont connector to 4 10cm wires (black, red, brown and white) and use these wires to connect the HC-05 Bluetooth module with the microcontroller through connecting VCC (red; HC-05) to 5V (red; microcontroller), GND (black; HC-05) to GND (black; microcontroller), TX (brown; HC-05) to RX (brown; microcontroller) and RX (white; HC-05) to TX (white; microcontroller) 15. Fit (double sided tape) HC-05 Bluetooth module to hard foam plate custom insert or B&W outdoor case wall 16. 3D-print sensor case and install PG9 cable fitting 17. Fit the 2nd end of the >10 wire cables (e.g., DSUB) through the PG9 into the sensor box 18. Solder from the >10 wire cable: a. 7.2V (red) wire to VCC (red) of the K30 FR b. 5V (red) wire to SHT41 and PAR sensor (red) c. GND (black) to GND (black; K30 FR, PAR sensor and SHT41) d. SDA (green) to SHT41 (green) e. SCL (yellow) to SHT41 (Yellow) f. A2 (blue) to PAR sensor (blue) g. Digital pin 12 (brown) to TX of K30 FR (brown) h. Digital pin 13 (white) to RX of K30 FR (white) 19. Seal solder connections using e.g. transparent nail polish; Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 37 20. Upload program code to microcontroller and check if all system components deliver values in expected range via Bluetooth using the MonksHillLab Logger App 2.3.1.5 Calibration: Although the CO₂ and RH sensors used in this system provide direct readings in parts per million (ppm) and relative humidity (%) respectively—eliminating the need for conversion from raw signal to physical units—performing a simple calibration or check-up remains good practice to ensure sensor accuracy over time. For the CO₂ sensor, this can be done in a lowcost and straightforward way using commercially available CO₂ cartridges (e.g., for carbonated water), which contain 100% CO₂. When used with a pressure regulator, a known amount of CO₂ can be injected into a sealed calibration vessel containing the sensor unit. The resulting increase in CO₂ concentration can then be compared to the sensor’s readings of the measured increase in CO₂ concentration due to injection, providing a quick and effective way to verify sensor performance. This method is especially useful in field conditions, where access to laboratory calibration equipment may be limited. While relative humidity sensors are more difficult to calibrate directly in the field, cross-checking with a trusted reference device under stable environmental conditions can help confirm their reliability. 2.3.1.6 Code: Please see Arduino IDE code example given in Appendix A to implement the manual lowcost DIY CO2 and ET flux measurement system. 2.3.1.7 References: Macagga R, Asante M, Sossa G, Antonijevic, Dubbert M, Hoffmann M (2024) Validation and field application of a low-cost device to measure CO2 and evapotranspiration (ET) fluxes. Atmospheric Measurement Techniques 17/4, DOI: 10.5194/amt-17-13172024 Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 38 2.3.2 Mesocosm System for Automatic CO2 and ET flux measurements 2.3.2.1 Purpose & Use Case: This system is a further development of the manual closed-chamber setup described earlier and is specifically designed for controlled greenhouse and mesocosm experiments. Its primary goal is to precisely and automatically measure CO₂ exchange and ET dynamics in a semi-controlled environment, minimizing interference with the natural physiological processes of plants. By automating chamber opening and closing via a motorized sliding door and continuously recording environmental variables - such as CO₂ concentration, relative humidity, and air temperature - the system enables detailed investigations into plant responses to varying experimental conditions, such as drought stress and fertilization treatments. The system is compatible with additional sensors (e.g. NDVI, soil moisture, or leaf temperature) to broaden its analytical scope. Building on the modular and flexible design of the manual system, the system’s automation and high-resolution data collection support advanced analyses, making it a valuable tool for plant ecophysiology research in greenhouse and mesocosm settings. 2.3.2.2 Bill of Materials: A comprehensive overview of all components required to build the mesocosm system for automatic CO2 and ET flux measurements (including the closed-chamber), needed quantities, recommended suppliers and approximate prices (in €) is provided in Table 2.3.2.2. Tab. 2.3.2.2: Components required for system assembly; includes quantity, typical suppliers, and approximate prices (no links provided due to frequent changes). Component Amount Supplier Price (approx.) Chamber body (customized) 1 Romid 600.00 € ATmega328-Board 1 AZ-Delivery, Amazon, Reichelt 9.00 € Datalogger module XD-204 1 AZ-Delivery, Amazon, Reichelt 6.00 € Boost converters step up/down (HW-140 DCDC) 1 AZ-Delivery, Amazon, Reichelt 5.00 € 2-Relay module 5V 1 AZ-Delivery, Amazon, Reichelt 3.00 € Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 39 Bluetooth module (HC-05) Wireless RFTransceiver module RS232 1 AZ-Delivery, Amazon, Reichelt 9.00 € Outdoor box (170*110*48 mm) 1 Amazon, Conrad, Reichelt 14.00 € Hard foam plate 5mm 1 Amazon 1.00 € 0.5 mm2/20 awg electrical wire, 7 colors 1 Amazon 2. 50 € Luster terminals 8 Amazon 0.80 € Rocker switch (2 connections) 1 Amazon, Conrad, Reichelt 1.00 € Mosfet (IRLZ44N model) 1 Amazon, Conrad, Reichelt 0.80 € Resistors (10k Ω and 200 Ω) 1 Amazon, Conrad, Reichelt 0.20 € SD MEMORY CARD (2 GB.10 MB/s) 1 Amazon 5.00 € 8 pin aviation connectors 1 Amazon 1.50 € Power jack socket 2 Amazon, Conrad, Reichelt 3.00 € 8 Core cable (1 m) 1 Amazon 3.50 € Rubber rope (1.5 m) - Amazon 1.20 € Self-adhesive hooks 20 Amazon 8.20 € K30 FR NDIR CO2 sensor 1 Driesen & Kern 80.00 € SHT31 module (waterproof) 1 Aliexpress 8.00 € PAR sensor 1 DIY (see handbook) 3. 45 € BMP280 (5V) 1 Amazon, Conrad, Reichelt 2. 00 € DC12V linear actuator 1 Aliexpress, Amazon, Reichelt 19.50 € Power supply 9v adapter 1 Reichelt, Amazon, Conrad 9.10 € Axial fan (92x92x25mm, 12V) 4 Reichelt, Amazon, Conrad 12.00 € Sum: 802.80 € 2.3.2.3 Wiring Diagram: A schematic overview of the wiring layout, showing all necessary connections between electrical components, is provided in Figure 2.3.2.3. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 40 Fig. 4.2.3: wiring scheme for the mesocosm system for automatic CO2 and ET flux measurements, illustrating the electrical connections between the microcontroller (e.g., UNO), HC-05 Bluetooth module, data logger module, relays module, mosfet, DC-DC converter, BMP280 module, K30FR, PAR sensor, SHT41, axial fans and linear actuator. The schematic focuses solely on the wiring layout required for sensor assembly, not placement of components and additional, non-electronical components within the sensor case. 2.3.2.4 Assembly Instructions: The following section provides a step-by-step assembly guide for constructing the manual system to measure CO2 and ET fluxes, detailing the integration of all components: 1. Prepare outdoor control case (170×110×48 mm); drill two holes into the back wall for mounting the case to the chamber door with screws. Drill an additional hole at the top for the linear actuator cable and one at the bottom for the 9V power input. 2. Install connectors into the case; fit one female DC power jack socket at the top and one at the bottom wall. Install an 8-pin aviation connector on the front wall to connect sensors and fans. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 41 3. Cut hard foam insert for internal compartment; use a hard foam plate and a carpet knife to cut one vertical and one horizontal insert, creating four compartments inside the case. 4. Fit components into the foam compartments a. Microcontroller + data logger shield → upper left b. Step-down converter → lower left c. Relay module → upper right d. MOSFET and aviation connector wiring → lower right 5. Install ATmega328 microcontroller with data logger shield; place in upper-left compartment, ensuring SD card slot remains accessible. 6. Install 2-channel relay module; mount in upper-right compartment and connect: a. IN1 and IN2 → microcontroller digital pins 2 and 3 b. VCC and GND → 5V and GND from microcontroller c. COM1 and COM2 → top DC jack socket (actuator power) d. NC1 to NC2, NO1 to NO2 7. Install step-down DC converter in lower-left compartment Connect: a. IN+ and IN− → 9V input from bottom DC jack b. OUT+ and OUT− → COM1 and COM2 on relay (to power actuator) 8. Wire the MOSFET (air exchange fan control): a. Gate (G) → digital pin 7 via 10kΩ resistor b. Source (S) → GND (common 9V ground) c. Drain (D) → negative terminals of air exchange fans d. Add 200Ω resistor between Gate and GND 9. Mount and wire linear actuator to chamber door: a. Mechanically attach actuator to sliding door b. Route actuator cable through top hole and connect to top DC jack socket 10. Install Bluetooth module (HC-05): a. TX and RX → RX and TX on microcontroller (cross-wired) b. VCC and GND → 5V and GND on microcontroller 11. Install K30 FR and SHT31 sensors; mount sensors in a 3D-printed housing on the inside of the chamber door. 12. Install fans for air exchange and mixing: Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 48 4. connect a female Luer-Lock connection to the 4 mm PTFE tube 5. connect the 2-1 luer-lock stopcock 6. wrap the adhesive joint with transparent adhesive tape (only to prevent the adhesive joint from breaking) 7. wrap the adhesive joint with parafilm (only to prevent the adhesive joint from breaking) 8. secure the round tape under the valve with additional transparent tape The following section provides a step-by-step assembly guide for constructing the dry air supply box, detailing the integration of all components: 1. prepare the toolbox to install all components (with wood or plastic) 2. attach the battery and pump in the box a. Connect the pump and battery with power cables (optionally with a switch that can be attached to the outside of the tool case) 3. prepare the glass bottle with desiccant (silica beads) a. drill a hole for the 1/4" and 4 mm PTFE tube in the lid of the bottle b. measure the 1/4" tube distance (the tube should be placed approx. 5 cm above the bottom of the bottle in the middle) c. Insert a 4 mm PTFE tube into the lid hole to ensure the air supply and prevent the silica beads from escaping d. Insert the 1/4" PTFE tube into the lid and fix the fine-mesh net at the open end in the bottle with a cable tie to prevent the silica beads from entering the tube e. Attach a tube connector to the other end 4. Connect a short piece of PTFE tubing to the pump inlet to connect the bottle with desiccant 5. connect a piece of PTFE tubing to the pump outlet a. Depending on the application, different numbers of outlets can be connected here with T-shaped hose connections (depending on the pump capacity) b. Connect the control valves to regulate the flow to the outlets Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 49 c. Optional: Add a one-way luer-lock stopcock to each outlet to be able to close them if necessary. 2.3.3.5 Calibration and Handling Recommendation: To calibrate the measurements, a three-point standard calibration should be used (similar to liquid water stable isotope measurements). These standards should encompass the expected isotope range of the samples in both directions. Our measurements over an entire cultivation period provided many insights into the handling of the described gas bag approach: 1. Regarding the described dry air supply box, the use should always be tested for the specific application, as a very high flow rate combined with very humid air could greatly affect the duration of possible use. 2. Using the gas bags, the manufacturer states that the valves should not be opened more than one turn (Sense Trading B.V., personal communication, 2024). However, our experience has shown that a quarter to half opening is already sufficient to fill the gas bags reliably. If the gas bags are opened too wide, leaks may occur, and the sample may be contaminated. In addition, great care must be taken not to fill the bags more than 90% to avoid material damage (as specified by the manufacturer). On the other hand, a larger sample is recommended to reduce any effect on the sample. 3. When using the bags in the field, it is necessary to record the source temperature at the corresponding depth during the measurement to be able to convert the isotopic signature of the soil water from vapor to liquid. In addition, it should be ensured that there is no liquid water in the soil probes, e.g. by flushing with dry air beforehand. Furthermore, it is advantageous to fill the bags in a protected box to avoid large temperature differences in the bag during filling (e.g. due to solar radiation in summer) and reduce the risk of damage to the gas bag, e.g. from sharp plant parts. The same applies to transport. 4. When reusing the bags, it was important that 1) the bags were rinsed ten times with dry air, 2) the additional connection including valve was built and 3) the bags and their valves (especially the seals) were regularly checked for damage. To avoid holes Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 50 in the bags due to frequent filling/emptying, areas of the bag that are heavily creased can be reinforced with tape to be on the safe side. 5. The subsequent measurement in the laboratory was easy to perform, but the combination of the vapor storage method with in situ probes requires that the temperature in the laboratory is higher than the source temperature during the measurement. Otherwise, condensation will occur in the bag, which can greatly distort the measurement result. 2.3.3.6 Code: None. 2.3.3.7 References: Dahlmann A, Marshall J D, Dubbert D, Hoffmann M, Dubbert M (2025) Simple water vapor sampling for stable isotope analysis using affordable valves and bags. Atmospheric Measurement Techniques, DOI: 10.5194/amt-2024-43 Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 51 3. Software Solutions 3.1 “MonksHillLab Logger App” 3.1.1 Purpose & Use Case: The “MonkshillLab Logger App” is an Android-based tool developed to support field deployment of low-cost DIY environmental monitoring systems presented in this Handbook. It enables wireless Bluetooth communication between mobile devices and DIY sensors, allowing users to retrieve logged data or trigger new measurements directly in the field. For systems like the handheld NDVI sensor (see 2.2.1), the app is essential, as data are stored solely on the mobile device. For other platforms such as the weather station (see 2.1.3) and the manual system to measure CO₂/ET fluxes (2.3.1), the app usage is optional but provides a convenient alternative to physical data retrieval. Beyond sensor communication, the app includes modes for manually entering field observations—such as leaf temperature or soil moisture. This allows users to directly digitize field data at the point of measurement, improving organization and reducing lateron transcription errors. All functionalities are structured into modular “modes” within the app, which users can select depending on the used low-cost DIY system or measurement task in field. 3.1.2 System Requirements: The “MonkshillLab Logger App” is developed using MIT App Inventor and is currently available as a standalone “.apk” file (~3.7 MB; Appendix C). It is compatible with most Android devices, typically Android 6.0 (Marshmallow) or higher. The app is not available on the Google Play Store and must be downloaded manually (see 3.1.3). For Bluetooth communication, the app is designed to work with HC-05 Bluetooth modules, which operate using the classic Bluetooth protocol. These modules are commonly used in low-cost DIY microcontroller systems and are supported by Android. iOS devices are not compatible with HC-05 modules and are therefore not supported by the current version of the app. To ensure proper functionality, the Android device must support: 1.) Classic Bluetooth (not only BLE) 2.) External storage access, which is needed to store “.csv” files in download folder 3.) Location permission, which is needed to scan for new Bluetooth devices Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 52 3.1.3 Installation & Setup: The “MonkshillLab Logger App” is distributed as a standalone “.apk” file (see Appendix C) and must be installed manually. The file can be transferred to the Android device via USB, Bluetooth, or email/cloud services. To install the app: 1.) Enable “Install unknown apps”; On most Android devices, navigate to: Settings → Apps & notifications → Special app access → Install unknown apps; Then allow your file manager (e.g., “Files” or “Chrome”) to install apps. 2.) Open the transferred “.apk” on the device and confirm the installation prompt 3.) Grant necessary permissions; Upon first launch, the app will likely request: a. Location access (required to scan for new Bluetooth device) b. Storage access (to save generated “.csv” data files to your Downloads folder) 4.) Go to Bluetooth settings on your Android device and pair with the HC-05 device (usually named “HC-05” or similar; Tip: rename your low-cost DIY devices for better organization); default pairing PIN is often 1234 or 0000. Once installed and paired, the “MonkshillLab Logger App” is ready to be used in the field. Users can select a measurement mode, connect to the device, and begin data retrieval or manual entry. 3.1.4 Usage Instructions: The “MonkshillLab Logger App” operates in six distinct modes, each tailored to specific measurement tasks: 1.) Weather Station (EVE): Collects via Bluetooth environmental data from the weather station (see 2.1.3) 2.) CO₂ & ET Sampling (Minion): Connects via Bluetooth to the manual device for CO2 and ET flux measurements (see 2.3.1) to initiate CO₂ and ET flux measurements and associate sampling location identifiers to the collected data. 3.) NDVI Sampling (WallE): Connects via Bluetooth to the handhold NDVI sensor, enabling measurement initiation and adding sampling location identifiers to the collected data. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 53 4.) GC Sampling: Used during NFT-NSS closed chamber measurements for gas chromatography (GC) vial sampling. This mode automatically records sampling date and times, allows selection of vial numbers corresponding to specific time points (t0, t1, t2, t3, t4) sampled, and adds those information together with the sampling location identifiers to the collected data. 5.) Analyzer Sampling: Serves as a backup protocol for multi-gas analyzer measurements by recording start and end times of measurements to facilitate matching gas analyzer data with measured sampling locations. 6.) Multi-Purpose: Supports manual entry of measurements such as leaf temperature or soil moisture. Users can input a numeric value along with the sampling location identifiers; the date and time is automatically recorded and added. For modes 1 to 3, Bluetooth connectivity is required to communicate with the respective systems. 3.1.5 Output Format & Interpretation: Each mode creates a CSV file named according to the mode and the date of data collection, saved in the download folder of the Android device. The CSV files include headers that identify the data columns, facilitating interpretation and further analysis. Subsequent data collected on the same date is appended to the existing file, ensuring continuous and organized record-keeping. 3.1.6 Troubleshooting & Known Issues: This section summarizes common issues encountered when using the “MonkshillLab Logger App” and provides practical solutions to help ensure smooth operation and accurate data collection: 1.) Bluetooth Connectivity: Ensure Bluetooth is enabled on the Android device and the target system (EVE, Minion, WallE) is powered on and within range. Retry connection if connections fail. Also, make sure the devices have been properly paired in the Android Bluetooth settings before. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 54 2.) Zero Values for the handhold NDVI Sensor (e.g. by completely blocking the lower sensor) might results in error warnings by the app as NaN NDVI values are calculated. Confirm proper sensor connection and adequate lighting conditions, and avoid measurements in situations likely to cause invalid readings. 3.) .csv files are not saved: Verify that there is sufficient storage space on the Android device and that the app has permission to write files to your download folder. Additionally, ensure the .csv files are not open in other applications (e.g., spreadsheet editors) during data collection, as this can prevent the app from saving new data. 4.) Confirm the Android device’s date and time settings are correct to ensure accurate timestamps for all recorded measurements. 5.) Double-check all manually entered values and sampling location identifiers to prevent data entry errors. Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 55 4. Real World Use & Regional Hubs 4.1 Philippine Hub – Reuse of Pineapple Residues 4.1.1 Location & Context: Philippine Hub was located on a smallholder pineapple farm near Calauan, Laguna (14°07'49.9"N, 121°18'28.2"E), within the humid tropics of Southeast Asia. Fig. 4.1.1: Deployment of low-cost sensor systems at the Philippine Hub (Calauan, Laguna, 14.13° N, 121.31° E), focusing on the reuse of pineapple residues under different nitrogen fertilizer strategies. (a, b, c, d, e) Field setup and sensor use on a smallholder pineapple farm. (f) Satellite imagery showing the experimental site location. (g) Experimental design comparing multiple residue management and fertilizer treatments. Integrated f) g) Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 56 mini weather stations and manually operated systems were used to monitor greenhouse gas emissions (CO₂), evapotranspiration (ET), and plant physiological responses (e.g., NDVI). From 2022 to 2023, the site hosted a field trial that tested different reuse strategies for pineapple residues in combination with various nitrogen (N) fertilizer forms. The research assessed their impacts on greenhouse gas (GHG) emissions, water dynamics, carbon (C) and nitrogen cycling, as well as crop yield. The trial addressed key challenges in enhancing soil health and productivity in tropical smallholder systems under resource-limited conditions. 4.1.2 Used Systems: The handheld system for measuring spectral plant indices such as NDVI was used alongside the manual CO₂ and ET flux measurement system presented in this handbook. Together, these tools enabled low-cost assessment of plant physiological responses and ecosystem gas exchange under harsh field conditions (e.g., temperatures up to 45 °C, high humidity, and heavy rainfall during throughout the year). 4.1.3 Deployment & Operation: The field trial was a key activity of the BMEL-funded project rePRISING, coordinated by Reena Macagga (PhD student at ZALF and Humboldt-Universität zu Berlin) in collaboration with the University of the Philippines Los Baños and a local farmer. Conducted from 2022 to 2023, the field trial covered a full pineapple crop growth period, lasting nearly 18 months on a smallholder farm near Calauan, Laguna. All measurements—using handheld systems for spectral plant indices and manual gas flux chambers for CO₂ and ET—were carried out on a biweekly basis during intensive field campaigns. Mini climate stations were installed permanently to ensure continuous environmental monitoring throughout the field trial. 4.1.4 Current Status: The field trial has been completed. A key outcome was that the addition of chopped pineapple residues into the soil prior to planting substantially increased yields across all treatments— unfertilized, mineral fertilized, and organically fertilized (chicken manure). During a farmer workshop, participants also noted that pineapples grown with residue addition tasted sweeter, though this observation remains subjective and unverified by compositional Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 57 analysis. Improved plant performance was reflected in higher CO₂ exchange and NDVI values. No consistent differences in evapotranspiration (ET) or water use efficiency (WUE) were observed. A central achievement of this trial was the development and successful testing of both used sensor systems under real field conditions. 4.1.5 References: Macagga R, Sossa G, Ayaribil Y, Bayot R, Sanchez P, Augustin J, Bellingrath-Kimura SD, Hoffmann M (2025) A new, low-cost ground-based NDVI sensor for manual and automated crop monitoring. Smart Agricultural Technology 11, DOI: 10.1016/j.atech.2025.100892 Macagga R, Asante M, Sossa G, Antonijevic, Dubbert M, Hoffmann M (2024) Validation and field application of a low-cost device to measure CO2 and evapotranspiration (ET) fluxes. Atmospheric Measurement Techniques 17/4, DOI: 10.5194/amt-17-13172024 Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 64 Appendix A — Full Code Listings List of complete Arduino IDE scripts: 1. Automatic_NDVI.ino 2. Manual_System_CO2_ET_fluxes.ino 3. EVE_offline_weather_station.ino 4. EVE_online_weather_station.txt 5. Mesocosm_System.ino Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 65 Appendix B — 3D-printing/PCB board files List of 3D-printing files: 1. PAR_Sensor_case.stl 2. NDVI_PRI_Sensor_case.stl 3. Automatic_NDVI_System_case_I.stl 4. Automatic_NDVI_System_case_II.stl 5. Automatic_NDVI_AS7262_3_case.stl 6. Automatic_NDVI_PRI_System_PCB.zip 7. Manual_CO2_ET_System_K30_FR_sensor_case.stl 8. Manual_CO2_ET_System_PCB.zip 9. NiMH_Solar_Trickle_Charger_PCB.zip 10. EVE_PAR_sensor_case.stl 11. EVE_SHT40_case.stl 12. EVE_Weather_station Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 66 Appendix C — Other Software List of Software: 1. MonksHillLab_Logger_APP.apk 2. Dashboard_Backend_Scripts (folder) 3. SQL_Scheme.txt Build-It-Yourself: Low-Cost Systems for Field Ecophysiology 67 Appendix D — Supporting Data List of Data sets: 1. 36h_PAR_comparison_test.csv 2. 48h_PAR_comparison_test.csv 3. EVE_Offline_Field_Deployment.csv 4. EVE_Online_Field_Deployment.csv