## 2 Structure of the Monitoring and Control System
### Sensor Node Design and Implementation  
#### 2.2.1 Infrared CO\({}_{2}\) Sensor Design and Implementation  
The structure of a CO\({}_{2}\) sensor node is shown in Figure 2. In order to ensure accurate CO\({}_{2}\) fertilization, the CO\({}_{2}\) sensor was designed and implemented based on infrared technologies with waterproof membranes prevention. The fabricated CO\({}_{2}\) sensor includes two main parts, which are the optical part and electrical part. In the optical part, the used heat source (IR55, HawkEye, Milford, CT, USA) was fabricated with micro-electromechanical system (MEMS) technology and covering the required wavelength range. The pyroelectric detector (LIM262) (InfraTec, Dresden, Germany), equipped with two inherent filters, has a detectivity (D*) of \(3.5\times 10^{8}\) cm\(\sqrt{\rm Hz}/\rm W\) and receives the light reflected by a special spherical mirror, which was also used as a light-collector. In the electrical part, firstly, the IR heat source is driven by a desired signal. This signal was generated by micro-controller to MOSFET (Metal-Oxide-semiconductor Field-Effect Transistor). In consideration of it being fast switching and low on resistance, IRF830 SPbF MOSFET was selected. Secondly, the generated signals by the detector are processed by a pre-amplifier (PA), a band-pass filter (BPF), a lock-in amplifier, and an analog-to-digital convertor (ADC). In the detection part, the pre-amplifier, whose core chip is INA116 (Texas Instruments, Dallas, TX, USA), plays a crucial role in suppressing noise and improving the signal-to-noise ratio (SNR). In the lock-in amplifier, the micro-controller was used to realize the digital phase shift function. The AD630 (Analog Device Inc., Norwood, MA, USA) works as the multiplier for the two-channel signals, with same frequency and phase. In the noise filter selection, an 8 th-order Butterworth filter MAX291 (Maxim integrated, San Jose, CA, USA) was prioritized. In order to further improve the accuracy of digital-analog conversion, the low power consumption, 16-bit ADC MAX1416 (Maxim integrated, San Jose, CA, USA) was used to replace the internal 12-bit ADC of the micro-controller. This chip owns an inherent \(\Sigma\)-\(\Delta\) with a digital filter. Finally, the measured data is packaged and sent to a 433-MHz wireless module. In the design and implementation of hardware, besides the basic function and performance, factors including energy consumption, scalability, and cost-effectiveness were significant reference indicators. For measurement of environmental factors, a temperature and relative humidity multi sensor (type: SHT15, Sensirion, Laubisruetistrasse, Switzerland) was selected due to advantages including calibrated digital output, high reliability, and excellent long-term stability based on industrial CMOS  
Figure 2: Structure of a CO\({}_{2}\) sensor node using NDIR (non-dispersive infrared) technology. processes with patented micro-machining (CMOSens(r) technology, Laubisruetistrasse, Switzerland), which is characterized by the fusion of the sensor element and the signal conditioning electronics on a single silicon CMOS chip. Under this technology, a \"micro-machined\" finger electrode system with different protective and polymer cover layers forms the capacitance for the sensor chip, which protects the sensor from interference. A digital 16-bit ambient light sensor (type: BH1750 FVI, ROHM, Tokyo, Japan) was selected for luminance measurement covering the desired ambient range from 400 nm to 710 nm, with a standby current low of up to 1.0 \(\upmu\)A. Before the light propagates to the detector, the light was filtered by two filter windows, corresponding to the detection channel at 4.26 \(\upmu\)m and reference channel 4.00 \(\upmu\)m. The detection channel and reference channel correspond to the strong absorption region of CO\({}_{2}\) and nearly no absorption region of CO\({}_{2}\), respectively. Because of the same background conditions, such as transmission path and light source, the environmental interference could be suppressed.