# LR-MPIBS: A LoRa-Based Maritime Position-Indicating Beacon System
## 2 Materials and Methods
### Design and Implementation of System Hardware
#### 2.2.2 Falling-Water-Detection (FWD) Module of the Position-Indicating Terminal  
The FWD module uses the conductive properties of seawater to detect drowning persons. The charging interface pins and FWD detection pins are exposed outside the PIT protective casing and come into contact with seawater. If the positive pole of the power supply is selected as the detection pin, it can form a cascaded circuit with the charging interface and generate interference current when a person falls into the water. Meanwhile, the power supply voltage fluctuates due to electromagnetic interference from seawater. The negative pole of the power supply is used as a detect pin instead of the positive pole, which can avoid the issues mentioned above. According to [PERSON]'s Voltage Law, seawater is a conductive medium that participates in the circuit's voltage division, resulting in an increased reference voltage in the detection circuit. Additionally, this voltage varies with different marine areas. The PIT trigger's automatic startup needs the voltage to drop below 0.85 V for more than 3 s. Therefore, randomness exists in using the negative pole as the detect pin for automatic startup in the FWD module. The FWD circuit shown in Figure 3 is designed to solve this randomness problem. After personnel fall into the water, the output voltage of U1 is controlled to a low level, and PIT triggers an automatic start. At the moment of startup completion, the output voltage of U1 is constantly controlled at a high level to ensure the stable operation of PIT.  
The emitter terminal (FWDP1) of transistor Q1 and the negative pole of power supply are selected as the trigger pins of FWD. During the immersion detection process, FWDP1 and GND will conduct and control Q1 to a saturation state [22]. The Q1 collector reference voltage exhibits instability due to changes in sea salinity values, meaning it cannot be directly used as a trigger pin. The voltage comparator (U1) is connected to the Q1 post stage to ensure that the output voltage of the control U1 (TAS) is always below 0.85 V in the Q1 saturation state.  
The switch transistor Q3 is added to the front stage of Q1 to prevent the frequent triggering of the automatic startup signal caused by personnel floating on the sea surface. After the start of PIT, Q3 enters a saturation state and controls Q1 to a cutoff state to ensure that the output voltage of U1 remains constant at a high level during the running of PIT.  
Equation (1) is used to calculate the circuit running current:  
\[I_{R}=\frac{3.3-U_{bv3}}{R_{20}}+\frac{V_{BAT}-U_{cc3}}{R_{12}+R_{25}}+10 \tag{1}\]  
\(U_{bv3}\) is the Q3 base-emitter conduction voltage drop, which has a value of 0.65 V. \(U_{cc3}\) is the Q3 collector-emitter saturation voltage drop, which has a value of 0.6 V. The resistance values of \(R_{25}\), \(R_{20}\), and \(R_{12}\) are in the range of 30-60 k\(\Omega\), and the operating current of U1 is 10 uA. The operating current of the FWD (\(I_{R}\)) is 90 uA. In the PIT shutdown state, the current consumption of FWD is the cut-off current \(t_{cb0}\) of Q3 and the operating current of U1, with a value of 10.1 uA.