CO Gas Monitor Research
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2. The NL-2007 module NL-2007 module is the main functional component of the terminal. It is composed of a GSM baseband processor, a power supply integrated circuit, a radio frequency circuit, and a flash memory. It is responsible for processing audio, data, and signals in GSM cellular devices. The embedded software part performs the functions of the application interface and all GSM protocol stations. The baseband processor contains all the analog-to-digital conversion functions of the cellular radio, in order to meet the growing demands of the GSM and PCS cellular subscriber markets. FR, HR and EFR voice and channel coding can be supported without external circuitry. The RF part is based on the SMARTi type circuit. The antenna cable inside the module is connected to the GSC type 50Ω connector. The NL-2007 module is suitable for the smallest power GSM cellular device. The application part of this cellular device constitutes a Human Machine Interface (MMI). The NL-2007 can be accessed through the serial port (RS232). The NL-2007 is connected to the cellular application via a 40-pin ZIF connection. The ZIF connector provides an application interface for controlling data, audio signals and power cables. The working voltage of the terminal system is 5VDC. Since the burst current consumption peak current of the NL-2007 can reach 3A, the external voltage regulator device must be able to provide the rated current. In this terminal, the LM2596 switching power supply is used to complete the conversion from 12V to 5V. As a power supply for the NL-2007 terminal. It must be noted that the LM2596's switching power supply conversion requires high-power inductors and capacitors to increase the energy storage capacity to meet the power requirements of NL-2007. 
3. Structure of CO Gas Monitor System The structure of a portable CO gas concentration monitor with GSM short message sending and receiving functions is shown in Figure 1. The monitor developed by the author is mainly used for the monitoring of CO concentration in the air in public places and certain production workshops. The battery is used as the power supply. The CO sensor N1 uses the NAP-505 electrochemical sensor manufactured by Japan Genchi Special Chemicals. The output current of the sensor has a linear relationship with the CO gas concentration (UOUT = 70 nA ± 10 nM/l x 106). A1 (OP90) ensures that the working and reference electrodes are at the same potential. Sensor output OμA ~ 70μA current through Aa (OP90) converted to 0V ~ 0.7V voltage, to ensure that when CO concentration 0 ~ 10-3 when the output of A3 is 0V ~ 2.5V to meet the MD converter UI (ADS7822) Input requirements. The OP90 has an internal nulling circuit that allows the instrumentation amplifier to provide true zero-input zero-output operation. The temperature characteristics of the NAP-505 are compensated with an NTC thermistor with a constant B of 3435K. After the temperature is compensated, the output can meet the accuracy requirements within a range of 10°C to 50°C. The reference voltage of Ul (ADS7822) is connected to Vcc through a 5 Ω resistor and capacitor power filter, which filters out the high-frequency noise of the power supply itself. The A/D converted data is sent to a U2 (89LV/51) microcontroller and stored in U7 (AT24C08). The liquid crystal display module of the CO gas concentration monitor can display the concentration value and concentration change curve and communicate with the control center through the short message transceiver module.
4. MCU control of NL-2007 module This system uses only short message sending and receiving functions. Therefore, only NL-2007 data input and output interfaces are used. It should be noted that NL-2007 is connected as a Data Communication Equipment (DCE) rather than as a normal modem as a Data Terminal Equipment (DTE). As shown in Figure l. NL-2007 BXDO corresponding to the microcontroller. RXD, TXD0 of NL-2007 corresponds to rXD of the one-chip computer. The TE35 data interface operates at CMOS level (2.65V) and the microcontroller performs level translation of the FC35 control and communication signals. In this system, the level conversion is accomplished through the Type 7404 OC gate circuit. After the system is powered on, in order for NL-2007 to enter the working state, a low pulse with a delay greater than: 100ms must be added to the IGT. The duration of the level drop cannot exceed 100ms. The IGT should remain high (3.3V) after startup. The NL-2007 supply voltage must not be lower than 3.3V when driving the IGT, otherwise the NL-2007 cannot be activated. The data input/output interface of the NL-2007 is actually a serial asynchronous transceiver conforming to the ITU-RS232 interface standard. It has fixed parameters: 8 data bits and 1 stop bit. No parity bit. The baud rate is 300b/s~115kb/s. The author set the system to 9600b/s. The ZIF connector provides 6 pins to the SIM card interface. CCIN is used to detect whether the SIM card is plugged in. The SYNC pin of the connector controls the status of the lamp. To judge the working status of the NL-2007. SCM control short message transceiver subroutine. The process shown in Figure 2. among them. The initialization work includes setting the serial port rate, wireless network login, and setting the short message mode to PDU mode.
5. Implementation of SMS Transceiving NL-2007 adopts AT Hayes directive. The MCU can receive and send initial messages through the correct AT command to the NL-2007 module. There are 3 modes for controlling short messages: Block mode, PDU mode, and Text mode. Using the Block mode requires the manufacturer to provide driver support. Currently, PDU mode has replaced Block mode, and Text mode does not support Chinese. Therefore, the system uses PDU mode to receive and send short messages. After NL-2007 starts, it will automatically send ~SYSSTART to the MCU to indicate that the module can work normally. Then the short message module is controlled by the following AT command: Network registration: AT+CREG? Set to PDU mode: AT+CMGF=0 Send SMS information (PDU) mode: AT+CMGS=0190DOA0891683108401505f011000d91683167866040f20008a9044f60597dla (transmit it to ASCLL code). The single-chip microcomputer sends AT+CMGL; the query command of O0D0A can check whether there are unread short messages. The return format is as follows:
+CMGL:2,0,,260891683108401505F0240D91683167866040F20008507032418095000665E94EOA597DOK
The format of the received data is explained as follows: +CMGL:2,0,,26,(2:2nd bar; 0:unread); 26: Length of short message 6ã€Conclusion The control center has movable, wide coverage, and real-time performance. Good, high reliability, good confidentiality. Taking the GSM network as the wireless transmission network, the short message service is economical and is suitable for the management of remote mobile monitoring central stations. It has a wide range of application prospects.