Design scheme of CO gas monitor based on TC35 module

1 Introduction

The CO gas concentration monitor is a portable intelligent instrument used to measure the CO content in the relevant ambient air. At present, the most common data communication between domestic CO gas monitors and control centers is through CAN bus, RS-485 bus or RS-232 bus. The communication distance of RS-232 bus is 12 m, and the maximum is 15.4 m; the communication distance of RS-485 bus is 1200 m. The direct communication distance of the CAN bus can reach up to 10 km. But either way has distance limitations. And ultimately determines the fixity of the control center. With the rapid popularization of GSM mobile communication networks and increasingly fierce competition. The GSM module serves as a major GSM network access device. Its applications are becoming more and more widespread, and a variety of applications with promising prospects have been developed. Such as GPS/GSM (SMS) mobile vehicle monitoring and positioning system; computer room monitoring and remote maintenance system: mobility data query, securities trading and information query; data collection system; mobile: POS machine and mobile charging system: mobile IC card telephone, fixed wireless Access system; mobility data and Internet access. However, the short message module has not yet been applied to CO gas detection instruments. This paper proposes a design scheme for a CO gas monitor that communicates with the SMS center through a GSM short message sending and receiving module.

2. TC35 module

The TC35 module is the main functional component of the terminal. It is composed of a GSM baseband processor, a power supply application-specific integrated circuit, a radio frequency circuit and a flash memory. It is responsible for processing audio, data and signals in GSM cellular equipment, and the embedded software part executes Application interface and functions of all GSM protocol stations. The baseband processor contains all analog-to-digital conversion functions of the cellular wireless part to meet the growing requirements of the GSM and PCS cellular user markets. It can support FR, HR and EFR voice and channel coding without external circuits. The radio frequency part is based on SMARTi type circuit. The antenna cable within the module is connected to a GSC type 50Ω connector. The TC35 module is suitable for minimum power GSM cellular equipment. The application part of this cellular device constitutes the Man-Machine Interface (MMI). TC35 can be accessed through the serial port (RS232). TC35 connects to the cellular application part through a 40-pin ZIF connector. The ZIF connector provides application interfaces for control data, audio signals and power lines. The end system operates at 5 VDC. Since the peak current consumption of TC35 can reach 3 A. Therefore, the external voltage stabilizing device must meet the conditions sufficient to provide the rated current. in this terminal. Use LM2596 switching power supply to complete the conversion from 12V to 5V. As the power supply for TC35 terminal. It must be noted. The switching power supply conversion completed by LM2596 requires high-power inductors and capacitors. to improve energy storage capacity. Meet the power consumption requirements of TC35.

3. Structure of CO gas monitor system

The structure of a portable CO gas concentration monitor with GSM short message sending and receiving function is shown in Figure 1. The monitor developed by the author is mainly used for monitoring CO concentration in the air in public places and some production workshops. Use battery as power supply. The CO sensor N1 uses the NAP-505 electrochemical sensor produced by Japan's Nemoto Specialty Chemicals Co., Ltd. The sensor output current has a linear relationship with CO gas concentration (UOUT=70 nA±10 nM/l×106). A1 (OP90) can ensure that the working electrode and reference electrode are at the same potential. The sensor outputs OμA~70μA current and is converted into 0 V~O by Aa (OP90). A voltage of 7 V to ensure that the output of A3 is 0 V to 2.5V when the CO concentration is 0 to 10-3 to meet the requirements of the MD converter U. I (ADS7822) input requirements. OP90 has an internal zero adjustment circuit. Allows the instrumentation amplifier to provide true zero-input, zero-output operation. The temperature characteristics of NAP-505 are compensated by an NTC thermistor with a constant B of 3435 K. After temperature compensation. Its output can meet the accuracy requirements within the range of 10℃~50℃. The reference voltage of Ul (ADS7822) is connected to Vcc through a power filter composed of a 5 Ω resistor and capacitor. Filter out the high-frequency noise of the power supply itself. The data converted by A/D is sent to U2 (89LV/51) microcontroller and stored in U7 (AT24C08). The LCD display module of the CO gas concentration monitor can display the concentration value and concentration change curve. And communicates with the control center through the short message sending and receiving module.

4. Microcontroller control of TC35 module

This system only uses the short message sending and receiving function. Therefore, only the data input and output interfaces of TC35 are used. It should be noted that the TC35 is connected as a data communications equipment (DCE), not as a data terminal equipment (DTE) like a general modem. As shown in Figure l. The BXDO of TC35 corresponds to the microcontroller. RXD, TXD0 of TC35 corresponds to rXD of the microcontroller. The TE35 data interface works at CMOS level (2.65 V), and the microcontroller performs level conversion on the control and communication signals of FC35. In this system, level conversion is completed through the 7404 type OC gate circuit. After the system is powered on, in order to put the TC35 into working condition. A low pulse with a delay greater than: 100 ms must be added to the IGT. The duration of the level drop cannot exceed 100 ms. IGT should remain high (3.3 V) after startup. When driving IGT, the supply voltage of TC35 cannot be lower than 3.3V, otherwise TC35 cannot be activated. The data input/output interface of TC35 is actually a serial asynchronous receiver and transmitter, which complies with the ITU-RS232 interface standard. It has fixed parameters: 8 data bits and 1 stop bit. No check digit. Baud rate is optional from 300 b/s to 115 kb/s. The author set the system to 9600 b/s. The ZIF connector provides 6 pins for the SIM card interface. Among them, CCIN is used to detect whether the SIM card is inserted properly. The SYNC pin of the connector controls the status of the light. Use this to determine the working status of TC35. The microcontroller controls the short message sending and receiving subroutine. The process is shown in Figure 2. in. The initialization work includes setting the serial port speed, wireless network login and setting the SMS mode to PDU mode.

5. Implementation of SMS sending and receiving

TC35 uses AT Hershey instructions. The microcontroller can initialize the reception and sending of short messages to the TC35 module through correct AT instructions. There are three modes for controlling short messages: Block mode, PDU mode and Text mode. Using Block mode requires driver support from the manufacturer. Currently, PDU mode has replaced Block mode, and Text mode does not support Chinese. Therefore, this system uses PDU mode to receive and send short messages. After starting up, TC35 automatically sends ~SYSSTART to the microcontroller, indicating that the module can work normally, and then controls the short message module through the following AT commands: Network registration: AT+CREG? Set to PDU mode: AT+CMGF=0 Send SMS message (PDU) mode: AT+CMGS=019 0DOA0891683108401505f011000d91683167866040f20008a9044f60597dla (convert it to ASCLL code when sending). The microcontroller sends AT+CMGL; the O0D 0A query command can query whether there are unread short messages. The return format is as follows:

+CMGL：2，0，，260891683108401505F0240D91683167866040F20008507032418095000665E94EOA597DOK

The received data format is explained as follows: +CMGL: 2, 0,, ​​26, (2: second message; 0: unread); 26: length of short message

6. Conclusion

The advantages of the CO gas monitor controlled by the TC35 module are that the control center is mobile, has wide coverage, good real-time performance, high reliability, and good confidentiality. Using the GSM network as a wireless transmission network, the short message service is economical and suitable for the management of remote mobile monitoring center stations. It has a wide range of application prospects.