With the development of science and technology and economy in China and the adjustment of national energy policy, the mode of “central heating and household measurement†has become the preferred mode in urban heating systems, so heat meters have been widely used in heating systems. However, because the heat metering charge of this model involves the direct interests of thousands of households, the heat use situation is complicated, which makes it more difficult for the heat company to charge. MFRC522 is a new non-contact IC card read/write base station chip from Philips, which is used in the 13.56 MHz contactless communication system. The MF RC522-based heat meter pre-paid module designed in this paper uses the RF IC card as the information carrier to store the purchased heat. The heat meter reads the data in the card and then clears the data in the card, thus realizing the “pre-purchase heatâ€. The consumption pattern after the use of heat has created conditions for solving the problem of heating charges, improving residents' awareness of saving and improving resource utilization.
1 Prepaid module composition and working principle
A heat meter is a meter used to measure and display the energy released or absorbed by water flowing through a heat exchange system. It calculates the heat released by the system by collecting the temperature of the water inlet, the water outlet, and the water flow. The prepaid module is set up to achieve "pre-purchase heat, after-use heat", and the prepaid module based on MFRC522 is shown in Figure 1.
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System working principle: When the card reading operation is required, the single-chip microcomputer wakes up the low-power mode from the low-power mode. After receiving the valid card reading information, the MCU opens the power of the RF card reading and writing circuit and requests the IC card to enter the RF field. When it is judged that there is an IC card of the system, communication is performed, the calorific value in the card is read, added to the original calorific value of the heat meter, and the purchased calorific value and the existing calorific value are stored, and the IC card is placed in the IC card. The calorific value is cleared, and the IC card number is displayed on the LCD display, the heat is purchased, and the heat is expressed in the heat. [1]. If the heat value in the heat meter is 0 before reading the IC card, the existing value is read. If the heating value is greater than 0, the power of the valve control circuit is turned on, and the operation of opening the water inlet of the heating pipe is performed. When the microprocessor detects that the valve is effectively opened, the power of the valve control circuit is turned off. The system turns off the power of the RF read/write module after scanning the RF field for a period of time.
When the existing calorific value in the heat meter is equal to 0 KJ, the system will open the power of the valve control circuit, perform the operation of closing the inlet valve of the heating pipe, and when the microprocessor detects that the valve is effectively closed, the power of the valve control circuit is turned off. .
2 hardware design
The hardware of the heat meter prepaid module is mainly composed of a microprocessor, an IC card reading and writing circuit, a valve driving circuit, a button and a display circuit.
2.1 microprocessor
Since the heat meter is a battery-powered instrument, one battery needs to work for more than five years, which imposes strict requirements on the low-power performance of the microprocessor. The MSP430F149 is a 16-bit ultra-low-power MCU from TI. Its supply voltage range is 1.8~3.6 V. The operating current is only 0.8 μA in sleep mode. It only takes 6 μs to wake up from sleep mode. It has 6 data ports, P1. And P2 port can be used for external interrupt processing, 2 serial communication modules, and both serial ports can be set to UART mode or SPI mode through software. 5 energy-saving modes are very suitable for use in ultra-low power products [ 2]. Therefore, MSP430F149 is selected as the microprocessor of this system.
2.2 RF IC card read and write circuit
The main function of the RF IC card read/write circuit is to complete the identification of the IC card, read the data in the card, and clear the data in the card. The circuit is mainly composed of the MFRC522 base station chip and its matching antenna. MFRC522 is a new non-contact low-power read-write base station chip from Philips. The card reader chip fully integrates all types of passive contactless communication methods and protocols at 13.56 MHz, supporting all layers of ISO14443A [3] It can easily read Mifare 1 S50, S70 and other cards. It has three interface modes: SPI mode, UART mode and I2C mode, which can communicate with various types of MCUs.
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MSP430F149 has SPI interface, so the communication between the RF base station chip and the microprocessor in this system adopts SPI mode, adopts the typical application circuit shown in Figure 2 [4]. The circuit is mainly composed of power supply control circuit and low-pass filter circuit. The receiving circuit and the antenna matching circuit are composed. This design antenna uses a direct matching antenna method. The power control circuit (R15, R16, R17, Q5, Q12) makes the on/off of the RF circuit power supply in a controllable state, and only supplies power to the RF IC card circuit when the card needs to be read, and does not need to read the card RF IC card circuit. The power supply is off to reduce system power consumption. The low pass filter circuits (L1, C17; L2, C20) are provided to suppress high-order harmonics generated by the crystal oscillator in the system circuit, and the receiving circuits (R25, R26, C13, C14) are used to receive the return signals, The values ​​of the resistors, inductors, and capacitors are recommended. The antenna matching circuit (C15, C18, C19; C23, C22, C21) is used to tune the antenna to the optimum. When selecting the value, the capacitor should be considered according to the length and width of the designed antenna and the inductance of the antenna. value.
2.3 valve drive interface circuit
The valve control circuit and the low-voltage electric valve cooperate with the IC card read/write circuit to realize the pre-paid function, which is the execution mechanism of the pre-paid system, and completes the opening and closing control operation of the heating pipe. The circuit is powered by a separate power supply, and the microcontroller only needs to provide relevant operating signals. Mainly composed of power circuit, drive circuit and low voltage electric valve, as shown in Figure 3.
The low-voltage electric valve uses the electric copper ball valve of Tianjin Zhufeng Valve Factory. The hot water working temperature is 0~100 °C, the working voltage is DC 3 V~6 V, and the valve control leads to 4 lines, which are the valve opening line and the off. Valve, ground and feedback lines are ideal for this system. The working process is as follows: when the valve is opened and the ground wire is connected to the power supply, the valve automatically rotates to the fully open position and the internal power is automatically cut off; when the valve line and the ground wire are connected to the power supply, the valve automatically rotates to the fully closed position and then the internal Automatic power-off; when the valve is opened or closed, the feedback line will output a high level; when the valve is opened or closed, the feedback line outputs a low level, so that it is very simple and efficient to control with a single-chip microcomputer.
The power supply part of the valve control circuit is mainly composed of a battery and a power supply voltage detecting circuit, and the power supply is powered by a 3.6 V battery. Since the minimum operating voltage of the motor is 3 V, the voltage detection chip of 3.1 V is used. When the power supply voltage is lower than 3.1 V, the microcontroller is interrupted, the system will close the inlet valve, and the user is prompted to replace the power of the valve control circuit.
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The driving circuit mainly controls the rotation direction of the valve motor. When the valve opening or closing operation is required, the corresponding circuit can be opened. When it is detected that the valve is open or closed, turn off the power to the circuit to reduce power consumption. As shown in Figure 3, the valve closing circuit (Q2, Q7, Q11) is controlled by the CLO_M terminal. When CLO_M is high level, Q2, Q7, and Q11 are turned on, the valve closing end is high level, and the valve opening end is low. Level, perform the valve closing operation. When it is detected that the level of the feedback terminal becomes low, the CLO_M terminal is low, and the power is turned off; the valve opening circuit (Q9, Q10, Q3) is controlled by the OPEN_M terminal, and the working process is similar to the valve closing process.
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