The answering circuit of the access control system designed with AT89C51

　　The RFID system is mainly composed of hardware devices such as readers, antennas, transponders, and software such as data acquisition and processing middleware. The transponder hardware part in this system can be specifically divided into: antenna, power circuit, receiving and sending circuit, control circuit, and memory.
　　
　　The following introduces the transponder circuit design that constitutes the access control RFID system.
　　
　　1. Design of transponder antenna
　　
　　Like the reader antenna, the transponder antenna is also replaced by a 10-turn coil with a diameter of 6.6cr±0.5cm. The antenna loop of the transponder is also required to work in a resonant state. By the inductor

　　2. Design of transponder power supply circuit
　　
　　This system requires the transponder to be passive. As mentioned above, the energy required for the entire transponder circuit to work must be obtained from the antenna, so the design of the power supply circuit becomes the key to the transponder design. The controller and peripheral circuits in the transponder require a DC power supply of about Sv. Therefore, obtaining a 5v DC power supply from the antenna design becomes the core content of the entire power supply circuit design. The circuit design is shown in the figure below.

　　The signals from the antenna are all AC components. To obtain the DC component, they must first be rectified by a rectifier bridge. Because the antenna operates at a high frequency , the diodes for the bridge rectifier should all be high-frequency diodes. In order to obtain a more stable 5V power supply V CC , the convenient adjustable shunt reference chip TIA31 is selected. It contains a 2.5V reference voltage . When the output feedback is introduced at the REF terminal, the device can control the output voltage through a wide range of shunts from the cathode to the anode. When the resistance values ​​of Rl and R2 are determined, the two introduce feedback to the voltage division of Vo. If Vo increases, the feedback amount increases, and the shunt of TL43l also increases, which in turn causes Vo to decrease.
　　
　　Obviously, this deep negative feedback circuit must be stable when VI is equal to the reference voltage. Vo=(l+Rl/R2)VI(ref). Selecting different values ​​of Rl and R2 can obtain any voltage output in the range of 2.5V to 36V. When R1=R2, Vo=5V. It should be noted that when selecting resistors , the necessary condition for TL431 to work must be met , that is, the current through the cathode must be greater than 1mA. Here, R1=R2=10kΩ is selected. From the formula Vo=(l+Rl/R2)V1(ref), it can be seen that vcc can be more accurately controlled to 5v, and it also plays a role in voltage stabilization.

　　3. Design of transponder receiving circuit
　　
　　For the receiving circuit, the same receiving circuit as the reader cannot be used directly due to energy issues. A circuit with low power consumption and as simple as possible must be considered.
　　
　　Like the signal received by the reader, the signal received by the transponder is also demodulated according to the change of energy on the antenna , so it also needs to be detected. The connection between the detection circuit and the antenna is shown in the figure below. The detection circuit is the same as the reader detection circuit, and the functions of each component are the same.
　　

　　After detection, the most critical thing is how to design a low-power and simple circuit to obtain the required signal from the detection signal. Here, a clever use of the chip CD4069 is adopted, as shown in the figure above. CD4069 consists of 6 low-power CMOS inverter circuits, and the transmission characteristics of the CMOS circuit are used here, as shown in the figure below. A 1MΩ resistor is connected in parallel between the input and output ends of one of the inverters. Since the resistor is a linear element, its characteristic curve intersects with the transmission characteristic curve of the CMOS circuit at one point. This point is the static operating point of the circuit, as marked in the figure below. It can be seen that the static operating point is in the linear turning area of ​​the CMOS circuit characteristic curve. As long as the signal has a slight change, the circuit will have a high gain output, that is, the signal is amplified. Then it is output through an inverter, so that the signal is in phase with the detection input signal and also has a certain shaping effect on the signal, so that the signal sent from the sjjs end can be directly read by the microcontroller . C501 is a coupling capacitor . Resistor R11 is used to adjust the potential of the output terminal when it is static. The connection method in the above figure makes the output voltage of the output terminal a negative value when it is static, and adjusts the pulse waveform obtained after adding the signal. The size of R11 can be determined according to the required waveform (usually a few megohms).

　　4. Design of the transponder transmission circuit As mentioned above, the modulation
　　
mode 　　used in this system is load modulation. For this reason, the transponder must change its own impedance to transmit the signal . The circuit design is shown in the figure below. 　　The entire transmission circuit uses only one NPN transistor, which is used to control the load size of the entire transponder. The TZXH termination controller outputs the modulation signal. When the modulation signal is "0" ( low level ), the transistor is in the cut-off state, and the load of the antenna is equal to the impedance of the entire circuit. When the modulation signal is "1" (high level), the transistor is turned on, and the two ends of the antenna are equivalent to grounding. The earth becomes the load of the entire antenna, which reduces the Q value of the circuit. It is through this process that the energy size of the transponder antenna coupled from the reader antenna is changed, and then load modulation is realized to complete the signal transmission.
　　

　　5. Design of control circuit and memory
　　
　　Because the energy required for the transponder is obtained by antenna rectification and voltage stabilization, the transponder should choose low-power chips as much as possible, considering the efficiency of the antenna and the energy demand of the entire circuit. For this reason, the controller of the transponder part selects AT89C2051 (2.7V-6V) with low power consumption and large voltage operating range. Its external crystal oscillator also selects 6M Hz . The connection method is the same as the reader. Because the transponder can only work normally when it enters the reader's working range, the single-chip microcomputer only needs to be powered on and reset. In addition, the transponder should have a storage function. Considering the energy problem, the low-power storage chip CAT24WC01 with a voltage operating range of 1.8V-6V is selected. Its pin arrangement is shown in the figure below.

　　
　　As shown in the figure below, the specific connection method of each pin of the controller is: P3.0 port is connected to the 24C01 clock control terminal SCL, and P3.1 port is connected to the 24C01 data transmission and reception terminal SDA. P1.5~P1.7 are respectively connected to the address control terminals AO~A2 of 24C01; P1.4 port is used for data reception and is connected to the sjjs terminal of the transponder receiving circuit; P1.3 port is used for data transmission and is connected to the TZXH terminal of the transponder transmitting circuit; the power supply Vcc is connected to the Vcc terminal of the power supply circuit.
　　
　　Conclusion: Through the design and research of the circuit system of the transponder and the experiments, the expected effect has been basically achieved.