Design of projector anti-theft device based on Attiny13

　　1 Introduction

　　With the development of electronic science and technology, the number of projectors purchased by universities, enterprises, and government departments has increased, and they are also required to have anti-theft functions. Design and implement a projector anti-theft device with low power consumption and good anti-theft effect. The built-in battery can work continuously for one year under normal circumstances.

　　2 Hardware Design of Projector Anti-theft Device

　　The projector anti-theft device mainly consists of two parts: one part is the anti-theft device, which is placed on the projector and has an anti-theft function; the other part is an electronic key, which prevents false alarms caused by users replacing batteries.

　　The two parts realize data transmission through custom three-wire communication. The electronic key sends a data volume to the immobilizer through the custom three-wire communication interface, and the immobilizer chooses whether to turn off the alarm function by judging the data volume. Figure 1 is the overall block diagram of the projector immobilizer system.

　　2.1 Anti-theft device

　　Figure 2 is a partial block diagram of the projector's anti-theft device. The design of the anti-theft device mainly considers low power consumption, simple implementation, and complete functions. Attinyv13 is selected as the MCU. The anti-theft device system mainly consists of a custom three-wire interface, an anti-theft circuit, a buzzer alarm circuit, and a battery. When the projector is moved, Attiny13 collects the voltage changes of the anti-theft circuit through the internal A/D converter, and Attiny13 sends a control signal to the buzzer and the wireless module to realize the alarm.

　　Figure 3 is a partial circuit diagram of Attiny13. Attiny13 is a low-power 8-bit CMOS microcontroller based on the enhanced AVR RISC structure produced by Atmel, with the characteristics of high performance and low power consumption. It has 1 K F1ash, 64 bytes of EEPROM, and 64 bytes of SDRAM on the chip. It also has 4 10-bit A/D converters on the chip. The system design chooses Attiny13 as the MCU mainly because of its low power consumption. When working at 1 MHz and 1.8 V, the current consumption is 240μA.

　　Figure 4 is the schematic diagram of the anti-theft circuit of the anti-theft device. This part is completed by using multiple mercury switches and normally closed switches, and at the same time, the internal A/D converter of Attiny13 is used to collect the voltage changes caused by circuit changes to determine whether an alarm is needed. [page]

　　Figure 5 is the burglar alarm circuit. CON2 is the power and ground wire of the 315 MHz wireless module. This module has many manufacturers and is easy to purchase. The power of the buzzer and Attinv13 is provided by 3 button batteries, and the power of the 315 MHz wireless module is provided by a 9 V battery. Both batteries are common batteries in the market, which is convenient for users to purchase and replace. In this part of the circuit diagram, the transistor VQ1 acts as a switch, and controls the power of the wireless module through a universal I/O interface of Attiny13. Because Attiny13 has only 8 pins, a custom three-wire communication is used to consider the reliability of communication. The connection circuit diagram is shown in Figure 6.

　　2.2 Immobilizer electronic key

　　In order to prevent false alarms caused by users disassembling the housing of the anti-theft device to replace the battery, the anti-theft device also designs an anti-theft electronic key. The electronic key is used to turn off the anti-theft alarm function, measure the battery power of the anti-theft device, and remind users to replace the battery in time. The anti-theft electronic key uses ATmega168 as the processor. The main considerations for using this MCU are its low power consumption, simple electronic key design, and easy implementation. The electronic key measures the battery power of the anti-theft device, mainly through the A/D converter on the ATmega168 chip.

　　3 Software Design of Projector Anti-theft Device

　　Figure 7 is the main program flow chart of the projector anti-theft device. After the system is powered on, Attiny13 continuously collects the anti-theft circuit voltage through the internal A/D converter. If the voltage changes, the alarm function is triggered.

　　The custom three-wire communication mainly uses the MCU interrupt pin as the clock signal, 1 general I/O for data transmission and data reception, and 1 general I/O as the trigger interrupt signal.

　　4 Conclusion

　　Taking full advantage of the characteristics of Attiny13 microcontroller, a low-power projector anti-theft device is designed and implemented. The anti-theft device is simple in design and low in cost, and can well complete the projector anti-theft function. The 315 MHz transmitter module is used to realize wireless signal transmission, which can adapt to the anti-theft host produced by many manufacturers. The host is easy to purchase and convenient to use in combination.