Dormitory intelligent fire alarm system based on RS-485 bus

0 Preface

Nowadays, in the dormitory of the school, students are stolen from valuables, fires and other accidents due to student negligence, which are "big problems" that have plagued students, academics and school security offices. Traditional preventive measures have many drawbacks. For example, when an accident occurs, protective iron gates and iron railings will become the biggest obstacle for the owner to escape. The use of intelligent fire alarm system can solve such problems. However, the price of fire prevention and anti-theft system on the market is too high for college students. This system is designed for the university dormitory and is designed from a low cost point of view.

1 system composition and working principle

The system is simulated by a monitoring room and two dorms, and communication is realized through the RS 485 bus. Considering that the monitoring room should be able to monitor the entire dormitory building in reality, the powerful ARM7 chip LPC2103 is used as the host control chip, and the slave machine adopts the low-cost 51 series MCU as the control chip. The pyroelectric sensor is used to detect the human body in the dormitory, the smoke sensor realizes the pyrotechnic detection, the infrared sensor realizes the detection of the number of personnel entering and leaving the dormitory, and the host monitors the slave in real time through the RS 485 bus. When there is an abnormality, the dormitory and the monitoring room The alarm device will sound at the same time. Only the dormitory members can cancel the alarm status by entering the password of the slave in this dormitory. The detailed structure of the system is shown in Figure 1.

2 hardware circuit design and related theoretical analysis

The fire alarm intelligent alarm system hardware is mainly composed of seven parts: the main part part processor circuit, the slave part part processor circuit, the sensor detection circuit part, the sound and light alarm circuit, the RS 485 bus interface circuit, the keyboard interface circuit and the display circuit.

2.1 Sensor detection and sound and light alarm circuit

The sensor module consists of a pyroelectric sensor, a smoke sensor MQ211 and an infrared sensor.

The internal resistance of the smoke sensor changes with the concentration of the smoke, so it is converted into a varying voltage signal, which is formed by the voltage comparator LM339 and several corresponding voltage divider resistors. The specific circuit design is shown in Figure 2. Shown. The internal resistance of the sensor measured under power-on is about 130 kΩ, and the internal resistance is about 6 kΩ when the smoke is thick. When the smoke is smokeless, the negative input of the comparator is about 2.5 V, and the positive end is about 1.2 V. The negative side of the smoke is 2.5 V, and the positive end is 3 to 5 V. This circuit can achieve level conversion well. The pyroelectric infrared sensor is composed of RE200B and signal processing component BISS0001 and a small number of external components. The circuit is shown in Figure 3.

The infrared sensor circuit is composed of an infrared emitting diode and a 1838B. The single chip is used to detect the sequence of the low level of the two sensors to judge the entry and exit of the person. The principle is shown in Figure 4. The sound and light alarm module is composed of buzzer, (red, green) LED and NPN triode drive circuit. The specific circuit diagram is shown in Figure 5.

2.2 RS 485 communication circuit

This module uses the MAX485, a low power, slew-rate transceiver for RS 485 and RS 422 communication. The MAX485's driver slew rate is unrestricted and can achieve transfer rates up to 2.5 Mb/s.

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