Design of a smoke sensing alarm circuit with emergency lighting function

In modern high-rise buildings or public places, the establishment of various safety equipment is essential. Smoke alarms and emergency lighting are two common safety equipment. Smoke alarms are generally composed of smoke sensors and smoke detection chips. Emergency lighting generally consists of a set of charging and discharging circuits connected to the indoor power supply. In buildings, they all need to be wired separately, which wastes resources and affects the appearance. If there is a product that can realize the emergency lighting function and take into account the smoke sensing alarm, then the problem is solved, as shown in Figure 1.

1 System Design

Since the system mainly realizes the functions of emergency lighting and smoke alarm, the system can be divided into several main parts and designed separately. The following is the system block diagram. The MSP430 microcontroller is used as the core control unit of the entire circuit, and it controls other modules to complete various operations. The power supply circuit uses an integrated three-terminal regulator, which not only completes the charging of the battery, but also meets the normal operation of the system. Many modules of the system are easy to use mature application examples, which will not be repeated here. The following focuses on the design of several important modules.

2 Central Processing Unit

This article selects the MSP430 microcontroller as the core device. The MSP430 series microcontroller is a 16-bit ultra-low power hybrid digital signal processor launched by TI. It integrates many analog circuits, digital circuits and microprocessors on one chip to meet the actual application needs. MSP430 is a 16-bit microcontroller based on the reduced instruction set (RISC) structure. When using an 8MHz crystal oscillator, the instruction speed can reach 8MIPS. The power supply of MSP430 microcontroller adopts 1.8～3.6V low voltage. The power consumption is only 0.1μA in RAM data retention mode, and the maximum leakage current of I/O input port is only 50nA.

MSP430 has normal working mode (AM) and 5 low power working modes (LPM0～LPM4). When the power supply voltage is 3V, the working current of each mode is AM: 340μA, LPM0: 70μA, LPM2: 17μA, LPM3: 2μA, LPM4: 0.1μA. The microcontroller can easily switch between various working modes. The ultra-low power consumption of MSP430 makes it show excellent characteristics in applications with very demanding power supply requirements such as alarm systems and portable devices. The MSP430 series of single-chip microcomputers combines TI's high-performance analog technology. Each member integrates a rich set of on-chip peripherals (depending on the model). These peripherals include multi-channel high-precision ADC and DAC, on-chip analog comparator, multiple timers with PWM function, on-chip USART, hardware multiplier, LCD driver, watchdog timer (WDT), on-chip digital control oscillator (DCO), a large number of I/O ports and large-capacity on-chip memory, etc. A single chip can meet the needs of most applications. The high integration of MSP430 allows application personnel to easily design a true single-chip system without spending too much energy on various peripherals.