Implementation of 3V photoelectric smoke sensor based on RE46C190

Photoelectric smoke detectors are one of the main detectors currently used in fire protection. There are two main ways to implement traditional photoelectric smoke detectors: based on a single-chip solution or using a dedicated ASIC solution. Dedicated ASIC chips are widely used due to their high integration and the small number of peripheral devices required. Currently, general-purpose ASIC chips on the market are generally powered by 9V, and the battery cost is relatively high. At the same time, most chips use peripheral discrete components to set related parameters, and the production and debugging costs are also relatively expensive. Microchip's industry-first 3V programmable photoelectric smoke sensor chip RE46C190 for photoelectric smoke sensors integrates smoke detection and boost drive functions on a single chip, and has programmable calibration and multiple working modes, which greatly simplifies the research and development and manufacturing of photoelectric smoke sensors. At the same time, the chip has low power consumption and can work stably for up to 10 years using a single lithium battery.

The photoelectric smoke sensor is an "active" detector. Its working principle is shown in Figure 1. When there is no smoke, the photoelectric receiving diode cannot directly receive the light signal emitted by the infrared light-emitting diode. When a fire occurs, the smoke diffuses into the maze of the detector and scatters the light emitted by the infrared light-emitting diode, so that the photoelectric receiving diode can receive the scattered light signal. The size of the light signal indicates the smoke concentration and the degree of fire burning.

Figure 1. Principle of photoelectric smoke detector

The smoke sensor main control chip identifies the output signal of the photoelectric receiving diode and amplifies it, and ensures that the alarm signal is triggered when the signal reaches a certain intensity, that is, when the smoke reaches a certain concentration. Figure 2 shows the internal functional block diagram of RE46C190. Among them, the driving current of the infrared photodiode, the amplification factor and integration time constant of the photoelectric tube amplifier, the smoke alarm threshold value, and the battery low voltage alarm threshold value can all be programmed, which provides great convenience for different designs. At the same time, the chip also provides personalized function settings such as alarm sound options, mute options, ten-year expiration alarm options, and baseline drift correction options, so that the same design can be applied to different product series. Since RE46C190 is powered by 3V, the device also integrates a boost controller to provide the voltage required for the buzzer alarm drive.

Figure 2. RE46C190 internal functional block diagram

Figure 3 shows the principle block diagram of the photoelectric smoke detector using RE46C190. The figure includes complete photodiode drive, photodiode signal processing, boost drive, alarm indication drive and networking functions. The circuit includes two status indication LED drivers, where the drive current provided by RLED is used to indicate low battery voltage, smoke alarm and silent mode; GLED is used to indicate the alarm register status, which is used for quick confirmation of alarm points when connected to the network.

Figure 3. RE46C190 block diagram

RE46C190 has a built-in crystal oscillator and control unit. In standby mode, the boost circuit will work for 10 ms every 10 seconds. Before the end of this 10 ms, the infrared emitting diode will work for 100~400 seconds according to the time set by the user programming; at the same time, the signal received by the infrared photoelectric tube is converted into an electrical signal, integrated and quantized in the chip, and compared with the limit value stored in the chip to determine whether there is smoke. If smoke is detected, the system will continue to detect. If smoke is detected three times in a row, the system will enter the alarm mode, and the boost circuit will provide sufficient driving capacity to make the buzzer sound an alarm of sufficient decibels, and the IO pin will be pulled high.

RE46C190 provides 11 programming and test modes. Different parameters can be programmed in different modes. All configuration words will be stored in the EEPROM inside the chip.

The average standby current of the entire circuit is about 8A. In order to achieve the requirement of 10-year operation of the 3V system, the internal resistance and self-discharge rate of the battery must also be considered. Usually, a battery of about 1500 mAh can meet this requirement.

in conclusion

RE46C190 provides a single-chip solution for 3V photoelectric smoke detectors. The programmable function of the chip makes the system design very flexible and provides a solution that can be designed to meet different product requirements.