● Principle of photoelectric detection
This scheme is based on the principle of scattered light/transmitted light to detect smoke, fully complying with the technical requirements of GB 20517-2025 for photoelectric detection.
Inside the optical maze, the emitting light path and the receiving light path form a certain angle (usually an obtuse angle). Under normal working conditions, the receiving end can hardly detect optical signals. When smoke particles enter the maze, the emitted light scatters on the surface of the particles, and some of the scattered light is captured by the photosensitive device at the receiving end. The higher the smoke concentration, the greater the scattered light intensity.
Photoelectric detection signal chain:
① Constant current drive: The MCU's built-in current generator (adjustable from 50mA to 360mA) provides a constant driving current for the LED, ensuring consistent light intensity and avoiding power fluctuations that affect measurement.
② Signal reception: The photosensitive device converts the scattered light received into a weak current signal.
③ Amplification and conditioning: The current signal is input into a dedicated operational amplifier (smoke AFE) built into the MCU, and after I/V conversion and voltage amplification, it is conditioned into a voltage signal suitable for ADC sampling.
④ Analog to digital conversion: The amplified signal is sent to a 12 bit SAR ADC and converted into a digital signal for MCU processing.
● Intelligent discrimination and anti-interference
① Temperature compensation
The built-in temperature sensor (with an accuracy of ± 2 ℃) compensates in real-time for the impact of environmental temperature changes on the optical path and circuit, avoiding false alarms caused by temperature drift.
② Dual optical path enhancement technology
For scenarios with higher anti-interference requirements, the solution can support dual path detection of red and blue light:
Two operational amplifiers process the scattered signals of red and blue light respectively. Different wavelengths of light have different scattering characteristics on smoke particles and interference sources (water vapor, oil fume). By comparing the ratio of the two signals, the true fire smoke and interference sources can be accurately distinguished, greatly reducing the false alarm rate.
③ Pollution monitoring
Long term monitoring of the baseline value when smoke-free, if the baseline drift is caused by dust pollution, a yellow pollution alarm will be triggered.
● Low power consumption operation
The system wakes up from STOP mode at a fixed cycle (e.g. every 10 seconds) and returns to STOP mode after completing one smoke detection. The single working time is only milliseconds, the average power consumption is extremely low, and it meets the requirements of long-term battery life. The wake-up source can come from the RTC automatic wake-up unit or external GPIO interrupts (such as buttons, infrared remote control signals).
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