The CN-0537 is a modern smoke detector with a design complying with the specifications outlined in UL 217. The design is based on fire data that researchers have collected at the smoke testing facilities of the Underwriters Laboratories and Intertek Group plc. The design uses the integrated optical sensor ADPD188BI and an optimized smoke chamber. It has a single calibrated device for sensing and measuring smoke particles.
The design also uses a smoke detection algorithm that UL has tested and verified. This facilitates OEMs in reducing their product development time and thereby delivering their product designs more quickly. The hardware design has a form factor resembling the Arduino board, and this includes an ADICUP3029 microcontroller development board apart from the CN-0537 smoke detector.
There are two basic designs popular for smoke detectors. One is the ionization type which uses radioactive materials to ionize the air while checking for electrical imbalances. The other is the photoelectric type that checks for current in the photodetector caused by light reflecting off airborne smoke particles and falling on the photodiode.
Although experts recommend a combined solution of both types, the improved reliability of the photoelectric smoke detector makes it more popular. It is faster in detecting common house fires and has a smaller response time to smoldering fires.
The optical module ADPD188BI is a complete photometric system. Its design is specifically meant for smoke detection applications. Rather than the conventional discrete smoke detector circuits, using the ADPD188BI makes the design significantly simpler. This is because the integrated package contains two LEDs and two photodiodes, along with an analog front end. The module utilizes a double-wavelength technique. The two LEDs emit light at different wavelengths—blue light at 470 nm, and infrared light at 850 nm. The LEDs also pulse at two independent time slots, and any particulate matter present in the air scatters the transmitted light back into the device.
The scattered light reaches two integrated photodiodes, which produce proportional levels of current. The analog front electronics digitize this output current. As the optical power from the LEDs is maintained constant, any increase in the ADPD188BI output over time indicates that airborne particles are building up.
The response of the ADPD188BI photometric sensor is a ratio of the input optical power to the transmitted optical power. The manufacturers refer to this as the power transfer ratio or PTR and express it as nW/mW. PTR is a more meaningful parameter than the raw output, as it is independent of the actual hardware settings.
The ambient temperature affects the response of the ADPD188BI system. As the shape of the temperature response curve can vary for the blue LED depending on the amount of current in the LED, it complicates matters further. The temperature response curve of the infrared LED is independent of the LED current.
The CN-0537 smoke detector has a temperature and humidity sensor that monitors the conditions, in real-time, within the chamber right next to the optical module ADPD188BI. This helps to determine the value of the relative response. The software helps with temperature compensation.
