Smoke Detection Using the C8051F990 and Si1141
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The Si114x is both a proximity detector and an ambient light sensor. As a proximity detector, the Si114x has multiple photodiodes allowing measurement of light in the near infrared and visble light spectra. As part of the proximity detector function, the Si114x can drive up to three external Infrared LEDs. The Si114x products can address a wide range of end applications, such as display control for handsets, range detection for lavatory appliances, and 3D gesture recognition for e-readers and tablet PCs.
Smoke detection is another application that can benefit from the Si114x product. Smoke detectors can either be of the optical or ionization type. An ionization type smoke detector uses an Americium 241 source. Given the radioactive source, this technology has fallen out of favor due to environmental concerns. Coupled with the lower cost and implementation simplicity, the majority of commercially-available smoke detectors are of the optical type.
Subtypes of Optical Alarms
Optical smoke alarms are further segmented into two general subtypes. The first subtype measures the opacity of the air between an emitter LED and its sensor. With this approach, the opacity of the air increases with increasing smoke particulates. The signal measured at the sensor decreases with increasing smoke density. The second subtype measures the light scattered by the smoke particulates. With this approach, the emitter LED is not directly in line with the sensor. The signal measured at the sensor decreases with increasing smoke density.
The main disadvantage of the opacity measurement approach is that the decrease in opacity is generally small compared to the incident emitter LED used. Most smoke alarms use the reflectance approach due to its relatively
simple design.
Proximity and Reflectance-based Smoke Detection
Ostensibly, the principles behind proximity detection and reflectance-based smoke detection are similar. However, the target object to be detected is different. In a standard proximity detector, the target object is typically a solid surface with diffuse reflectance properties. On the other hand, smoke consists of hundreds of miniscule particles per cubic centimeter. The increase in smoke particulate reflectance is generally much smaller than the increase of a single, large diffuse surface.
Another difference between a standard proximity detection application and smoke detection is that a smoke detector is expected to remain in a “no-detection” state for long periods of time. Today's smoke detectors are typically implemented with a combination of an MCU, IR sensor, and IR LED. The optical components are mounted in detection chambers in a way that prevents ambient light from artificial light sources from interfering with the measurement.
Smoke Detection Design Solution
The high sensitivity and ambient light rejection capability of the Si1141 in combination with averaging algorithms run in a low-power, high-performance MCU, such as the C8051F990, can work together to allow smoke detection without complex and costly chambers, further simplifying smoke detector design and production. Silicon Labs provides a hardware solution and example code to demonstrate this feasibility.
Note: Smoke-detection-related specifications for Europe include EN 14604:2008 for household or residential; EN 54-7 for industrial and public places