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Smoke Detector

Smoke Detector

Smoke Detectors are designed for 3 basic types of system implementations, Standalone, Wireless Connectivity, and Fixed or Wired Connectivity, the first two being battery powered implementations.
Smoke Detectors have 3 main functional blocks; The Sensor and Signal Chain, Processor, and the Communication Interface. Where the processor and communications interface typically are not needed in your very basic standalone detectors.

Power consumption is a very critical aspect in wireless smoke detector because they need to be able to run for very long periods of time on a battery. This makes microcontrollers like the MSP430 ideal for the application; their high level of system integration also simplifies the design and reduces system cost. An infrared (IR) diode and IR receiver are used inside a smoke chamber to detect the presence of smoke. The IR diode is pulsed periodically, and the IR receiver signal is examined to determine if smoke is present in the chamber. An operational amplifier is used to magnify the IR receiver current as a transimpedance amplifier, so it can be sampled by the ADC in the MSP430. Between sampling periods, the operational amplifier and IR circuitry are shut down, and the microcontroller is in a standby mode, consuming less then 1-mA current.

When selecting an external operational amplifier for the application, it is important to balance cost vs. settling-time performance, while minimizing current consumption. Settling time is important in allowing the detector to provide multiple reports of a smoke event in a short period of time, so as to minimize any false alarms. To further reduce current consumption of external components, some may be powered directly from an MSP430 port pin, even though it has a shutdown feature. This will take the current consumption of the amplifier to zero, when the MSP430 is in a standby state, significantly increasing the battery run time of this application.


Communication Interfaces

RF Transceiver: Range, network configuration and power consumption are important factors when selecting a Low Power Wireless (LPW) solution. Range is affected by output power, sensitivity and selectivity, which in turn impact the jamming of other signal sources and the ability to distinguish the desired signal from local interference. Selectivity is also important in RF design, especially when designing products in the 2.4GHz band, where interference from other equipment is likely. When making the RF radio selection, it is also important to understand the network configuration is which the smoke detector will be use: Point to Point, Star or Mesh Network, as it may impacted the radio, processor, memory and power requirements of the system.

Line Powered: Power over Ethernet (IEEE 802.3af) integrates data and power over standard LAN connection. It provides uninterrupted 15W max (13W load), 48V nominal supply to the devices connected to the system. The power requirement for smoke detectors is well below the ~12.5W limit for powered devices and can be easily powered from PoE. This sort of implementation removes the need to run AC power to sensor locations, and reduces the cost of the power supply in the detector by requiring only DC/DC power conversion.

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