SMOKE DETECTOR : SMOKE DETECTOR By
Faiza tabassum Bahria University Islamabad Slide 2: A mechanical device that is sensitive to the presence of smoke or particulate material in the air that transmits a signal to a measuring or control instrument. Smoke detectors consist of two basic parts: a sensor to sense the smoke and a very loud electronic horn to wake people up. screwed to your ceiling always. Slide 3: Types of smoke detector
Photoelectric smoke detector
Ionization smoke detector
Aspirated smoke detectors Slide 4: Photoelectric Smoke Detector The photoelectric type of smoke detector utilizes light as a detection mechanism. There are two basic types of photoelectric smoke detectors
• Light sensing (scattering).
• Light obscuring (blocking). Slide 5: Light scattering smoke detector Light sensing photoelectric smoke detectors depend on the ability of small airborne particles to scatter light.
Scattering is due to absorption, reflection, refraction, polarization, and diffraction. Slide 6: the light emitted from the light source is not detected by the photo sensor.
When smoke particles enter the chamber, some of the light is scattered toward the photo sensor.
The amount of smoke that enters the chamber increases, more incident light is scattered toward the photo sensor.
When the concentration of particles reaches a predetermined threshold, an alarm will sound. Cont. Light obscuring smoke detectors : Light obscuring smoke detectors A projected beam photoelectric smoke detector can use a single light source and photosensor to monitor a very large area.
Use an IR LED or laser diode as a light source.
smoke detectors cover 900 sq. ft. Slide 8: Detection of smoke by attenuated light.
Uses a separate emitter and detector.
Signals are processed and compared to a reference level.
When two consecutive signals exceeding the reference level are received.
Time delay circuit triggers the switch to activate the alarm signal. Ionization smoke Detector : Ionization smoke Detector Have ionization chamber inside the detector
Ionization chamber is simply two plate chamber with voltage across them along with radioactive source of ionization radiation.
The alpha particles generated by the americium ionize the oxygen and nitrogen atoms of the air in the chamber. Slide 10: Ionize means to "knock an electron off"
free electron are created.
The negative electron is attracted to the plate with a positive voltage, and the positive atom is attracted to the plate with a negative voltage.
The electronics in the smoke detector sense the small amount of electrical current that these electrons and ions moving toward the plates represent.
When smoke enters the ionization chamber, it disrupts this current.
The smoke particles attach to the ions and neutralize them. The smoke detector senses the drop in current between the plates and sets off the horn. Ionization smoke detector (Cont.) Comparison of ionization & photoelectric : Comparison of ionization & photoelectric Ionization
Detects fast, flaming fires
Detects smoke produced by flammable liquids, loosely packed light combustibles, and kitchen grease.
Effective for slow burning, smoldering types of fires.
Detects smoke caused by cigarettes burning in furnishings and bedding. Aspirated smoke detectors : Aspirated smoke detectors Active detector system.
Central detection unit draws air through a network of pipes to detect smoke.
The sampling chamber is capable of detecting the presence of smoke particles suspended in air by detecting the light scattered by them in the chamber.
The air samples are captured and filtered, removing any contaminants or dust to avoid false alarms.
ASD systems actively draw smoke to the detector
If smoke is detected, the systems alarm is triggered, and signals then are processed through centralized monitoring unit . Slide 13: Architecture Transmitter PIC Microcontroller Tone Generator Receiver Smoke Sensor Temperature Sensor Low Battery Sensor System Overview : System Overview Hardware
Tone Generator (LM555CM)
Smoke Sensor (276-142, 279-143)
Temperature Sensor (LM235AH)
Low Battery Sensor
PIC Microcontroller (C Language) PIC MICROCONTROLLER : PIC MICROCONTROLLER Block Diagram Slide 16: Temperature Sensor
Temperature threshold of 125°F reached? Receiver
RF signal received from another detector’s transmitter? Smoke Sensor
Smoke detected? Low Battery Sensor
Battery measured below 3.5V? PIC Microcontroller
Temperature sensor, receiver, smoke sensor, or low battery sensor has been set off? Transmitter Tone Generator No Yes Yes Always No No Yes No Yes Importance : Importance Deaths occur from breathing in smoke and toxic gases from a fire, not from being burned by flames and Poisonous gases created during a fire, like carbon monoxide, can quickly cause a person to become confused and disoriented
Smoke alarms are purposely built to warn and protect those inside buildings from potential fire accidents
Playing the Numbers
Night time Fire Safety
Invisible Threats : Statistics show that death or serious injury can be avoided in households
where smoke alarms are installed and maintained.
SMOKE ALARMS SAVE LIVES!
PROTECT YOURSELF. Slide 19: ANY QUESTION