31-01-2012, 12:33 PM
SMOKE DETECTOR
[attachment=16891]
INTRODUCTION
A smoke detector is an important safety device in homes, offices and other enclosed structures. It can save lives especially when fires occur during the most unexpected times. Thousands of people are killed every year as a result of suffocation from thick smoke caused by fires.
Smoke detectors in homes can either be the hard wired or the battery-powered units. Most states, however, require the installation of hard wired smoke alarms which are more reliable. This type is powered by 120-volt electricity in the home and is interconnected with other smoke detector units in the house. They are normally wired into the electrical system of the home. Most homes install several units with each room and hallways having a separate alarm to make sure that all members of the family are warned in case a fire occurs.
BLOCK DIAGRAM DESCRIPTION
2.1.1 Stimulant
The stimulant here is the smoke which acts as the input to the circuit.The intensity of the smoke determines the activation of the photodiode and in turn affects the working of the circuit.
2.1.2 Sensor assembly
The sensor assembly consists of a photo diode and a white LED. The white LED makes sure that photodiode gets enough intensity of light in the absence of smoke. The photo diode is a photoelectric device and it is connected in the reverse bias mode so as to work in the photoconductive mode of operation.
2.1.3 Switching circuit
The switching circuit consists of an NPN transistor BC549. It switches on the main component, the NE555 timer. The transistor gets on when no smoke is there and it gets off when a smoke of desired intensity gets into the sensor assembly.
2.1.4 Bistable Multivibrator
The bistable multivibrator consists of a NE555 timer which is designed in the bistable mode. The popular timer ICNE555 is used in the Bistable mode to generate the alarm when the sensor assembly detects smoke. The output of IC1 has two states (Bistable), either high or low. This Bistable states are determined by the state of its reset pin4. When the reset pin is high, output turns high and when the reset pin is low, output turns low.
2.1.5 Audiovisual Components
The audiovisual components are mainly aimed at giving the audiovisual response on the detection of smoke. They include a red LED and a buzzer. When smoke is detected, the visual response is first indicated by the glowing of the red LED and then the buzzer sounds for giving the audio response.
3.1 CIRCUIT DESCRIPTION
3.1.1 Power Supply
The supply to the circuit is a 12 v supply. This is provided by connecting either a transformer to the circuit or a battery. When we use a transformer, we have to convert the ac power to dc.This is done by using the two diodes and they perform diode rectification. We can also use a battery as the power supply. As far as a smoke detector is concerned, using a battery is more reliable as current failure may occur due to fire which results in the failure of the circuit.
3.1.2 Photodiode
The major component of the sensor assembly is the PIN Photodiode. Photodiode is a type of light detector capable of converting light energy into either current or voltage, depending upon the mode of operation. Photodiodes are similar to typical semiconductor diodes except that they may be either exposed or packaged with a window to allow light to reach the sensitive part of the device.
[attachment=16891]
INTRODUCTION
A smoke detector is an important safety device in homes, offices and other enclosed structures. It can save lives especially when fires occur during the most unexpected times. Thousands of people are killed every year as a result of suffocation from thick smoke caused by fires.
Smoke detectors in homes can either be the hard wired or the battery-powered units. Most states, however, require the installation of hard wired smoke alarms which are more reliable. This type is powered by 120-volt electricity in the home and is interconnected with other smoke detector units in the house. They are normally wired into the electrical system of the home. Most homes install several units with each room and hallways having a separate alarm to make sure that all members of the family are warned in case a fire occurs.
BLOCK DIAGRAM DESCRIPTION
2.1.1 Stimulant
The stimulant here is the smoke which acts as the input to the circuit.The intensity of the smoke determines the activation of the photodiode and in turn affects the working of the circuit.
2.1.2 Sensor assembly
The sensor assembly consists of a photo diode and a white LED. The white LED makes sure that photodiode gets enough intensity of light in the absence of smoke. The photo diode is a photoelectric device and it is connected in the reverse bias mode so as to work in the photoconductive mode of operation.
2.1.3 Switching circuit
The switching circuit consists of an NPN transistor BC549. It switches on the main component, the NE555 timer. The transistor gets on when no smoke is there and it gets off when a smoke of desired intensity gets into the sensor assembly.
2.1.4 Bistable Multivibrator
The bistable multivibrator consists of a NE555 timer which is designed in the bistable mode. The popular timer ICNE555 is used in the Bistable mode to generate the alarm when the sensor assembly detects smoke. The output of IC1 has two states (Bistable), either high or low. This Bistable states are determined by the state of its reset pin4. When the reset pin is high, output turns high and when the reset pin is low, output turns low.
2.1.5 Audiovisual Components
The audiovisual components are mainly aimed at giving the audiovisual response on the detection of smoke. They include a red LED and a buzzer. When smoke is detected, the visual response is first indicated by the glowing of the red LED and then the buzzer sounds for giving the audio response.
3.1 CIRCUIT DESCRIPTION
3.1.1 Power Supply
The supply to the circuit is a 12 v supply. This is provided by connecting either a transformer to the circuit or a battery. When we use a transformer, we have to convert the ac power to dc.This is done by using the two diodes and they perform diode rectification. We can also use a battery as the power supply. As far as a smoke detector is concerned, using a battery is more reliable as current failure may occur due to fire which results in the failure of the circuit.
3.1.2 Photodiode
The major component of the sensor assembly is the PIN Photodiode. Photodiode is a type of light detector capable of converting light energy into either current or voltage, depending upon the mode of operation. Photodiodes are similar to typical semiconductor diodes except that they may be either exposed or packaged with a window to allow light to reach the sensitive part of the device.
