CHINTAN H.PATELAHMEDABAD-380009UNI NO:ROLL NO:247

INDEXCONTENTS PAGE NUMBER

INTRODUCTION 3

CIRCUIT DAIGRAM 6

CIRCUIT WORKING 7

COMPONENT DISCRIPTION

10

APPLICATIONS 12TESTING OF CIRCUIT 12

TROUBLESHOOTING 13

INTRODUCTION:Smoke alarms, when properly installed and maintained, can

literally be lifesavers. These detectors provide an early warning in 2 | P a g e

the event of a fire, allowing residents more time to escape it. Although smoke alarms are commonplace in the majority of homes, they are typically found to not be in working order.

Ionization alarms react more quickly, warning residents sooner than photoelectric alarms. This is essential when a fast moving fire occurs. Ionization alarms are triggered by a change in electrical current that comes from the presence of smoke. Photo electric smoke sensors respond either to a reduction in light or the scattering of light caused by smoke. Smoke detectors should ideally be interconnected — meaning that if one detects smoke, all smoke detectors in the home should sound.

Dual sensor smoke alarms combine the best of both technologies. Since no one can predict which type of fire will start in their home, it is best to purchase dual sensor smoke alarms or install both photoelectric and ionization alarms. Smoke alarms must be certified.

Smoke alarms for the hearing impaired are also available. These alarms may either flash or vibrate — with some models doing both — to alert hearing impaired to smoke or a fire. A fire department can provide information and assistance about any type of fire alarm to members of the community.

Smoke alarms are easy to find, as they can be bought at hardware stores, home supply and repair stores, and many mass merchandisers. Battery powered smoke alarms are easy to install as well. These types of smoke detectors may have self adhesive material to stick them to the wall or ceiling, and some require a

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screwdriver to secure them. If the smoke alarms are hardwired into a home, an electrician will be needed to install them.

The problem for many people lies in keeping the smoke detectors in good working order. Batteries for most smoke alarms should be replaced every year — it is recommended that they are changed at the beginning of the year, or on the day that begins day light saving Time. If the smoke alarm begins to chirp, it means that the batteries should be replaced. The entire unit should be replaced every 10 years, if not more frequently.

Hardwired smoke alarms are very different, and should be tested monthly. batteries for these alarms should also be replaced every year, or when the alarm sounds the low battery warning . hard wire smoke detectors may have backup power in the home's electrical wiring, but the battery should still be in working order at all times.

Many people remove the batteries from their smoke alarm if it sounds during cooking or if something burns, then forget to replace the batteries. Although this is a common mistake, it is potentially very dangerous. Batteries should not be removed if something burns during cooking. Instead, open all the windows and door, turn on the fan over the stove, and wave a towel or broom around to clear the smoke.

LIST OF COMPONENTS:IC 1 NE555

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BASE 8-pin

R1 470 ohm

R2 4.7 K ohm

R3 1 K ohm

R4 47 K ohm

VR1 1 K ohm

C1 0.04µ F ceramic

C2 0.01 µ F ceramic

C3 100µF/25V electrolyte

It also has photo interrupter module MOC 7811, speaker 8 ohm, 0.5 watt, 9V supply and PCB.

CIRCUIT DIAGRAM AND PART PLACE PLACEMENT DIAGRAM:

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The smoke alarm circuit presented here is based on readily avaliable photon-coupled interruter module and IC NE555. The photo interrupter module is used as smoke detector while timer

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555 is wired in astable configuration as as AF oscillator for sounding alaram via loud speaker. In absence of any smoke the gap of photo interrupter module is clear and light from LED falls on the photo-transistor through slot. As a result the collector of photo transistor is pulled towards groound. This causes reset pin no.4 of IC555 to go low. Accordingly the timer is reset and hence the alaram is reset and it doesnot sound. However when smoke is present in gap of the photo interrupter module the light beam from led to phototransistor is obstructed. As a result the photo transistor stops conducting and pin no 4 of IC555 goes high to activate the alaram.

The basic idea behind circuit operation is to turn ON the alaram as soon as it detect smoke. Resistor R1 and preset VR1 provide current path to light up internal LED in normal operation,internal LED of photo-interrupter emits photons which continuously keep internal phototransistor in saturation region owning which its collector Reset input pin no.4 of IC1 are pulled towards ground. As reset pin of IC1 NE555 is active low it continuously reset the timer and it doesnot produce any kind of output in such a situation.

Now consider the second scenario, when a room filled by smoke and slot of photo interrupter gets engulfed in smoke which obstructs the path of light rays from LED to phototransistor. Phototransistor’s base being Reverse biased causes it to cut-off and hence as a result its collecter and reset pin of IC1 are held high via transistor R2. Thus reset condition of Timer is disabled by 7 | P a g e

smoke IC1 working here in astable mode generated square wave output at its pin no 3. The frequency of this wave should be in range of 20-20KHZ and its dependent upon the values of resistor R3,R4 and capacitor C1. Capacitor C3 effectively passes the output from IC to loud speaker which then converts electrical signals to sound. Capacitor C2 connected at pin no 2 provide immunity from any type of false triggering.

Photo interrupter module used here as smoke detector consists of an internal LED and photo transistor. In normal phototransistors the LED on condition provided base biasing to photo-transistor in form of light which helps in changing its state from cutt-off to saturation. But this photo-coupied interrupter

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module is slightly different as provision of window gap is also provided here. The variation in intensity of smoke at which detection is wanted can easily be done using preset. The circuit operates normally at 9-12 V dc .

WORKING MODEL:

COMPONENTS DESCRIPTION:

NE555 IC:

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Pin 1 (Ground):Connects to the 0v power supply. 

Pin 2 (Trigger):Detects 1/3 of rail voltage to make output HIGH. Pin 2 has control over pin 6. If pin 2 is LOW, and pin 6 LOW,  output goes and stays HIGH. If pin 6 HIGH, and pin 2 goes LOW, output goes LOW while pin 2 LOW. This pin has a very high impedance (about 10M) and will trigger with about 1uA. 

Pin 3 (Output):(Pins 3 and 7 are "in phase.") Goes HIGH (about 2v less than rail) and LOW (about 0.5v less than 0v) and will deliver up to 200mA. 

Pin 4 (Reset):Internally connected HIGH via 100k. Must be taken below 0.8v to reset the chip. 

Pin 5 (Control):A voltage applied to this pin will vary the timing of the RC network

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(quite considerably). 

Pin 6 (Threshold):Detects 2/3 of rail voltage to make output LOW only if pin 2 is HIGH. This pin has a very high impedance (about 10M) and will trigger with about 0.2uA. 

Pin 7 (Discharge):Goes LOW when pin 6 detects 2/3 rail voltage but pin 2 must be HIGH. If pin 2 is HIGH, pin 6 can be HIGH or LOW and pin 7 remains LOW. Goes OPEN (HIGH) and stays HIGH when pin 2 detects 1/3 rail voltage (even as a LOW pulse) when pin 6 is LOW.  (Pins 7 and 3 are "in phase.") Pin 7 is equal to pin 3 but pin 7 does not go high - it goes OPEN.  But it goes LOW and will sink about 200mA. 

Pin 8 (Supply):Connects to the positive power supply (Vs). This can be any voltage between 4.5V and 15V DC, but is commonly 5V DC when working with digital ICs. 

APPLICATION OF CIRCUIT:

Most of smoke alarams available in market use one or another kind of smoke sensor which increases the cost and also 11 | P a g e

require complicated interfacing circuit around it. But this smoke alaram cirucit uses readily available photon-interrupter module as smoke detector. The function is to continously detect smoke and causes the alaram to ring when excessive smoke is detected. When the smoke is gone the alaram stops and resets itself for the further detection of the smoke.

TESTING THE CIRCUIT:

After assembling and soldering of components on PCB connect the external DC source of 9 V

Intially slot of photo-interrupter module being clear there wont be any sound from speaker.

For testing purpose manually close the slot opening by inserting some piece of wood or wood in between gap. A loud sound will be heard in this case.

See whether the circuit automatically resets on removing the obstacle.

Now set the intensity using preset VR1. Now burn a match stick or insanse stick and allow the smoke

to enter the module.

TROUBLESHOOTING: If the circuit does not detect the smoke

while it is on we need to calibrate the preset given in the circuit

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as the detection of the smoke depends on the resistance preseted in the preset.The preset should be set in such a manner that evena small amount of smoke can be detected.Now if the circuit breaks down while working then the following steps should be taken:

1. Problem in input suppy.2. Check the batteries3. Check the shouldering whether it is proper or not. 4. Check the IC and all the other components in the circuit with

the help of multi-meter whether they are in working condition or not.

5. Now check the sensor whether it is working or not, there might be some problem with the sensor also.

6. Check the speaker whether it is in working condition or not.

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