Our Air Flow Detector circuit works on the concept of change in electrical resistance value of the lamp's filament when air is passes over it. Indirectly it is a temperature on tungsten which changes resistance on change in temperature. when temperature around tungsten gets low then resistance of filament also gets low which ultimately decreases voltage at positive terminal of op amp and led glows.
In normal condition when temperature of tungsten is high i.e. when no air is flown over bulb then electrical resistance value of lamp increases and thus there is high voltage at positive terminal of op amp due to which the output of opamp is also of that positive terminal that is 5V which is coming out of the IC-7805. Now if u see below circuit you will find on one end of led there is this 5V from O/P of opamp and on other end you again get almost same voltage .As a result of with net potential difference across led will be 0V and thus led not glow.
In Cool Condition, when Air is passed over the bulb then in that case the electrical resistance value of lamp decreases and thus there is low voltage at positive terminal of op amp as compared to voltage at negative terminal. due to which the output at that opamp comes out to be that of negative terminal .And it is obvious that this negative terminal voltage that is coming out from the output of opamp will be less compared to the voltage at other end of led.therefore,in this case there will be potential difference across led and thus it glows in this case.
During construction of this circuit one of the challenging task is breaking the bulb to get filament (show in a figure below). steps of how to extract filament out of bulb is shown on left portion of second figure.
Now How to Break a Bulb
1) Wrap a Bulb with Tape
2)Hold a Bulb in Bench wise
3)Rotate it slowly slowly and don't let it fall on ground.That is rather catch it on your hand.
4)Remove the Tape and you find the filament part as shown below
Now How to Break a Bulb
1) Wrap a Bulb with Tape
2)Hold a Bulb in Bench wise
3)Rotate it slowly slowly and don't let it fall on ground.That is rather catch it on your hand.
4)Remove the Tape and you find the filament part as shown below
Once u have build ur circuit after that u need to calibrate ur circuit with the help of 10k potentiometer .As i did with screw driver in video .
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