Small change of light the smoke detector will be activating. Initially photo-conductor resistance shall have to high value. Total Circuit designing. Smoke Detector is a detector which is activating by the smoke. It is another practical application of a voltage-level detector. Smoke Detector is working with the change of voltage. This circuit can be used in fire alarm.
Smoke Detector of Figure the lamp and photo-conductive cell are in an enclosed chamber that admits smoke or dust but not external light. The photo-conductor is a light-sensitive resistor. In the absence of smoke or dust, very little strikes the photo-conductor and its resistance stays at some high value, typically several hundred kilohms. The 10-kΩ sensitivity control is adjusted until the alarm turns off.
Any particles entering the chamber cause light to reflect off the particles and strike the photo-conductor. This, in turn, causes the photo-conductor resistance to decrease and the voltage across R1 to increase. As Ei increases above Vref, Vo switches from –Vsat to +Vsat, causing the alarm to sound.
In initial condition photoconductor resistance must contain high resistance otherwise the detector will be always activated. When the voltage of inverting terminal and non-inverting terminal are same output show 0 V.
Output voltages not more then supply voltage. The resistive network at the input of the op-amp forms a Wheatstone bridge. This circuit can be used to monitor the level of dust particles in a clean room.