Which of the following is used in fire alarm ?

Question

UnlockIAS · Accepted Answer

Correct — A, a photo cell. A photoelectric smoke detector — the commonest type of fire alarm sensor — houses a light source (an LED) and a light-sensitive photo cell set at an angle to it. In clean air no light reaches the cell; when smoke from a fire enters the chamber it scatters the light onto the photo cell, the cell's output changes and the alarm is triggered. The photo cell (a photodetector such as a photoresistor/photodiode) is therefore the sensing element that 'uses' light to detect a fire.

Why the other options are wrong — (b) Water is the extinguishing agent released by a sprinkler system to put out a fire — it does not detect the fire, so it is not what a fire alarm (the detector) uses to sense one.  (c) A solar cell (photovoltaic cell) is designed to convert sunlight into electrical power, not to sense the small amount of scattered light in a smoke chamber; it is a power-generation device, not a smoke-detection sensor.  (d) Incorrect, because a photo cell is in fact the sensor used in photoelectric fire/smoke alarms — so option (a) is valid and this cannot be the answer.

Concept: A photoelectric (optical) smoke detector works on light scattering. A photo cell is a component whose electrical behaviour changes when light falls on it (photoconductive cells/photoresistors lower their resistance under light; photodiodes/phototransistors produce a current). Pairing a light source with a photo cell lets a device 'see' smoke: smoke particles scatter light onto the cell and change its signal, tripping the alarm.

State PCS 2020 Q91 — Science & Technology PYQ with Answer

State PCS 2020 Q91 — Science & Technology PYQ with Answer