Eli5 how do fire alarms detect fires?

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Eli5 how do fire alarms detect fires?

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One type of fire alarm uses a little laser beam. It shines onto a sensor, and completes a circuit. If the laser beam gets broken by smoke, the light can’t reach the sensor, and the circuit breaks. That makes the sound go off.

They detect smoke

Your classic smoke detector has a small bit of radioactive americium that shoots alpha particles into a chamber and figures out the charge in it. If there’s smoke or other fine particles in the air it’ll absorb the alpha particles, change the reading, and sound the alarm

The other type is photoelectric. Shine a light at a light sensor, if there’s smoke in the air it’ll scatter the light, reduce how much is sensed, and sound the alarm

Each type is good at detecting specific types of fires

In addition to the earlier comments about ways that some detectors can detect smoke (and sometimes are triggered by steam or dust in the air); some detectors also have an infra-red sensor that triggers if it detects something too hot.