How you build a fire detecter?

Answer 1

Actually they are smoke detectors, they detect the result of the fire, smoke. A smoke detector works as follows: There is a chamber with a small nuclear source in it. Smoke particles are hit by the radiation and are ionized, ie, an electron is knocked off the particle leaving the particle with a charge on it. The chamber has two parallel plates in it with a voltage across the plates. The ionized particles are attracted to one of the plates, causing an electric current to flow. This current is amplified and detected and sets off the alarm.