The utility model discloses a smoke alarm which comprises a serial feedback photosensitive circuit, a semiconductor tube switching circuit and an integrated alarm circuit. The serial feedback photosensitive circuit comprises an infrared light emitting diode VD1 and a photosensitive triode VT1. The semiconductor tube switching circuit comprises a diode VD2, a triode VT2 and a triode VT3. The integrated alarm circuit comprises a music chip IC, a triode VT4 and a loudspeaker B. Compared with the prior art, the smoke alarm is advantageous in that: the smoke alarm is a set of independent alarm equipment, and can operate through driving by a 6V power supply; and the smoke alarm further has advantages of simple structure, convenient mounting, practical function, and high suitability for various kinds of new and old buildings.