Patent ID: 7659986

Claim:
A smoke sensor comprising: a light-emitting device emitting light; a first chamber; a smoke passage providing communication between inside of the first chamber and outside of the first chamber; a second chamber having a density of smoke of a predetermined value; a first light-receiving element receiving light which has traveled in the first chamber after being emitted from the light-emitting device directly to the first chamber or light which has traveled in the first chamber after being emitted from the light-emitting device and entering the first chamber through only a place having a density of smoke of zero; a second light-receiving element receiving light which has traveled in the second chamber after being emitted from the light-emitting device directly to the second chamber or light which has traveled in the second chamber after being emitted from the light-emitting device and entering the second chamber through only a place having a density of smoke of zero; a first switch receiving and selectively outputting a signal from the first light-receiving element and a signal from the second light-receiving element; an amplifier section amplifying a signal from the first switch; and a density detecting section detecting a density of smoke in the first chamber on the basis of a first amplified signal which is a signal from the first light-receiving element amplified by the amplifier section and a second amplified signal which is a signal from the second light-receiving element amplified by the amplifier section.