The detector, HS-WTK55, is combination smoke and heat detector. The Photoelectric type smoke part of detector is based on he photoelectric sensing principle to make up. When fire break up and smoke enter the light-tight box, the particles of smoke scatter the infra red light, and this scattered light goes into the receiving element, there byemitting an electric current, and while the electric current is up to the fixed extent, the fire alarm will be activated. The fixed temperatured heat detector part stabilized performance, long-term durability and least-malfunction are ensured by employinga bimetal mechanism, which meets UL verification.