A compact, low cost particle sensor utilizing a photodetector (31) which directly collects light scattered by particles (33) entrained in a fluid traversing a beam of light (32). The beam of light (32) is aligned such that it is in close proximity to the photo detector (31). The beam of light (32) is typically provided by a laser and associated focusing/collimating optics. The beam of light (32) intersects a portion of the fluid flow permitting a low pressure drop system and fluid flow generated by a low cost, low pressure device such as an axial fan (50).