A method includes forming a housing having a ceiling portion and a smoke permeable wall portion. The ceiling portion of the housing has a first aperture and a second aperture. The method includes mounting a circuit board to the housing so that the housing and the circuit board collectively define a smoke chamber that is bounded by the ceiling portion, the smoke permeable wall portion and the circuit board. The method includes placing an emitter in the first aperture and placing a detector in the second aperture. The emitter defines an emitting region in the smoke chamber and the detector defines a detecting region in the smoke chamber. In a typical implementation, the detecting region at least partially intersects the emitting region in the smoke chamber, and the emitter and the detector are disposed substantially outside the smoke chamber.