A photoelectric switch is sealed against the infiltration of contaminants, for example, oil mist within a combustion-powered tool comprising a switch. In a housing structure sealed by an O-ring when assembled, two photoelectric devices define a line of sight therebetween. A plunger is biased to an inoperative position, in which a shutter portion thereof blocks the line of sight. In an operative position, the shutter portion unblocks the line of sight. The plunger has a stem portion sealed by another O-ring. Where electrical leads from the devices extend through apertures defined within the back member, the apertures are sealed by solder used to connect leads to copper layers, which may be screen-printed upon an outer surface of the back member. A cap mounted upon the back member covers the contacts and admits electrical wires soldered to the layers.