Abstract: An electric switch and signal output device for use in fluid systems. A light source and photosensitive pickup are disposed in aligned positions in a cross bore of the switch body at opposite sides of a main bore that reciprocally receives a piston element. If fluid subjected to differential pressures is introduced to the main bore at one side of the piston or if fluid is introduced to opposite sides thereof, pressure pulses or changes in differential pressure in the fluid system cause movement of the piston away from or toward an interfering position with respect to the light path between said light source and pickup. Incident light at the pickup, which may be a photocell resistor, provides an output electrical signal change that is usable for control, position indicating, counting or logic purposes. The actuation sensitivity is changed by use of different weight pistons and/or by use of adjustable resilient means.