A fiber optic position sensor utilizing visible light sources microprocessor and a selective absorption/reflection technique is presented as a replacement for conventional limit sensing technology including more recently developed opaque shutter-type limit switches. Because the technique to be presented applies the ratio of inversely varying returned light signals as a criterion for switching it is highly tolerant of losses induced by variability in cable length termination quality and microbending. In addition because both light signals are modulated by the sensor the operational state of the sensor is monitored. A sensor of this type using visible light sources plastic fiber and microprocessor circuitry has been built. The sensor has been commercialized and has found application in the cosmetics industry. The sensing technique is covered by a U. S. Patent. The theory of the sensing technique will be presented. A liquid level sensing application which exploits the loss tolerant self-monitoring nature of the sensor will be discussed. This application induces switch actuation via pressure change in a dip tube. It offers advantages of the ability to sense a wide range of liquid viscosity and immunity to coating buildup effects.