Analog photosensor operating on the power from a standard 4-20 ma instrumentation current loop

摘要

A photoelectric sensor interfaces and operates directly on the power available from a standard current loop without requiring an independent power supply. The sensor includes an emitter and receiver for measuring a parameter such as the distance between a target and the sensor. The intensity of light received by the receiver controls, as an inverse function, the amount of current drawn by the emitter. As the target becomes distant and the intensity of received light decreases, the sensor directs more current to the emitter, which also serves to enhance the sensor's range. As the target becomes closer to the sensor, the intensity of received light increases and the sensor directs less current to the emitter because less current is required to produce a signal of sufficient strength for the receiver to detect. With less current directed to the emitter, sufficient current from the current loop is still available to operate the remaining circuitry in the sensor.