Abstract: First and second spaced apart optical emitter-receiving pairs are located along a liquid carrying conduit. A regulator circuit is coupled to each emitter-receiver pair to control the operation of that pair inside a maximum response range regardless of the transparency of the liquid. The regulator circuit is so arranged as to supply the emitter with a voltage which is a function of the difference between a voltage supplied by the receiver portion of an optical-receiver pair and a reference voltage. A differential circuit produces a differential signal representing the difference between the signals produced by the receiver portions of the optical emitter-receiver pairs and a comparator circuit is connected to the differential circuit in order to generate an alarm signal as a function of the differential signal. The detector is particularly useful as an air bubble detector in a transfusion or perfusion system, and can also provide a container-empty alarm in such systems.