Abstract: Apparatus for controlling a load has a load operating circuit, two radiation sources, a photodetector which is responsive to radiation from one of the sources to change the condition of the circuit, and optomechanical transducer which defines a path for radiation from the sources and has a mirror which reflects radiation from the one source to the photodetector. The mirror is movable between first and second positions in one of which the quantity of reflected radiation suffices and in the other of which the quantity of reflected radiation does not suffice to change the condition of the circuit. The apparatus also comprises an electrical conductor or other suitable means for changing the intensity of radiation from the other source in response to changes in the condition of the circuit so that each change in the condition of the circuit is displayed by the transducer.