A Light Oscillation Control Unit System (LOCUS) has been developed for the purpose of monitoring Reflexion High Energy Electron Diffraction (RHEED) oscillations and to use the oscillations to control the crystal growth with single atomic layer resolution. The system can be used for phase-locked epitaxy, i.e., shutters controlling the growth are actuated when monolayers are completed. To reduce the usual damping of the RHEED-oscillations, the system can also use the method of synchronization of nucleation, i.e., periodically varying the surface supersaturation by changing the substrate surface temperature during each oscillation. Four shutters can be operated automatically and the superlattice composition can be chosen in a number of different ways. LOCUS gives fast response on intensity oscillation maxima and minima beyond a programmable discrimination level. Relevant crystal growth parameters can be changed during the run of the program, which implies that even quasi-periodic superlattices can be grown. There is also a possibility to switch from automatic to manual control. The shutters and substrate heater have been designed to reduce the influence of shifts of the diffraction pattern as the growth conditions are changed.