A method and a device for gradually controlling the illuminating light intensity of light-emitting diodes by modulating the width of current pulses of defined duty cycle, using a DC-to-DC voltage converter and a control circuit for switching the supply current of the light-emitting diodes. Prior to switching the power supply of the diodes the DC-to-DC converter is initiated (A), so as to generate voltage pulses of defined duty cycle; the steady state of each pulse is determined (B), so as to generate a voltage pulse (CP) calibrated to a voltage level substantially corresponding to the steady-state level; and then the light-emitting diodes are powered (D) by applying each calibrated pulse to the light-emitting diodes by means of the switchable control circuit.