The present invention relates to a detection circuit (100) capable to detect a rectified phase-cut or sinusoidal wave-form using its duty cycle or average value and in response, to select the respective dim mode amongst the linear phase-cut and step-dimming. The circuit (100) receives the rectified waveform with its duty cycle, which is derived through a comparator (22, 24) and converted into a DC signal. The latter which is controlled by the duty cycle is then compared to a reference level (40) through another comparator (20) that, in response, supplies a signal controlling a switching device (30). The switching device (30) will be thus automatically connected either to one set signal level when the DC signal is greater than the reference level (40), namely when the circuit (100) detects a rectified sinusoidal waveform, or to the same level as the DC signal when the DC signal is less than the reference level (40), namely when the circuit (100) detects a rectified phase-cut waveform.