A light circuit includes a light unit (7), a control unit (3), a differentiating circuit (4) and a switching circuit (6). The light unit includes a plurality of light branches (L1, L2 . . . and Ln) connected in parallel. The light branches are controlled to light by the switching circuit that is enabled and disabled by PWM waves from the control unit. The PWM waves are differentiated into differentiated waves by the differentiating circuit before outputted to control the switching circuit.