System and method for controlling one or more light emitting diodes. For example, the system includes: a phase detector configured to process information associated with a rectified voltage generated by a rectifier and related to a TRIAC dimmer, the rectified voltage corresponding to a first waveform during a first half cycle of an AC voltage and corresponding to a second waveform during a second half cycle of the AC voltage, the phase detector being further configured to generate a phase detection signal representing a first time duration during which the first waveform indicates that the rectified voltage is larger than a predetermined threshold and representing a second time duration during which the second waveform indicates that the rectified voltage is larger than the predetermined threshold; and a mode detector configured to process information associated with the rectified voltage.