A circuit includes an emergency lighting system and an AC/DC converter. An output terminal of the emergency lighting system is electrically connected to a LED load, and the other output terminal DIM of the emergency lighting system is electrically connected to a switch S2. The switch S2 is controlled by the emergency lighting system. The emergency lighting system includes an AC input detection module, a switching time detection module and a lithium battery. The AC input detection module is electrically connected to a neutral wire VN and a live wire VL. The switching time detection module is electrically connected between an output terminal of the AC input detection module and the terminal DIM. An output terminal of the AC/DC converter is electrically connected to a positive electrode of the lithium battery, and the other output terminal of the AC/DC converter is electrically connected to the LED load.