No batteries and external power supply it will illuminate after water flows down. Automatic light switching. Green red & blue 3 color are displayed automatically when water flow.. Transforming the stream of water into a beautiful waterfall of light. Brightness is such enough even no other lighting fixtures need. Fits all standard connectors easy to install. Offer you a delight bath & a wonderful experience. - Temperature detectable for child and elderly safety - No batteries required - Thread Diameter: M24 24mm - Easy Installation - When water temperature at 0~32'C will emit green light automatically - When water temperature at 33~41'C will emit blue light automatically - When water temperature at 42~45'C will emit red light automatically