A light sensor device is provided. It is controlled with a dual-mode master-and-slave microcontroller unit (MCU) application. An MCU is embedded into a light sensor chip. The original dual-mode master-and-slave dual-CPU architectures are combined to be operated as a single-CPU architecture. Since the original circuit pin design is followed, it is possible to be compatible with the old circuit design. The present invention uses a single-CPU architecture to directly control light sensors. Through the configuration of RAM, an inter-integrated circuit bus (I2C I/F) can be redirected to an internal non-volatile memory to switch the operational mode of the light sensor chip from a slave machine to a host machine which switches off the interrupt pin and, then, turns to a GPIO pin. Thus, the present invention provides a simple single-CPU architecture with easy use and effectively-lowered cost.