An ambient light sensor includes a photoelectric conversion unit, a current control unit, a signal conversion unit and a comparison unit. The photoelectric conversion unit provides a photo current having a magnitude corresponding to illuminance of incident light to a first node. The current control unit generates a control current exponentially proportional to an elapse time based on a clock signal, provides the control current to the first node, and outputs a digital code corresponding to the elapse time in response to a reset signal. The signal conversion unit generates a comparison voltage logarithmically proportional to a sum of the photo current and the control current. The comparison unit generates the reset signal by comparing a magnitude of the comparison voltage and a magnitude of a reference voltage. The ambient light sensor generates the digital code effectively representing illuminance of ambient light that eyes of a human being perceives.