A photo detective LCD device includes a light pen. The light pen includes a body, a driving pulse generating module and a light generating module. The body has a pen shape, and an end of the body includes an opening through which light exits. The driving pulse generating module is disposed in the body and generates first and second driving power pulses having first and second frequencies, respectively. The light generating module generates first and second light in response to the first and second driving power pulses, respectively. The first and second light flickers in a third frequency and a fourth frequency, respectively. The power consumption is reduced, and the brightness of sensing light is enhanced. The light pen generates light having at least two different frequencies, and the display device recognizes light generated from the light pen effectively. Therefore, the display device may operate without failure.