A control circuit is applied into a projector and an operation method is provided for the projector. The projector includes a photo sensor provided for generating a sensing voltage according to light intensity sensed by the photo sensor. The control circuit includes a first voltage-comparing unit, a reference voltage generating unit and a second voltage-comparing unit. The first voltage-comparing unit is provided for comparing the sensing voltage and a first reference voltage, to generate a first comprising result. The reference-voltage generating unit is provided for generating a second reference voltage and determining whether adjusting the second reference voltage according to the first comparing result. The second reference voltage is relatively larger than the first reference voltage. The second voltage-comparing unit is provided for comparing the sensing voltage and the second reference voltage, to generate a second comparing result.