An apparatus for detecting either a zero crossing or a voltage amplitude of an Alternating Current power source voltage inputted to an electronic device, the apparatus comprising: a first calculation unit for comparing the AC power source voltage to a first reference voltage to determine whether or not the AC power source voltage is smaller than the first reference voltage; a second calculation unit for comparing the AC power source voltage to a second reference voltage smaller than the first reference voltage to determine whether or not the AC power source voltage is larger than the second reference voltage; a pulse output unit for outputting a pulse if the AC power source voltage is smaller than the first reference voltage but larger than the second reference voltage; and a detecting unit for detecting a zero crossing or a voltage amplitude by analyzing time duration between two consecutive pulses which have been outputted from the pulse output unit, characterized in that the detecting unit comprises: an interrupt generating unit for generating an interrupt when the pulse which has been outputted by the pulse output unit dies; a time checking unit for checking a time duration from when the pulse which has been outputted by the pulse output unit dies to when the pulse output unit outputs a first different pulse afterward; a comparing unit for comparing the time duration checked by the time checking unit with a pre-set reference time; and a discriminating unit for discriminating a zero crossing within the time duration checked by the time checking unit if the time duration checked by the time checking unit is shorter than the pre-set reference time, or discriminating a voltage amplitude if the time duration checked by the time checking unit is longer than the pre-set reference time. a first calculation unit for comparing the AC power source voltage to a first reference voltage to determine whether or not the AC power source voltage is smaller than the first reference voltage; a second calculation unit for comparing the AC power source voltage to a second reference voltage smaller than the first reference voltage to determine whether or not the AC power source voltage is larger than the second reference voltage; a pulse output unit for outputting a pulse if the AC power source voltage is smaller than the first reference voltage but larger than the second reference voltage; and a detecting unit for detecting a zero crossing or a voltage amplitude by analyzing time duration between two consecutive pulses which have been outputted from the pulse output unit, an interrupt generating unit for generating an interrupt when the pulse which has been outputted by the pulse output unit dies; a time checking unit for checking a time duration from when the pulse which has been outputted by the pulse output unit dies to when the pulse output unit outputs a first different pulse afterward; a comparing unit for comparing the time duration checked by the time checking unit with a pre-set reference time; and a discriminating unit for discriminating a zero crossing within the time duration checked by the time checking unit if the time duration checked by the time checking unit is shorter than the pre-set reference time, or discriminating a voltage amplitude if the time duration checked by the time checking unit is longer than the pre-set reference time.