TWO-MODULUS VARIABLE FREQUENCY-DIVIDER CIRCUIT

An original document detecting device includes at least a pair of elements made up of a light emitting element and a light receiving element oppositely arranged on both sides of an original placing platform which is used for placing an original document, for example, on a copying machine. When no original document is present on the platform, the light receiving element receives the light from the light emitting element. When an original document is placed on the platform, the light is interrupted because of the presence of the original document. The detecting apparatus is provided for detecting whether the light is received by the light receiving element or the light is interrupted by an original document. Because the light is interrupted, the presence or absence of the original document is detected. There is also provided a sensor for detecting the intensity of disturbance of light and drive control apparatus for controlling the intensity of the input value of the light emitting element, so as to control the intensity of the input light of the light receiving element dependent on the output signal of the sensor. The apparatus can also be used for determining the size of the original document when an original document is present on the placing platform.