JP2580756B

PURPOSE:To exactly generate a laser scanning light detection signal by measuring the time when the output of a comparator is in a low state by a counter circuit. CONSTITUTION:A level shift circuit 3 shifts the level of the output of a photosensor 2 with respect to the output of a photosensor 1 and the comparator 4 compares the output of the level shift circuit 3 and the output of the photosensor 1 and outputs a signal. The counter circuit 5 emits an instruction in such a manner that a judgment circuit 6 outputs a high state when the input from the output of the comparator 4 is in a high state. The time when the input from the output of the comparator 4 maintains the low state is measured by the counter circuit 5. The counter circuit 5 send the signal to output the input from the output of the comparator 4 to the outside when the specified period of time or longer time elapses. The false signal by noises and the signal of the photosensor are, therefore, discriminated. The generationn of the laser scanning light detection signal is exactly executed in this way.