PURPOSE: To improve the linearity of a sensitivity adjusting volume to facilitate the adjusting work and to obtain a high gain with a small response difference by applying a DC potential to one side of a variable resistance together with input of an amplification signal.
CONSTITUTION: A photo diode PD and a resistance R 1 constitute a light reception circuit 1. An operational amplifier IC 1 constitutes an amplifying circuit 2 together with resistances R 2 and R 3 and a capacitor C 2 , and a DC potential V d is given to a sensitivity adjusting volume VR by a resistance R 4 . A threshold V p as the reference voltage of a comparator IC 2 is determined by resistances R 5 and R 6 , and a hysteresis is given to the comparator IC 2 by a resistance R 7 , and they constitute a comparator circuit 5. Consequently, the adjusting work is facilitated because the rotation angle of the sensitivity adjusting volume VR and the detection extent are linearly changed.