Photometric circuit incorporated in objective provided with automatic diaphragm for closed circuit television (CCTV) camera

摘要

A control circuit for automatically opening and closing the diaphragm of the objective lens of a closed circuit television (CCTV) camera, including a variable gain inverting amplifier for amplifying and inverting video signals from the camera, a voltage doubler and rectifier coupled to the output of the inverting amplifier, a reference voltage generator, a diaphragm control assembly for comparing said reference voltage with signals output from said voltage doubler and opening and closing said diaphragm in response to the results of said comparison, and a series circuit including a clipping diode and variable resistor coupled between the input and output of the inverting amplifier. The clipping diode clips negative going portions of signals output from said inverting amplifier and adjustment of said variable resistor selects different light modes by controlling the degree of clipping.