Constant image brightness automatic adjusting circuit

The invention relates to the image brightness automatic control technology field and particularly relates to a constant image brightness automatic adjusting circuit. According to the adjusting circuit, a set gray value signal and an output signal of a position feedback loop connected with an input end of a contrast module A are respectively received by two input ends of the contrast module A, an output end of the contrast module A is directly connected with a position loop operation network, two input ends of a contrast module B are respectively connected with the position loop operation network and an output end of a rate feedback loop, an output end of the contrast module B is connected with a rate loop operation network, an output end of the rate loop operation network is sequentially connected with a digital analog converter (DAC) and a power amplification module, and a motor driving control signal is outputted. Through feedback proportion coefficients and a lead-lag network of the two loops in the adjusting circuit, the system realizes high dynamic following and no concussion. The adjusting method is stable and reliable, the camera image can be constantly maintained at the set gray value, and the response speed is fast.