Attenuators used to regulate optical signals are disclosed. According to one aspect of the present invention, an assembly including an IR source and an IR sensor is provided to sense blockage of optical signals transmitted between two collimators via a fixed common light blocker that is driven by a stepper motor. The movement of the light blocker is sensed by a sensing assembly including an IR source and IR sensor. By detecting the photocurrent from the IR sensor and a feedback circuit, the attenuation of the optical signals can be well controlled.