Hi all, I have this sensor which monitors the moving of a steel bar (see attached pic). The green part is moving backward and forward (the arrows), the gray is stationary. What I would like to know is the principle of this sensor. At first I thought it uses a small led in the gray one and the green passing through will gives signal. But I can not see the led. It may be based on magnetism, but the two are made of plastics... and I am puzzled. Any input is appreciated. Thanks.