How does a steel bar position sensor work?

[pixel01]

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.

 

[Danger]

Clarify something for me, please. You said initially that the thing measures the movement of a steel bar, then later that the parts are made of plastic. Do I take it, then, that the green bit is a plastic part that's attached to the steel?

 

[pixel01]

Clarify something for me, please. You said initially that the thing measures the movement of a steel bar, then later that the parts are made of plastic. Do I take it, then, that the green bit is a plastic part that's attached to the steel?

Yes, the green part is screwed to the steel bar that is moving.

 

[mgb_phys]

It probably is an optical switch but uses an infrared led.
The LED and photdiode will be inside the grey plastic. Thin plastic is fairly transparent to near IR and it will keep the optical parts clean.

 

[pixel01]

It probably is an optical switch but uses an infrared led.
The LED and photdiode will be inside the grey plastic. Thin plastic is fairly transparent to near IR and it will keep the optical parts clean.

Thank you. I have checked and it's a photomicrosensor.