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Problem: An infrared light barrier based on U2531B

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tomeeh

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Hello.

Does anybody have any experience with the IC: U2531B?

I bought an infrared light barrier (a Conrad kit; produced by Kemo); the receiver is using that IC. I've read the instructions carefully, tested it twice, but there is a problem.
The receiver detects the IR light (that is T2 is on, LED is on), but the interruption of the IR doesn't change anything - the receiver still "thinks", there is an IR light (T2 doesn't go off). Actually I can say, the receiver is 'locked'. I even tried to decrease the gain by lowering the R3 value (from 150kΩ to 1kΩ), but it didn't help. The receiver locks still.
Does anybody have had such a problem?

The transmitter is a typical 25kHz generator supplying the IR transmitter diodes.
Below, there is a receiver schematics.
 

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