whit this circuit the led blink normal for about 5-10 min a after that dont turning off just the light is slower the port cant full turn off the transistor
I know you are not an electronic engineer by profession, but you should try to draw a proper schematics. There are inexpensive to free programs for drawing schematics - you can for example try http://tinycad.sourceforge.net . The schematics as is now, does not show us the type nor polarity of the transistor, you even don't have a current limiting resistor in series with the LED shown on the diagram (actually, is there any?)
We also don't know what particular 8051 derivative you used and other whereabouts of the application.
Although it's hard to tell "remotely", there are also chances that after the 5-10 minutes your application "runs away" for one cause or other, and what you see as "dimmed" LED light is simply a fast flickering LED, for example due to (nonexistent) external memory accesses.
@manish12:
I don't understand any of your suggestion, expect possibly replacing a defective processor.
Assumed, the transistor was meant to be NPN, the LED current limiting resistor missng in drawing is existing nevertheless (I guess, the 4k7 was only misplaced), and the 8051 is a standard type with open drain P0, then the circuit should operate exactly as it is.
wek you rigth, I'm not el. enginer the circuit that is on post are made simple on Corel Draw ) i use professional software and the led is with resistor I just put it to know there is consumer )
about power supply it is on 5V the wrong is mine , we change 2-3 transistors and start to work normal today i will chek there is not problem.