Make a level detector circuit that rectifies the output from the filter. the output from the level detector is a DC voltage that can light the LED.

 

look... i built the filter circuit... and i get half voltage on the op.amp output (2.5v , i use 5v)... i think the filter is good.... but the problem is that when i transmit or not - i get the same 2.5 output voltage... and it's not good .. because i can't set '1'\'0' ...

do you have msn messenger?