Which signal voltages do you get?
The LED is fed with current/voltage from the signal, so it's not possible to give more juice without dragging the signal even lower. The given resistors are choosen carefully to give about 4V signal voltage (ATmega specs 3V as minimum for logic "1").
If you want a bright LED you'd have to add an amplifying transistor (and loose the K.I.S.S. principle somewhat).
The LED is fed with current/voltage from the signal, so it's not possible to give more juice without dragging the signal even lower. The given resistors are choosen carefully to give about 4V signal voltage (ATmega specs 3V as minimum for logic "1").
If you want a bright LED you'd have to add an amplifying transistor (and loose the K.I.S.S. principle somewhat).