Hello keith1200rs, FvM,
Thanks for your all discussion and
keith1200rs: Ya, your advice is work try adding a resistor from base to ground but we still can play the R1, and R2 value around rather than base pull-low. The transistor is not too slow to come out of saturation because i did try Lo ~800ns pulse for it and the transistor able to saturate.
Sorry, for never reply for so long... because i'm spent much time on doing testing until yesterday nite. Finally i get the thing work. May be late by to day I will capture the waveform with using scope then we analysis the result again and see why the BC817 can't saturate.
Thank a lot...