@buiquangdinh I say you could find it vry helpful reading through POST LINKED HERE .Oh God, I have try many times, but it's still not working .... I need some solution for it. I seen some video and they did it very simply. And I don't know why ??? I try to do with the same way at the first time, I watched a lot of videos about tutorials but something wrong, something I don't know how ?
So here's some other thoughts:
Whole trick is enhancing feedback from secondary (which as practical matter means giving it reactive load) but even radiation feedback from like _antenna action_ of wire connected to high Z end of secondary can sometimes make Osc run. Also cuz current is vry low (like < 1μA) you shouldn't expect big obvious sparks. You can get idea if it's doing anything by putting linear fluorescent lamp near secondary looking for glow in dim room! Once you see that (which means Osc is running) you can _tweak_ circuit for better performance. But just remember it's super low power circuit which is good for safety but basically vry boring for effects