iPhone 3gs has broken power button, can't restore it

I have a iphone 3gs I just restored it to 6.1.3 and now that i dont have assisitive touch, Im at the activation screen on the phone and I won't read my sim and I cant put it into recovery mode. Is there a way to put it into recovery mode by letting it die then rebooting it up? I want to restore it back to 6.1.2

Thank You