I made 3 different designs.

1) This design will detect and extend the piston instantly. The problem with the soul sand is that the mob will fall for 0.3 ( more or less ) seconds when spawns, so the system is too fast, and the mob will fall only with the second shift.

http://ift.tt/1LEkTGK

2) With a repeater the mob will fall at the first shift, but the system is a bit slower than the first.

http://ift.tt/1TwVJAj

3) The mob will fall at the first shift. The system will stop if it happens. If not it will continue as long as the mob will fall.

http://ift.tt/1LEkTWZ

I'm testing the 2 and 3 designs, the first is totally safe. Another problem is that if the witch is on the edge of 2 lines it will not fall cause the timing is different. When the first line is pushed the second is stable etc. They are not farms yet, just ideas I came out with.

Feedback is appreciate.