How do I create a swinging blade trap?

I want a blade that swings back and forth when a trap is triggered for a platform game. When the player hits the trap, the blade should start rotating until it reaches 180 degrees anti-clockwise, and then rotate 180 degrees clockwise, and then rotate another 180 degrees anti-clockwise, etc. I'm having a little trouble figuring out the events required for this. If it could have a little momentum on the swings to make it slow down on the upswing and accelerate on the downswing that would be great.

Can anybody help?

Here you go dude, hope I made it clear what I did. Feel free to ask some questions if you don't.

Thank you very much! I'm new here, so how do I give someone +rep for being awesome?

There is no rep system but I'll accept the compliment as a form of rep :^)