Algodoo

A rough timer made using gears. I had it timed to 3 minutes in the video but it varies because there is no escapement.

Right click the box and turn it so that the white pointer turns with it, and turn it until it touches the green box to start.
The red box has a script that sets the sim.frequency to 600. Lower that if it runs slow for you but it will become unreliable.

Video at:
https://www.youtube.com/watch?v=z1kd4-QvkIA

Check out the algodoo subreddit:
http://www.reddit.com/r/Algodoo/