how to make a motorized pendulum mallet

Hi all,

I hope I've come to the right place. I need to make a large scale pounding mallet that can repeat a pound every 5 seconds or so. I'd be using a mallet approximately 300mm in diameter. It would pound away at a large cube of compounded dirt - a mud brick of sorts - that is roughly 1.5sqm or so.

So, I hope it could come out looking a little like this -

http://www.electronics-lab.com/blog/?p=6799

I also assume that some weight bar will be required on the end of the mallet stick to allow for a pendulum like motion.

If I've come to the wrong place, sorry. If not, I hope you can help. So sue me for being an artist.

Best