[lm741]

These are really easy and fun to put together. I'd highly recommend it for a weekend project.

Here's one that I built running with a sample of Americium 241 from a cheap smoke detector: http://bit.ly/UDWpc0

Here's a different one running with a bit of Thorium containing lantern mantle http://bit.ly/1Annrp5

You can also try with thorated tungsten welding rods and other household sources of radiation.