I just read an article on hackaday.com called "amazingly cheap oscilloscope" (about 5euros)
check it out, it's perfectly applicable for this.
One note though,
I don't agree on calling this an oscilloscope, the sampling rate is waaaay to low for that. I think it's better to call it a data acquisition device or similar.
This would become too complicated for newbs, they could think "why would I pay 500€ for an o-scope when I can make myself for 5€ ?!)