Hi there Ma'ams, Sirs,
I would go directly ask my question about PN100.. I dont know what was really its purpose in some of the circuits, like Mr. OddBot's IR eye compound.. I dont know how it works... I tried to search it in google (http://www.fairchildsemi.com/ds/MM/MMBT100.pdf) ..yeah, I'm sorry for your disappointment that I just dont know about those stuffs to be honest.. Then I tried to search in youtube, but I got no answer.. (http://www.youtube.com/results?search_query=PN100+tutorials)
Hoping for you're answers..
-VerDe