Bus btwn Puigcerda Spain and Andorra

I'm contemplating a day trip to Andorra next summer, as I move about Europe "collecting countries". I'm a train freak, so I am aware of a neat way to go between Barcelona and Puigcerda, which I'd like to do also. But I have not had much luck finding info on how to get between Puigcerda and Andorra by public transportation, bus, etc. Can someone help me please?