building the Single-Sided Serial Board, but don't have a BC547.
can I use a 2N3904?
Yes; the transistors are very generic, and you can use pretty much any general purpose NPN and PNP transistors.
Be careful of pinout, however. It seems that many "American" transistors have "EBC" pinouts, while European/Japanese transistors have "ECB" pinouts. So you might have to twist a different-numbered transistor around to get the right leads on the right pads.
Thankyou as always westfw! 