Sounds cool. I have decided to build one of these. It's a very simple signal injector and audio probe on a single board. Not as handy as a scope, but still useful. I just need to check some datasheets to see which transistor I can sub. I don't have any BC547s on hand. I do have some other NPN small signal transistors (S8050 and others) that should work. Maybe breadboarding and some suck it and see engineering will be faster than reading datasheets