I've build a signal injector to debug a guitar pedal that was not working. It was a nice little journey in itself. The astable multivibrator produces so much harmonics that I could hear it all the way back from the input jack, where it was supposed to be silent. Heck, I could hear it just by putting the probe nearly close to the circuit. The signal pushed through the circuit like Juggernaut breaking walls. Learned a lot about filters and was able to produce a nice sine wave out of it, it worked great.