Solidstateoscilloscope

No substitute for a professional oscilloscope by far – but still could be a fun project to have a go at –

probably the best advantage is its very small size and the fact that it can run off the power supply of the circuit being tested. Although it has a low frequency range, it can still be used for most circuits. Its poor resolution will still allow for most waveforms to be visualized.”
[…]
It uses a matrix of 100 LED’s for a display, and does suffer from being slow and having rather poor resolution. Still we could display a sine wave running at 500Hz without trouble

Solid State Oscilloscope

Related:
Video: Dave Clausen and his LED oscilloscope project