Would you like a cheap oscilloscope? Do you have an Arduino and a LCD display to hand?
Some videos of Arduino Scopes
Some other people’s projects
And an unfortunate fail
Julian’s Kit Build (FAIL): Banggood £15 ($23) Oscilloscope Kit
Julian’s Kit Build: Banggood £15 ($23) Oscilloscope (update)
Banggod are just the distributor, the original site is the JYE Tech DSO 062 DIY kit.