After some frustration with other Arduino-based oscilloscope projects Martin Vaupell came up with this own design. I really like his video because he shares the process he went through to get to his final design.

Clearly the limitations are 0-5v but that is fine, what I needed to know is either wave type and frequency, and this is sufficient. It also gives me an indication of the amplitude which is just a nice bonus. But what I’m most happy with, is the two channel feature, I often use the second channel as a reference point to be able to differentiate the main channel on the screen.

Source: Arduino: A simple DIY Oscilloscope with Arduino… | element14