Over at the Mitchtech blog are the instructions for making a very, very simple oscilloscope using an Arduino and a host PC. How it works is easy - the values measured by analogue pin zero are sent to a host PC via the serial output, which are then interpreted by a processing sketch which displays the value over time. Note that you can only read DC waveforms between zero and just under five volts DC, but it could be useful as a simple logic analyser or just for fun.
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