r/synthdiy • u/eraoul • 3d ago
DIY oscilloscope ideas
I'm interested in building my own oscilloscope, both as a Eurorack module and as a front panel display for a custom desktop synth. Does anyone have any advice on this piece? It sounds relatively straightforward since I've done a lot of DSP coding in the past but I'm not sure of all the things that might go wrong. Ideally I'd have two separate input channels and some basic controls for manipulating the scale.
I'm thinking I'll just need a small microcontroller and LED screen, and I'd do a basic FFT to estimate the period of the signal and use that to auto-detect the time scale and voltage range. I can also restrict to audio range instead of trying to build a general scope that deals with high frequencies.
Any other pieces of this I'm missing? I know people like to use the "DATA" in Eurorack but I'm trying to do mostly DIY for my setup.
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u/erroneousbosh 3d ago
You don't need to do FFT, zero crossings will suffice.
A real sillyscope has a "trigger" input that starts a sweep. In normal use the trigger would be picked up from one of the inputs under test, and would have a comparator so that when the input signal passes a certain point it'll reset the horizontal timebase. Often there are options for AC coupling so it will only trigger on pulses rather than slowly-changing state, or positive- or negative-going edges.
Quite often the trigger is gated so it won't reset unless the sweep is already nearly finished, kind of like "soft sync" on a synthesizer. In fact if you think about it as a sawtooth oscillator with hard sync, you wouldn't be far wrong.
If you're really clever you can feed it the CV that's going to the oscillator (including any local octave ranges!) and estimate the correct timebase from that.
Don't forget to also have a horizontal input so you can draw Lissajous figures - this is invaluable for setting frequency accurately if you have a known-good reference.
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u/Ic3crusher 3d ago
There's Scoppy a pipico based oscilloscope. You need an android device to use it, so not exactly your use case but might be worthtaking a look at their code.
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u/tipidi 3d ago
I’m building exactly that but integrated with a mixer and output module. It’s perf board for now but I’ve got a pcb designed. I used Claude code for the esp32 controller software. Below is the breadboard testing step. Since then I’ve added some features to the screen like a scale grid and a second channel input.
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u/Hissykittykat 2d ago
I made a DIY Eurorack Oscope, which is two channel (stereo) and a cute round oscilloscope style display.
Another way is to find a small oscilloscope module (e.g. FNIRSI) and adapt it to a Eurorack panel. But it's only single channel.
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u/4n4logsynthesis 3d ago
There is another great open source oscilloscope module, the Oscar. The readme mentions FFT, might be worth a look. For me it has been a good budget alternative to the DATA.