It's not difficult. You need a camera and a pinhole. With my Speed Graphic, I tape some tinfoil over a lensboard and poke a hole in it with a pin. If you know roughly the size of the hole you can make a good enough estimate of the exposure time. Then it's basically point and shoot.

Or a scanner: http://dwarmstr.blogspot.com/2011/02/adventures-in-large-for...

With a digital camera, a person could go out in bright sunlight, play with ISO and/or shutter speed, then calculate fstop using Sunny 16 and looking at histograms.

I was think more about buying a sensor and an arduino....

When I looked, most parts-house available sensors are small and even a 1" format sensors are pretty dear. MFT, APS-C, etc. were a multiple of a complete entry level interchangeable lens camera. For a pinhole camera with a small sensor, it's going to be hard to frame a specific subject because the sensor's field of view is tiny. Even a wire viewfinder will probably struggle with framing due to parallax. Basically the low cost sensor for pinhole photography is photo-chemical film.

A hacky alternative for a might be modifying a flatbed scanner. Just step the photo array slowly enough that there's reasonable exposure at each point. The good part is that a flatbed scanner is a stupid big sensor and there is some literature for hacking them. But for a pinhole, I'd expect the exposures to be rather long even for pinhole photography.

This post [1] describes how to use the electronics in a webcam. But you're essentially just replacing the webcam's lens with a pinhole.

What you're basically asking is how to build a webcam from components. Which I assume is possible but non-trivial.

[1] https://shutterhub.org.uk/how-to-build-a-digital-pinhole-cam...