After stumbling across a load of old copies of Practical Electronics and Everyday Electronics (now merged), I found some rather interesting articles on hardware projects for Sinclair machines. Most are for the ZX81, but some are for the Spectrum.
Rather than keep these goodies all to myself, I have decided to scan them all in and upload them for your delight and delectation. Please note that they are straight scans – I haven’t OCRed them at all – so they are fairly big (about 300-400k per page). And don’t complain to me if you fry your ZX81s