The ZX81 was contemplated with a great amount of peripherals of diverse developers, such as memory cards, sound generators, modems, HIRES cards and disk controllers. Here will be presented diverse hardware projects destined to extend the limit of these computers and also the schems of the computers and peripherals, when available.
TK82 with 8Kb ROM but without SLOW circuit
The schem has some imperfections :-(
Implementing the SLOW on the NEZ-8000 (Interface Magazine, sorry the article is in portuguese):