Re: Sinclair ZX81
The ZX81 keyboard was always made down to a price. There's being original, and there's masochism!
I would be tempted to get fifty or so large "tact" switches and assemble a "proper" -- well, more proper than the original, anyway -- keyboard using copper strip breadboard or just plain matrix board with point-to-point wiring.
The ZX keyboard is split into eight half-rows (space, ., M, N, B; newline, L, K, J,H; P, O, I, U, Y; 0, 9, 8, 7, 6; 1, 2, 3, 4, 5; Q, W, E, R, T; A, S, D, F, G; shift, Z, X, C, V) each with five keys. Each of the common connections of each half-row is grounded in turn, and any input connected to a key within that group which is being depressed will read "0".
The extra keys can be used for refinements such as a right-hand SHIFT key and a "space bar" with a suitably-sized piece of plastic bridging across multiple key switches.
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If I have seen further than others, it is because I was standing on a pile of failed experiments.
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