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Mixes the input in analogue fashion, then tries to pull low if the mix result is either low or high, but not in between. The last transistor cleans up, inverting in the process.
Huh! When I made it, I didn't think I'd done a proper job. I wrote "Could probably be better," but it looks like it's much better than I thought. The first two transistors can have almost any gain* and it still works. I'm keeping the last transistor as somewhat high-gain anyway, but it too can vary a lot.
*: Very low is bad. I think only some big power transistors have gain low enough to be a problem here, and big power is certainly not needed in those stages.
Maybe later I'll try giving it the 4 inputs I wanted to.
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