Terry Pratchett told us a tale of making a crude vision system out of one of these. If I recall correctly, he figured out the memory modules were light sensitive, and if exposed to a focused image, the image could be 'read' straight from memory. I THINK he mentioned part of this in his speech at Penguicon 2.0 (Novi, MI -- 2003)... Actually, he DID!!! I just found a recording of his speech:

http://www.archive.org/details/Penguicon_2.0_Terry_Pratchett...

He starts talking about playing with the ZX-81 about two and a half minutes in, and at a little after five minutes, starts talking about making one "that can see things." This so impressed fellow guest Eric S. Raymond, that he came charging out of the banquet hall after the speeches to demand to hear more!

I had something of a front row seat to this conversation, as I'd already approached Mr. Pratchett to ask about how he'd wound up too radioactive to enter a nuclear facility (also mentioned in the speech, but kind of glossed over). So there I am on the inside of a crowd of onlookers as Terry's elaborating on the radiation story, with a flabbergasted ESR staring in disbelief, and each of us standing not much more than a meter from each other.

It was... an interesting experience.

I do wholeheartedly recommend that speech. Terry Pratchett is every bit as funny at the podium as he is on paper. He had us rolling!