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Old 22nd Apr 2017, 10:00 pm   #10
cmjones01
Nonode
 
Join Date: Oct 2008
Location: Warsaw, Poland and Cambridge, UK
Posts: 2,669
Default Re: What is it and how to use?

That does look like a home-made light pen. There was a time when they were quite popular for drawing on computer screens. The phototransistor picks up the light pulse created as the electron beam scans the CRT, and the computer's graphics circuits use the timing of the pulse to work out where on the screen the pen is pointing. It doesn't work if the screen is blank, of course.

Believe it or not the hardware of the BBC Micro supported this natively: there was a pin on the analogue port to which you could connect such a light pen (I made my own with an old felt tip pen and a phototransistor) and it was possible to write software to use the pen to control the cursor, do drawing and so on.

There were plenty of other machines that could work in the same way, but light pens never really made it beyond some specialist applications: Tektronix graphics/CAD terminals and the Synclavier synthesizer spring to mind.

The 'gun' on various video arcade games of the 1990s, which usually seemed to involve shooting zombies, worked the same way but with a lens on the front of the phototransistor so it worked at a distance rather than in contact with the screen.

Chris
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