Light pens?

[Tony Duell]

> 
> > > 2. Does anyone have any light pens for sale or trade?
> >
> > No, but it's not hard to make one if you don't require high resolution.
> > There was at least one design in 80-micro for the Tandy 1000. There were
> > plenty of designs for the Beeb (most of them use the Honeywell 'Sweet
> > Spot' devices). Thoese were not as good as the Torch one I mentioned, but
> > they do work.
> 
> There are a couple on ePay at the moment, too.

It may be worth mentioning that microcomputer light pens tend to be 
somewhat interchabeable.

They need either +5V, +12V, or both as power, The actual light pen signal 
is a pulse that occurs when the light pen detects light from the CRT 
screen, it's almost always TTL level, and the only possible problem here 
is that it's the wrong polarity, which can be trivially fixed with a 
74x04 inverter. If there's a pushbutton switch on the light pen, most 
likely it just grounds the appropraite wire when pressed.

DEC lightpens (at least the one on my GT40) are different. They're just the 
phototransistor, the amplifier, etc, is inside the display. Microcomputer 
lightpens tend to put the electronics in the pen or its interface box.

-tony