Sol-20 light pen project

**Hugo Holden** · November 2nd, 2019, 09:49 PM

I have just completed my light pen project for the SOL-20. The article describing the Hardware and the Software is here:

http://worldphaco.com/uploads/LIGHTPEN.pdf

A short video is here:

https://www.youtube.com/watch?v=awPFZCJFguM

As noted in the article, there were some interesting technical challenges. The article also details the design and explains the circuitry. I had never tried a light pen until doing this project, it was a lot of fun designing the hardware with the software control in mind and the notion of what is better done in hardware vs software.

**Chuck(G)** · November 3rd, 2019, 05:56 AM

Interesting, Hugo! The light pens that I recall of yore had a switch activated by pressure on the pen tip. Clearly, these were made for selection and not drawing. It's worth noting that several LSI controller chips, such as the Moto 6845 and Intel 8275 already have light pen capture circuitry on them. Depending on the phosphor used, it's approximate at best.

I've wondered if a flying spot "locator" could be simulated on a modern LCD display.

Does anyone remember the touch-screen project that Steve Ciarcia (I think it was him) rolled out back in the CP/M days? Basically a rectangular frame with LEDs on one side and phototransistors on the other. Your finger interrupted an X- and a Y-axis beam that would indicate your choice. I think the resolution was only about 8x8, but was sufficient for a "pick one" functionality.

**ziloo** · November 3rd, 2019, 06:52 AM

Originally Posted by Hugo Holden

I have just completed my light pen project for the SOL-20. .....

Brilliant work Hugo...

How many man-hours did it take from start, research, design, build, troubleshooting, and .....finish...

ziloo

**Hugo Holden** · November 3rd, 2019, 08:39 AM

Originally Posted by Chuck(G)

Interesting, Hugo! The light pens that I recall of yore had a switch activated by pressure on the pen tip. Clearly, these were made for selection and not drawing.

This particular pen is an interesting one. The buttons on it do not physically move, it detects the presence of the finger with a light touch. Also, in front of the photo-transistor, there is an objective lens, so a pixel gets focused when its a couple of inches from the screen surface. I think its a good idea because it doesn't have a chance to mark the CRT's faceplate. It has a resolution of one pixel.

**Hugo Holden** · November 3rd, 2019, 08:46 AM

Originally Posted by ziloo

How many man-hours did it take from start, research, design, build, troubleshooting, and .....finish...

ziloo

I didn't count it up but on and off over about 3 months at weekends etc doing a little here and there making the card. Most of the time for me was figuring out the software side of it.