¢FUTURA'S HARDWARE WAREHOUSE PRESENTS¢¢REDISCOVERING THE LIGHT-PEN¢¢WITH THE TECH-HEAD¢¢¢ The light-pen is one of those things you may have seen on Tomorrows World but never come across in real life. This input device, never very popular in the first place, has now been replaced by the mouse for serious applications. Even its games oriented relative, the light-gun, is not so common.¢¢ So what is a light-pen? In brief, it is a device that can sense where it has been placed on the monitor screen. The light gun is the same except that it uses a focusing lens so it can be used at a distance from the screen.¢¢ The Atari computers have always had the ability to use a light-pen or gun, but very few commercial devices and programs to use them have appeared. A light-gun and a few games are still available from Micro Discount, as is a light-pen and graphics cartridge.¢¢ However, these items are not cheap and as I discovered it is possible to build your own light-pen quite easily. ¢¢ I had recently come across issue six of The U.K. Atari Computer Owners Club magazine, later known as Monitor. This contains an article about the building of a light-pen. Knowing that this club had close connections with Maplin, the well known electronic supplier, it came as no surprise to find that the light-pen was available as a kit from them.¢¢ Unfortunately, it had just been deleted from their 93/94 catalogue. However, I did have the article and some suitable spare components, remnants of an earlier electronic project, so I decided to have a go.¢¢ The printed circuit board from the kit was still available, and was purchased along with the few components I needed to complete the project.¢¢ Departing from the suggested design, I built the entire device in a small plastic box, with the photo-transistor emerging at the front, the trigger at the side and connector at the rear. This enabled connection to the computer with a joystick extention lead.¢¢ Construction could not be easier, just solder the components to the PCB and install in the box after drilling a few holes for the external components.¢¢ Three sample programs were included with the article, one of which is used to adjust the sensitivity of the device. This took quite some time as I discovered that correct adjustment was critical to achieve satisfactory operation.¢¢ Once this was done, the pen worked quite well, but was by no means error free. The very simple programs provided demonstrated drawing, menu selection, and a musical instrument.¢¢ So having built your light pen, in order to use it you must now write or modify your own programs, as there seems to be no software available. There is also a small problem to be overcome when writing these programs.¢¢ The Atari stores the horizontal and vertical co-ordinates of the light pen in locations 564 (LPENH) and 565 (LPENV). The problem lies in the horizontal register. The positions are numbered from 78 at the left of the screen, increasing to 227 as you move right. It then resets to 0 and increases again to 7 as it reaches the extreme right of the screen.¢¢ There are 96 vertical positions, numbered from 16 at the top to 111 at the bottom, no problems here! The registers are updated whenever the trigger is pressed, all four ports can be used.¢¢ In view of the drawbacks I have outlined above it is easy to see why the light-pen did not catch on. And I think mine will soon be gathering dust when the novelty has worn off.¢
