Cathde Ray Tube Monitors - or "CRTS" are large, glass vacuum tubes that contain (an) electron gun(s) that sweep from left to right across the inside of the front glass of the tube from top to bottom, which basically light up phosphors that show the picture you see on the screen of the monitor about 30-75 times a second - or Hertz "refresh rate". The phosphors are lit up so fast it looks like a constant image, but it's really just one pixel at a time being lit up by the electron gun(s). Monochrome monitors have a single Electron Gun and the phosphors determine the color of the pixels. Color monitors have 3 guns - Red, Green, and Blue - and the phosphors are coordinated to a shadow-mask - sort of like a window screen that separates the three colors for each "dot" - which allows the red, green, and blue to be mixed at different intensities to generate different colors. So each "dot" is lit up - usually around 60 times a second.

So how the light pen works in all of this is a Phototransistor at the end of the pen is placed near/on the front of the screen, when the pixel underneath is lit up, it sends a signal back to the computer to tell it where the pen is on screen, and then can tell the software to act accordingly. So say, if you have a a drawing program, once the dot passes under the pen, it draws that dot or group of dots on screen, and then moving the pen causes it to draw dots along with the movement of the lightpen.

Another similar device we will talk about here, is also the "Light Gun" - as I may be messing with adapting a Nintendo Zapper to work with the Tandy 1000 as well (hehehe, Tandy 1000 Duck Hunt!). The Light gun works in a similar way. It's basically a triggered light-pen. Have you ever noticed when you play Duck Hunt on the original Nintendo on a CRT TV that when you pull the trigger, the room flashes. No, that's not a placebo thing from the "Kapang" frorm the Zapper, that's really the TV flashing for a frame to a completley black screen with a white square where the duck is located. If the Zapper's phototransistor is aimed AT that VERY intense white square, it registers a hit, and the dog does not laugh at you. This is also why you can cheat by pointing the Zapper at an incandacent light bulb.

Both technologies WILL NOT WORK WITHOUT A CATHODE RAY TUBE TV/MONITOR! THey also will not work with VGA on such a simple scale either for similar reasons. Most PC's that had Lightpen support - including the Tandy 1000, used CGA, or "Color Graphics Adapter", which had the ability of tracing it's own pixels this way as tthe support was wired into the supporting circuitry.

First off, LCD Panels, even today's high end panels, are not capable of generating an intense enough white light for things like the Zapper to register, and Light Pens tend to have problems with this as well. Cathode Ray Tubes are basically fancy light bulbs capable of multiple colors, LCD panels are similar, but they either generate their colors using a series of filters and shades known as "newumatic twists" (STN/DSTN) that block light/colors, or TFTs that use very small transistors that generate light that are incapable of generating powerful enough light to be noticed by such a device.

To add to it, these don't work with LCD's because of how LCD's produce an image. LCD's produce images differently, in that the pixels are permanantly lit up, and generate a solid image - to make for a simple answer. Ever notice on TV or YouTube when someone is filming a CRT that there is a diagonal or straight line going up and down the screen, or the screen seems to be blinking really fast - that's because you are visually seeing the screen being redrawn, and it's playing havoc with the camera's own shutter. However, you notice on old laptops, you get a nice, solid, sometimes more vibrant image than in real life using a camera....that's because the LCD panel does not work in the same fashion.

That said, it did have software, which I was able to obtain both via TV Dog's Tandy 1000 Archive and Mike Brutman's PC Jr. Page forums. One is a configuration utility which is what I was using to test out some early prototype light pens I tried to design for the Tandy 1000 back in 2018-2019ish when I first started this series of experiments. The other was "Penmouse" which I was somewhat able to get "kinda-sorta-working" in Turbopaint (a TGA compatible graphics software program I use sometimes).

It seems the heyday for light pens was just around the time the PC was first coming out - sort of circa 1978-1983 or so, with the earliest examples being in the 1950's on huge, building sized mainframes, and some of the last being units for the Vectrex game system, and probably the Warp Speed Light Pen itself. See, light Pens do have one particular problem, when appearing on a dark (black) section of theh screen, it's basically "Blind". This is why most drawing programs that are orientated to a Light Pen, start with a lighter color background, so the light pen can easily "see" where it is on screen, as black is the absense oof any pixels being drawn whatsoever as the electron guns are off when they hit that part of the screen.

Right off the bat I could figure out in general, how this was working (somewhat). +5vdc is there to both drive any active circuitry on the lightpen (or it's associated adapter box) itself. Pin 2 of course is ground and the negative lead for power as well (-5vdc), the "light pen in" sends back the signal from the photodiode/phototransistor back to the CGA card in the Tandy to tell it where to carry out actions on screen from said lightpen/light-sensitive device, and then the "trigger/switch/button" pin (pin 4) is for one side of the push-button/switch/trigger/etc to either act as a mouse button would, or a trigger for a light gun, or even possibly trigger the lightpen itself.

However, it's been difficult to figure out how to get this working on a Tandy 1000 because there's little to no documentation on a CGA-based Lightpen from the 80's anywhere. The majority of what I have found has been for Vectrex, Apple II, and Atari computers. There also was one for the Tandy CoCo as well which used a Tape Deck for an amplifier.

Atari 8-bit DIY LightPen - This was the first schematic I took a look at. It looked simple enough, a Phototransistor with a resistor on one side of the plug. This was definatley some 80's hackery here because they did not even bother listing what's the Emitter, Collector, nor Base on this thing IIRC. What I do like though, is that they talk about using a regular ballpoint pen casing for this, which woudl be something I would be up for. One of the primary reasons I'm doing this is so that I can do sort-of-like a "Bob Ross/Joy of (CGA) Painting" thing on my YouTube channel with such device once I'm done (might even don the big curly wig again, it has more purposes than Halloween contests as Neal Schon of Journey or Paul Dean of Loverboy ya' know!). I did try this circuit using a disused Mouse infrared sensor and a resistor and a cable from an Atari joystick and managed to get some response from it, though sometimes that response would go insane (probably because I was holding the bare wires with my fingers like an idiot). Seemed like we were already on the right track.

Nerdly Pleasures Blog Post on Thte PC Jr & Tandy 1000 Lightpen - Next we look at this one. Though it seems more like it'd be better for later on should I decide to program my own, original driver for the Tandy 1000 - maybe for better accuracy of the pen based on graphics mode, since it talks not at all about lightpens, but rather the CGA Graphics registers (and actually, I'm rather intnerested in using this to draw in the AGI Graphics mode as well).

Old Lightpen VCFED Thread - This was another old thread on VCFED which I can't even find now, probably need to look in the archives somewhere to find it (archive.org?). IIRC it had some useful information on somewhat, but I can't remember. I think it even had a schematic on it. I'll have to look around for it.

Basics of Lightpen Operation - Atari Magazines - Basically, some more information on operation of a Lightpen in the sense of Atari's unit. This one has a more elaborate schematic with a comparator (LM3900) and a 10K Linear trimpot for fine-tuning theh gain of the Phododiode 2N577. Sttill a relatively simple circuit though, but finding discreet components has been hard.

DE9 Lightpen Schematic On A Wiki Site - This one goes into detail on multiple lightpens on multiple platforms including Amiga, Atari, Commodore 64, and others. It includes a pinout table that shows the common wiring between devices. Out of all of these, this seems to be the most useful port of reference of the lot.

Looking at various products on e-bay, including pens for the Apple II, told me that some of these had a basic pen assembly, but needed a secondary box to work. I believe one version of the WarpSpeed device was like this as well. So I'm kind of all-over-the-place when it comes to what the circuitry in one of these things should look/function like.

So I tried a few different ideas. One was the Atari one, then came another one where I used a CD40106 Hex Schmidt Trigger based on an actual CGA/EGA Lightpen schematic which kinda worked as well. One trip in Silicon Valley lead me to one of the last standing Fry's Electronics where I bought some Photodiodes and a Phototransistor and made some attempts that way, all with varying degrees of success. Ultimatley I stopped with it because I had other, more pressing projects to get into.

At one point I found an "AMF Light Pen" advertised on E-bay for dirt cheap. S I bought it, and it turned out instead, to be a Barcode reader pen (!!!). That said, that opens up a whole new can of worms - maybe possible to make a multi-purpose device that can read barcodes, act as a light pen, and double as a "zapper" on the Tandy as well - a Tri-purpose photosensitive device. I could totally see somehow using that lightpen register redirected somewhere for barcodes - maybe to an app for DOS that could pull up SKUs off the internet for info retrieval or archiving/collection tracking? I did manage to re-wire the pen using a DB-9 and a breadboard and got it to light up and take some kind of input at least, though it did not work as a lightpen as far as I could tell. I would need a way to switch that LED off for Lightpen/lightgun use.

Another idea I have looked into, very recently (2022) is adapting the NES Zapper for the Tandy 1000, which looks to be one of the simplest for a "Light Gun" solution. Basically, I'd just get a NES extention cable, hack off one end for a DB-9, and rewire the same exact pins to match the pins on the Tandy - +5 to +5VDC, blue wire to the Lightpen in, Ground to ground, Trigger to Switch. This should allow me to use a NES Zapper on a Tandy 1000 computer - so if nothing else, could make some kind of crazy Duck Hunt style game for the Tandy and shoot at objects on screen no less, and possibly develop my own dedicated Tandy Light Pen off of this.