Electronic – How CRT allows use of ligh-guns or pens etc


While reading about CRT on Wikipedia, an information (CRT Allows the use of light guns/pens; https://en.wikipedia.org/wiki/Cathode_ray_tube#Advantages_and_disadvantages) suddenly triggered a a vast lots of memories.

Around early 1990's when I was a toddler, in a local "mela" or fair each year, there was a stall for video games .

They had several big boxes (maybe CPUs), several tiny to large sized, portable monochrome televisions with a mesh of cables. They provided any 1 of 2 kinds of input devices, 1. a game-control with arrow-keys & fire-key etc; as-well, 2. a toy Gun-shaped device.

Game in CRT with a gun-like input device

(The above diagram drawn completely based-upon memory.)

In the game, to destroy the enemy (a rocket, or an octopus, or a ghost etc), the tip of the toy-gun were almost-touched on that object on screen, and on pressing the trigger, the selected rocket were destroyed.

It was, specifically surprising to the kids, because that was a pre-touchscreen era. The television set, acted quite like a touchscreen.

Now I want to know, how that systems worked?

Which-one is acting as the sensor (of the particular place where the rocket was fired)? the toy-gun or the CRT? What is the mechanism to identify a particular spatial-position (of target-object such as rocket) out of many-other objects (other rockets)?

Best Answer

The CRT TV picture was generated by deflecting an electron beam fired from the back of the tube in a raster scan as shown in Figure 1. Two sawtooth oscillators control this: one for the vertical and one for the horizontal - the horizontal one running much faster than the vertical. For PAL used in Europe there are 625 lines (horizontal) refreshed at 25 frames/second (vertical). (It's a little more complex as the lines are interlaced to reduce flicker. The vertical scan runs at double the frequency alternately drawing the odd and even lines.)

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Figure 1. CRT TV raster scan. Source: Wikipedia Analog Television.

The electron beam excites the phosphor on the inside of the tube making a sudden step change in the brightness of the dot. This then decays until the next refresh. The persistence of the dot was a balance between keeping bright between refresh and "motion smear" if it was too slow.

The gun contains the required optics to view a small spot on the screen and a fast light sensor. Some analog logic is required to keep track of the vertical and horizontal dot position relative to the position of the on-screen "target" and match this with the step change in light level by the gun sensor. If the two coincide at the moment the trigger is pressed a "hit" is registered.

Note that this system could only work with bright targets as there would be no dot impulse on black or very dark areas of the screen.

I think @Passerby's answer is more accurate and obviously a much simpler method of working without the timing issues of my answer in the case of the gun. I predate video games. He obviously had a mis-spent youth - although it has paid back on this occasion.

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Figure 2. The cutting edge - once upon a time. This would be easier to date if we could see how wide his trousers flares were.

The method I have described is how the light pen worked - to the best of my recollection.

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