My crude Commodore 64 scanner

The drum scanner was my first attempt at hacking together a crude 'peripheral' for the commodore 64. I really didn't need to do any electronics for it - I hooked up a CDS cell to the paddle port as the light sensor, and the rest was all mechanics and software.


    The drum scanner
    The incredibly crude and slow scanner in action  

A drum scanner was the most logical choice, as it did not require producing any sort of back and forth motion for the scan head. The scan head itself was just a single CDS cell. To restrict the amount of area that it gets light from, I used the 'pinhole camera' approach. I used a black BIC pen lid, drilled a small hole in the tip of it, and stuck the CDS cell in the back. This was then positioned about 1 mm from the paper, and two 2 watt automotive bulbs to either side illuminated the paper.

A problem was that the cardboard drum, which used to be a paper towel roll, was not quite round, so I had to put the whole sensing mechanism on a 'sled' so it followed the variations in the drum. That way, the pickup did not get all fuzzy as the drum moved too far away, or all dark as the drum got too close and blocked the hole.

This worked much better than I had expected in terms of optical resolution and contrast. What I had not known was that a CDS cell is actually quite slow, especially under low light conditions. This makes it inherently unsuitable, but I originally suspected the paddle ports of the Commodore 64 instead of the CDS cell. Even if I had known it was the CDS cell, a photo transistor or photodiode was not easily come by in northern Ontario. So I stuck with the CDS cell and ran the scanner at a really slow speed.

I wrote a program on the Commodore 64 which kept reading the paddle port at a constant interval, and picked one of 4 grey levels, and drew a pixel on the screen. Once one pixel column was done, it would wait for the 'sync contact' on the drum to close, which indicated that the top of the image was reached again, and it was time for the next pixel column.

The whole thing worked pretty crudely, and the speed and quality, compared to the cheap scanner I used to scan in the photo for this website, it was pretty useless. Still, a fun toy. My brother ended up building a similar scanner, by almost the same design, out of non mechano set parts, and its still up in the attic somewhere.

I later figured that a back and forth scanning mechanism might be more flexible. With a drum scanner, you are stuck with the drup circumference as one of your maximum dimensions. If you scanned a line back and forth along the drum's axis, you could control the length of the stroke and thus optimize for different picture sizes. I built such a mechanism out of fischer technik. It consisted of a gearbox which had a slow forward, and a fast reverse speed. The carriage would, on reaching the right side, knock the gear stick into reverse. The carriage then sped back, and before it hit the left margin, a string from the gear stick to the carriage pulled the gear stick back into the forward gear. With enough tinkering, I actually got this to work semi-reliably, but I never did attempt to use it for a scanner. It was just too complicated to be practical.

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