Linear CCD Sensor And Other Curiosities In A Fax Machine

For SGVHAK’s regular first Thursday of the month meeting for June, Emily brought in an old fax machine abandoned by the side of the road.

The plastic enclosure yellowing from age is not a surprise, but its heft was: it was far heavier than it looked. Judging by the collection of debris this machine had gathered, it had been left outdoors for some time. It was not a surprise when it failed to power on. Which is fine as we had no use for a fax machine and little interest in fixing it, but we were curious what was inside one of these anachronisms.

Most of this device’s mass came from a single beefy cast aluminum frame inside the machine. Select portions have been machined to precise tolerances. Why was this necessary? Emily hypothesized the robust frame was necessary to hold optical scanning components in precise alignment. The optical path was more complex than we had expected. Illumination came from a wide LED strip sitting under what looks like a glass rod slice lengthwise. It began to emit visible yellow-green light at roughly 15 volts (while drawing less than 200mA) and we cranked it as high as 20 volts (just under 500mA) but no further as we didn’t know the strip’s limits.

That light bounced off a few front surface mirrors before reaching the document, whose reflected light is picked up by yet more mirrors and finally a lens assembly that focused onto a sensor. A web search for TCD102D only found the first page of this device’s data sheet. But it was enough to tell us it was a line of 2048 photodiodes designed specifically for this purpose of scanning a line of a document scrolling past sensor optics.

For the output side of this device, there was a roll of thermal paper and a thermal image print head that worked much like the sensor in reverse: a line that heats a sheet of paper rolling past it to create an image. Digging below them both, we find the mechanical pieces making paper scroll. There was a stepper motor driving rollers for source document, and another stepper motor driving rollers feeding thermal paper for output.

Beyond the two stepper motors, few components had prospect for reuse though some (like the front surface mirrors) were kept for novelty. Unfortunately this disassembly also evicted an insect from its now-demolished home.

The biggest win was a lens assembly that formerly sat in front of the linear CCD. It has the right optical properties to be used as a small macro lens for an equally small cell phone camera. Emily plans to design and 3D print a bracket to hold this lens at the proper location and distance so we should see more close-up shots of small electronics components in the future.

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