**** Cassettepunk ****

Technological Surrealism / Alec Nerds Out

DIY Projector Conversion


This year’s culture crawl was a rousing success! Thanks to everyone who came, not least the food, alcohol, and film crews. But more about that in another post to come shortly.

I procrastinate terribly, which is why most of these projects exist — by working on these I’m usually avoiding something else. The last sprint leading up to the culture crawl became its own target for procrastination, so I wandered into a thrift store and got one of these…

Bored easily? Jump to the end and watch the video. Otherwise, stick around for some history and hacking.

It’s a LaBelle 75 slide projector, made in Oconomowoc, Wisconsin. Googling around turns up a lot of thrift store listings but nothing much about the company — beyond the fact that they may still exist, making LED signage. Or it might be someone else who took up the name.

Anyway, it’s beautiful. A small plaque on the back references US patent #2503239, filed in 1945. Google Books turned up a reference to the projector from 1955.

So obviously I wanted to make it into a working video projector. People have made DIY projectors ever since LCD displays became available; the normal way to do it is to separate the built-in backlight from the LCD itself, which is transparent, and use a high-power light source instead. I’ve done this before, as documented back on the old Free Geek Vancouver Volunteer Blog. It worked — once — then a friend put a couch on top of it. (Hi, Andrew.)

The Achilles’ heel for this technique is heat. A projector bulb generates a lot of heat, and LCDs don’t like variations in temperature. The brighter the light, and the smaller the LCD, the worse it gets.

In recent years we’ve gained a very good solution: high-power LEDs. These are stunningly efficient, meaning low power and less heat — but another less obvious improvement is that they really only output light on the visible spectrum. Other kinds of projector bulbs typically output light all over the spectrum, which the LCD absorbs and has to dissipate as heat.

So instead of a 500W bulb you can throw a 10W LED in, mounted on a modest heat-sink, and basically forget that heat was ever a problem. A 10W LED isn’t going to give anyone a tan, but it’s bright enough for my purposes. I could’ve probably worked with a 20W LED just as easily, but none were in stock locally.

I decided to work with the optics of the slide projector as unmodified as possible, and the existing physical limits of the slide mount were roughly 45x45mm, so I opted for an Adafruit 2.2″ LCD. This is similar to the OLEDs I used for the Viewmaster Video — it’s supported by the same set of Linux kernel drivers — so it was an easy choice. The aspect ratio is not preserved because the whole display doesn’t fit exactly in the frame; it’s more like a traditional 4:3 aspect ratio.

I stripped out the old electrics from the projector, Dremelled out a hole for mounting the LCD, and otherwise got to it.

Stripping the backlight off the LCD is finicky work, but not hard; the backlight and LCD are lightly glued together into a plastic frame, which was in turn adhered to the controller PCB. A little patience suffices. The backing of the plastic frame can be removed entirely, leaving a see-through holder perfect for the stripped LCD; I applied a little bit of hot glue to keep the LCD in the frame, then did the same with the frame to attach it to the projector. The ribbon cable linking the LCD to the controller board has flex to spare. These LCDs are cheap enough that a catastrophic slip of the knife wouldn’t have cost much. Well, maybe a finger.

With the display mounted, I built a short ribbon cable connecting the SPI interface for the LCD to the 40-pin Raspberry Pi header. I mounted the 10W LED to a heat-sink and attached this to the chassis with cable ties — not elegant, but effective, as it holds everything in place adequately but also permits adjustment. I mounted a switch through the side of the chassis, where the slide advance lever used to be, permitting separate on/off switching of the LED and also a fan mounted above the heat sink.

For power, I internally mounted two wall warts — a 5V supply for the Raspberry Pi and LCD, and a 12V supply for the LED and fan — underneath the optics where the drive motor used to sit. These barely fit; I had to cut the 120V pins off the wall warts and solder to the remaining nubs in order to save a half-inch.

The rear panel had a power switch and two power jacks, apparently for chaining devices. I removed the power jacks, installed a standard C13 power socket in one, and added dual RCA audio outputs to the spot vacated by the other. I preserved the function of the power switch, which cuts 120V power to the two wall warts.

For adding storage media, I added an extra USB port on the top, where the slides used to be loaded — this preserves the function of this part of the chassis, even if the original deck of slides looks nothing like a USB memory stick. Just in case, I added a USB hub that could easily be accessed through the top of the chassis. I may later decide to add wireless keyboard and mouse or something, and the Raspberry Pi’s built-in USB ports aren’t easy to access, pointing downwards as it’s currently mounted. A USB network adapter currently occupies one.

The verdict? It’s quite watchable. The resolution isn’t stunning (320x240x18-bit colour for the original display, minus a little bit of that due to cropping) but I didn’t find it distracting. The aesthetic worked best showing something classic — I chose Chaplin’s “Modern Times” for the demo.

I did some research into processing the video for subpixel rendering in order to extend the effective resolution of the display — this is an especially attractive option for showing a black & white movie on a relatively low-resolution colour display. I don’t think this has been done, though, and would probably not combine well with lossy video encodings. This might be worth some more experimentation.

Overall this was a satisfying and surprisingly accessible project.


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3 Comments

  1. […] did you know it’s not that hard to convert an old slide projector to digital? [Alec Smecher] shows us how with a 1950’s LaBelle 75 slide projector, and the result is pretty […]

  2. larry says:

    Awesome build and one of my favorite silent movies.

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