Making Slides Come to Life

Neuschwanstein Castle, Bavaria, Germany

My wife and I took our son to Europe around 45 years ago. He was two at the time and, we at least, have good memories of that trip and the great places we visited. One outstanding place was the Neuschwanstein Castle in Bavaria. 

This past Christmas, for a variety of reasons, he managed to take his family back and revisit the castle. So, to help us follow his journey, we pulled out our old slides and tried to find our own photos of the places he visited. Of course, in those days, well before GPS cameras, the notable places were the only slides we could be sure were taken at the correct place. Unfortunately in the middle of this, my slide projector bulb lamp blew.

Now I had purchased that projector on our return to Australia from that trip so it has seen a few good years. The cost of a new bulb is between $6 and $30 on eBay but I decided the time has come to replace the halogen bulb with a LED. That seemed like a good idea at the time!

On YouTube there is a video of someone replacing a Kodak slide projector bulb with a 20w LED. It’s not exactly the same projector as mine though it is similar enough, and he required an external power supply and switch to make it work. But using a 20w LED to replace a 150w halogen lamp in my view, would be far too dim. 

So I searched eBay and found a 50w LED which works off 240v AC. You can see its specifications here. At 240 volts, no extra power supply was needed and if I worked it right, I could use the internal switching. In addition the LED cost $2.50 delivered to my mail box. According to the sellers documentation it should produce between 100 and 120 lumens per watt. So a 50w LED will produce between 5000 and 6000 lumens. To help you understand what that light measurement means, the original lamp is supposed to produce 6000 lumens when it’s new. So that sounds like a good deal. Next, I tried to measure the light from the halogen bulb but unfortunately, both the replacement bulbs I had, worked for only about 3 seconds. (they may have been 12v lamps!)

Fitting the LED looked fairly easy at first but it soon became obvious that the LED with newly attached heat sink, would be too big to fit in the original opening for the lamp. I tried shaping the heat sink so it would fit into the mirror hole since there was no point in having a mirror there anymore. But it got too messy and difficult so I chose another way. 

The optical assembly with condenser lenses but rear mirror removed

Now I don’t know a real lot about optics. I can recognise a magnifying glass. I also know from the documentation that came with the projector, the filament of the bulb is reflected by a concave  mirror behind it to double its apparent size. I assume the x36 printed on the side of the optic unit must mean that it magnifies the filament 36 times which would make it slightly larger than the 35mm slide when it reaches the back of the slide.

The LED lighting block unit on the other hand was only slightly smaller than the slide itself. So it needs just a small amount of magnification to cover the entire slide. After a experimenting with placement, I discovered there was too much magnification for a simple replacement to work. So, I concluded the first condenser lens was not needed. That gave me much more space for the LED and its heat sink and whilst there is probably a critical place to have the LED to get maximum light, I found that placing it around where the big condenser lens was seemed to work well. The other lens seemed to provide a small amount of magnification and so it looked like it would all work.

Unfortunately, the light falling on the slide was not even and it had dark corners. So after another couple of hours of trying different placements, I found the big condenser lens does even out the light falling on the slide so I out it back in. That meant I had very little room for the LED and its heatsink but with a couple of hours of mechanical adjustments, I made it work. And as far asI can see there isn’t 240v anywhere near metal pieces.

Like most of these projects, the greatest amount of time is spent on mechanical issues. Those include finding and shaping a heat sink (the LED unit gets surprisingly hot), making sure the LED is mounted in the right place, making sure the 240v connections are not causing a safety risk and generally making things look good. It is important to have the light emitting part of the LED block centred on the condenser lens so adjusting its position is important. In my case it had to be lower in the lens assembly than I first thought. That meant it pressed on the thermal sensor below the lens block so I had to lower it too.

Eventually after many false starts I made it work! The slides look great – maybe because I chose a cool white version rather than a warm white LED. So does it produce the same amount of light? I think the Chinese specifications for light output were a bit optimistic so maybe I’ll spend $4 and purchase a 70w version. Or maybe I’ll put the slide projector back in its box and leave it in the cupboard for another 40 years.

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