Slide viewer LED upgrade

Recently I have been going a bit crazy putting LEDs in things.  Probably because it is an easy way to get “runs on the board” as I get my confidence back.  This one was upgrading a hand held slide viewer for my father.

The slide viewer itself was awesomely retro looking.  It ran off two C sized batteries and it also feels lovely and solid.  The big downside was how it actually functioned.  There is a torch bulb sitting maybe 10mm behind a partly opaque sheet of white plastic.  The blub had an awful orange tint and the screen was not at all uniform, as the photo below shows.


Aside from the slide viewer I also had a Nokia N93 with a broken LCD screen.  The LCD screen seemed like a perfect fit for the back of the slide viewer case.


First step in getting to the LED backlight diffuser was to pull off the front glass/polarizer.  You should be careful if you are going to do this, as bits of glass can fly around when getting it off.  Wear safety goggles I guess.   After the slightly difficult task of getting the front glass off, the sandwich of the LCD glass can easily be folded back.  The flexible PCB acts kind of like a hinge to fold it back.  Just get to it with a pair of scissors because we only want to keep the back part with the LEDs in it.  Just don’t damage the thin bit of flexible PCB leading to the plug.  We need that to feed the mA down.

You can find pinouts for the N93 LCD here on Andy Browns web site.  It shows that the four LEDs are in two strings of two LEDs each.  One on pins 11 and 14,  and the other on 12 and 13.  I hooked them up with some wire wrapping wire and forced 20mA into the wires.  At this stage I took a photo of the light bulb next to the LEDs for comparison.

I could not easily hook the LEDs up to the batteries in the slide viewer.  Four white LEDs in series would be 12 or more volts.  I only had 3v from batteries, so needed some kind of boost converter.  Andy Brown to the rescue again.  I had previously gotten some of his Nokia E73 LCD driver boards for testing with a hand held video game project.  I hacked off the side with the LED driver.  Soldered on the parts, and wired it up to the blue wires from the previous step.  The extra blue wire you can see going from pin three to pin five on the SOT23-5 IC is to hard wire the Enable pin.

A thin layer of Araldite glue was applied to the back of the LCD and to the soldered up connector.  This was allowed to dry, as it was to act as insulation to stop shorts, not as an adhesive at this stage.  Then once the first batch of Araldite was dry, more was used to glue the driver PCB and the connecting wires securely to the back of the LCD.

Now came the irreversible step of hacking the slide viewer.  An Xacto razor saw was used to hack of the plastic dome as well as the copper strip that held the lightbulb.  The LCD/Driver part was then glued with still more Araldite to the back of the newly hacked plastic.

And this is now what it looked like reassembled.  The improvement in quality was amazing.  Brightness, colour and uniformity was in a different class than before the mod.  Sadly photos did not do it justice at all, so I will leave it with just the photos of the white screen which does look fabulous compared to the original light.

Also shown is a photo of the bits left over to be thrown away.

How about the efficiency?  Well that can be worked out from the NCP5007 LED driver datasheet.  Looking at formula on page 37 of the datasheet, and you can see that a much better idea is to just get out a crappy $7 multimeter and measure the current.

140ish mA. That is about 10mA less than the incandescent bulb was drawing, but the light produced is much, much brighter.  Better still it will stay the same brightness all the way till the batteries are totally flat.


BTW – If you enjoyed this retro slide viewer upgrade, and you like retro video gaming, you should check out our Uzebox DTV kickstarter campaign.

Edit: Welcome Hack-a-Day viewers

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