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October 11, 2025, 07:47:51 PM

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3D-printed enclosure for a home-made LDR/LED combination

Started by aquataur, Today at 09:41:11 AM

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aquataur

After being inspired by @Phatt's notes, I looked into Mictester's RCC compressor. In earlier days I would have mounted the LDR/LED combination the way it is frequently suggested, namely by a heat shrinking tube. This has drawbacks which I will address further down.
3D-printers are becoming increasingly common, and since I have access to such a very versatile thing, I quickly made up a small case that does it.

I had some gray filament on the spool, and you would think a 1mm thick non opaque plastic would be light-tight, but no. To my surprise it was translucent enough to be registered by the LDR.
So the filament HAS to be pitch-black. You can test the result by shining your workplace light on it – there must be no reaction by the LDR.

I used a common 8-20k, 1M (dark) LDR and two square LEDs that were left over from a VU-display. I believe they are all very similar in shape, but otherwise you can take the idea and adapt the case.

Note that in my routing program the lid is not shown. You see it in the 3d view. Neither are the LEDs nor the LDR. You see the finished assembly on the fotos, before closing of course. The footprint has grid lines for you to extract the measures (25 mil).

I found that the epoxy PCB, although being opaque, transports enough stray light which shines through the inevitable mounting holes to irritate the sensor.

The flanges on the ,,coffin" are solely there to provide more shading. I also made polygon areas (that are not connected to anything) around the pins for the same reason. For the prototype, I took a permanent marker and covered the area around the pins in black on both layers.
(Note the photo shows the prototype that does not yet have those improvements).
This stops all unwanted light intrusion.

Alternatively, after soldering, one could fill the mounting holes from the top with ahem, something, which unfortunately I failed to imagine. It would need to be black I guess. JBWeld? Suggestions are welcome.

Before you close the lid and glue it (be careful with super glue – it tends to produce deposit on nearby surfaces, I used fast curing epoxy), power up the unit and make sure both LEDs light up. Don´t expect to see a blinding light, you only see a tiny spark.

So back to why heat-shrink tubes are problematic:

    • during heat shrinking the components might shift place
    • the butt ends must be light-tight
    • the whole contraption can become pretty clumsy.

The depicted principle can be adapted to any LDR/LED combination.
The RCC now produces a subjectively very deep compression (too deep to be useful in the extreme setting).

Unfortunately, I cannot upload the STLs here. Find them on my server.

J M Fahey

Hey, cool!!!!

You modern kids with your newfangled stuff often awe us oldtimers  :tu:

Me?

I would have made that case out of wood, held together with some hot hide glue, kept warm in a water bath over my wood fired stove.
Already used to the stink.  :lmao:

Serious, LDRs are very sensitive to the minutest speck of light reaching them, even if invisible to the naked eye.