The market is full of projectors with a relatively low cost, but if we look closely at the best models, the investment they require is still very high. Add to this the fact that they don't get along very well with mobile devices, so many people decided to explore the DIY path, creating their own projectors with a shoebox and a magnifying glass.
Why build a homemade projector for your smartphone?
Now, why make a homemade projector for smartphones? First, it's an interesting project. One thing we'll learn is the theory behind each projector, and what the weak points of this device are. Second, it doesn't put a hole in the budget. Buying the wrong projector can be very expensive, while in this case, the only thing we lose is a shoebox. And third, it has a wide margin for optimization. The smartphone is just a starting point, and the design can be adapted to incorporate tablets, and even LED monitors. I remember very well that some adventurers used old CRT screens and combined them with Fresnel lenses, getting very striking results... but I'm getting sidetracked.
How to build it
Basically, you take the shoebox, cover its interior with black paint or a suitable dark material, make a hole to install the magnifying glass, prepare a base compatible with your smartphone so that it keeps it in a straight profile, and place it inside the box. Then it's necessary to block/disable the automatic orientation of the smartphone, set the screen brightness to maximum, position it "upside down" on the base (the magnifying glass will correct the image), start playback and calibrate the position between the smartphone and the lens, then do the same between the projection box and the wall. Turn off all the lights, and you have your homemade projector ready. If you're wondering about audio, nothing prevents adding a small hole in the box to connect speakers to the smartphone (or its charger).
Pitfalls to watch out for
What are the pitfalls? One of them, and if the theory doesn't fail me, is that the image will come out "mirrored", ruining any subtitle or text that appears in the movie (it can be corrected with a mirror at 45 degrees). The original tutorial from Photojojo (link below) clearly teaches this with the image of David Bowie at the end, because the painted lightning bolt on his face goes down in the opposite direction. The other pitfall is general brightness. No one in their right mind can expect the backlight of a smartphone screen to be on par with a lamp prepared for projector use, so eliminating external light sources and containing all the smartphone's light inside the box is a critical step for better image quality. Even so, it's well worth experimenting. Larger or smaller boxes, the lens of an old camera, and greater or lesser distance are all part of the list.
Photojojo
