- Age:8+
- Time:30 min
- Difficulty:Medium
- Mess level:Low
- Supervision:Yes
Types of Projectors commonly used
There are three types of projectors we can commonly find around. Two since one is slowly going to history. Those types are DLP (Digital Light Processing), LCD (Liquid Crystal Display) and CRT (Cathode Ray Tube). They are different based on the technology they use to project images.
CRT (Cathode Ray Tube) Projectors
CRT projectors were one of the first projectors commonly used. With today’s technology, they are getting obsolete and are less in use than their more advanced counterparts. They use 3 tubes, one for each of three additive primary colors: Red, Green, and Blue. If you want to know more about colors, check the article about mixing colors and learn all about it.
The biggest downside of CRT projectors is their large size, low light output and frequent need of aligning the three tubes so the image is correct. Because of this reason, they are mostly replaced with more friendly LCD and DLP projectors.
LCD (Liquid Crystal Display) Projectors
LCD projectors use polarized mirrors that reflect and pass through only certain colors of light. First, the red, green and blue colors are separated and go through the LCD panel that controls the intensity and saturation of each color. After passing through the LCD panel, the colors re-converge with the help of prism and we get the picture we want.
DLP (Digital Light Processing) Projectors
There are two types of DLP projectors based on the number of chips they contain. There are one-chip and three-chip models. Each chip contains millions of mirrors that reflect light thousands of times per second. One-chip projectors can produce more than 16 million colors while the three-chip projectors can produce more than 35 trillion colors. Impressive indeed! That makes them suitable for the production of more lifelike and natural images.
They are capable of creating a more crisp and fluid image than LCD projectors due to the more dense closeness of each mirror. That also makes pixels closer together and it is almost impossible to see any space between them.
DLP Projectors use millions of mirrors contained in the Digital micromirror device (DMD) unit.
Factors influencing image quality
Brightness
Projector brightness is measured in lumens. The size of the room and the size of the picture we want to project are the biggest factors that determine the lumen rating we need for a clear picture. In small rooms and when we are projecting on a small screen, the lumen rating between 1500 and 3000 will suffice. But in large rooms and on large screens (think of cinema) we need a lumen rating of 10000 to 28000 for a clear picture. Also, the ambient light influences the lumens we need. The higher the ambient light, the more lumens we need to see the clear image.
Lens
As with brightness, distance and screen size also play a major role in the lens we need. If the projector is close to the screen, we will need a short-throw lens. If the projector is casting on a large screen and is placed in the back of the room, we will need a long-throw lens. All projectors have a lens that specializes in one of those functions. They all can be adjusted and focused to some extent but can never play a good role in both long and short throwing of an image.
The lens helps to focus the light so we can get a clear image.
Science Behind Homemade Projector
How come the video on the wall is much bigger than on the smartphone? That’s the whole point of the projector, right? Well, the explanation lies in the lens of the magnifying glass - convex lens.
The convex lens is thicker in the middle than on the edges. The light, coming from your smartphone is going in the straight line. Then, when the light reaches the focal point, it bends and converges.
From the focal point, light disperses in the opposite direction, stopping at the surface where our video is projected. That’s why the video gets bigger the further the projector is from the wall. However, that also makes the video less sharp.
That is also the reason we see our video upside down! The light coming from a focal point is projected in the opposite direction. So that is the reason why we need to turn our smartphone upside down to get a correct image.
Did you know that the human eye works similarly? Images we receive by our eyes are upside down! The lens in our eye is focusing light from the environment on the retina, where a similar process occurs. Our brain then makes the necessary correction so that we perceive the world around us in its regular shape and direction.
After passing through the Focal point, the image becomes upside down.
You slide the whole projector further away from the wall. What happens to the picture?
Make your prediction, then tap an answer to check!
Materials needed for homemade projector
All you need is a shoebox, scalpel, magnifying glass, and a smartphone. All the other is optional.
- Magnifying glass
- Shoebox
- Scissors
- Scalpel
- Cardboard or Styrofoam
- Pencil
- Smartphone
- Duct tape
- Optional: Hot glue
Instructions for making a Homemade Projector
Be sure to check out the video on how to make a homemade projector, or if you prefer to read, continue with a step by step instructions below.
👨👧 Adult supervision needed
Cutting the lens hole calls for a scalpel or craft knife, which is very sharp and easily slips on curved cardboard. An adult should do the cutting, always slicing away from fingers on a cutting mat or spare piece of cardboard.
1. Put the magnifying glass on a shoebox and trace the shape with your pencil. Cut it out with a scalpel or scissors.
Cut the hole in the cardboard box by making the circle around the magnifying glass.
1.1. Now you can attach your magnifying glass to the hole. If it’s too loose, you can use duct tape or hot glue to secure it
2. Time to make a stand for our mobile phone. Use cardboard or styrofoam to make a smartphone stand. It would be ideal if it perfectly fits the box (from edge to edge). Also, the stand must be mobile so we can focus and sharpen the picture.
3. Put your smartphone on the stand (attach it with the duct tape), make sure it’s turned upside down. Set the brightness to maximum and turn auto-rotate to off.
Make a stand from cardboard or styrofoam and attach the smartphone upside down.
4. Put your stand inside the box.
5. Play the video, turn off the lights and enjoy the show! You can make the video sharper by moving your stand until you find the proper distance.
✨ Pro tip
This projector is dim, so a few tweaks make a big difference. Make the room as dark as possible, set the phone to maximum brightness, and turn off auto-rotate and auto-lock so the screen doesn't dim or spin mid-video. Wipe the magnifying lens clean, and if the picture looks washed out, move the projector a little closer to the wall for a smaller but brighter, sharper image.
What will you Develop and Learn by making Cardboard Projector
- Engineering skills
- Designing skills
- Motor skills
- Oculomotor coordination
- Physics (Optics)
We hope you enjoyed making your projector and that you will enjoy even more watching videos with your friends.
Key takeaways
- A magnifying glass is a convex lens - thicker in the middle, so it bends light rays together at a focal point.
- Past the focal point the rays cross and spread out, so the picture on the wall is enlarged and flipped upside down - that's why you mount the phone upside down.
- Further from the wall = bigger but dimmer and blurrier; closer = smaller but brighter and sharper.
- For a watchable image you need a dark room and maximum screen brightness.
- Your eye works the same way: its lens throws an upside-down image on the retina, and your brain flips it.
Frequently Asked Questions
Why is my projected image upside down?
That's the lens doing its job. Light rays cross over at the lens's focal point, so the image comes out flipped both top-to-bottom and left-to-right. The fix is simple: mount your phone upside down (and turn off auto-rotate) so the projected picture lands the right way up.
Why is my projector image blurry?
The image is only sharp at one specific phone-to-lens distance. Move the phone stand slowly back and forth inside the box until the picture snaps into focus. A clean lens and a phone that's exactly parallel to the wall also help a lot.
Why is the projected picture so dim?
A single magnifying glass spreads your phone's light over a big area, so it's naturally faint. Turn the phone to full brightness, make the room as dark as you can, and don't project too large - a smaller image closer to the wall is much brighter and clearer.
What kind of magnifying glass works best?
A large, strong (high-magnification) magnifying glass gives the biggest, brightest image. A bigger lens gathers more light, and a stronger lens has a shorter focal length, so you get a good-sized picture without needing a huge box. A cheap page-sized Fresnel magnifier works well too.
Does the smartphone projector work with any phone?
Yes - any smartphone works, since all you need is a bright screen. Just make sure to turn brightness to maximum, disable auto-rotate and auto-lock, and mount it upside down. Bigger, brighter screens naturally give a better projected image.
Why do I have to turn off auto-rotate?
Because the phone sits on its side, upside down. With auto-rotate on, the phone keeps trying to "correct" the picture as you tilt it, flipping your video the wrong way. Locking rotation keeps the image steady and correctly oriented.
If you search for even more engineering ideas, you will probably be interested in making your Potato Battery. Also, check out how to make a Cardboard Clock and how to make Cardboard Castle from toilet paper rolls.
