A DIY overhead projector uses just a handful of components: a bright light source, a Fresnel lens, a flat glass stage, and a mirror angled at 45 degrees to redirect the image onto a wall or screen. You can build a functional version with salvaged parts and basic tools, or go even simpler with a smartphone and a shoebox. The approach you choose depends on how bright and sharp you need the projected image to be.
How an Overhead Projector Actually Works
Every overhead projector follows the same optical chain. A lamp sits inside an enclosed base and shines light upward. That light passes through a Fresnel lens, which acts like a giant flat magnifying glass, gathering and focusing the light into a uniform beam. The beam then passes through a glass stage where your transparency or image sits. Above the stage, a second lens (the objective lens) and a mirror angled at 45 degrees redirect and focus the image forward onto a wall or screen.
The Fresnel lens is the key component that separates a working projector from a glorified lamp. It looks like a flat sheet of plastic covered in tiny concentric rings. Those rings do the same job as a thick curved lens but in a fraction of the space. Without it, you’d get a dim, uneven wash of light instead of a crisp projected image.
Parts You’ll Need
For a traditional overhead-style projector, gather these components:
- Light source: A high-output LED chip (sometimes called a COB LED). For a visible image in a dimmed room, aim for at least 4,000 to 5,000 lumens from the chip itself. After light loss inside the projector housing, a 100-watt LED typically delivers around 1,000 to 1,500 lumens of actual projected brightness. A 50-watt chip will work in a very dark room but will look washed out otherwise.
- Fresnel lens: A plastic Fresnel condenser lens, available from surplus optics suppliers or salvaged from old projectors. These are often sold as a double lens (two sheets cemented together), one with a long focal length and one with a short focal length. The short-focus side must face the light source. If you mount it backwards, you’ll get a round bright spot instead of an evenly lit square image.
- Glass stage: A flat piece of tempered glass or heat-resistant glass, roughly 10 by 10 inches. This is the surface where you place your transparencies. Standard window glass works for LED builds that don’t generate extreme heat.
- Objective lens: A second, smaller lens mounted above the stage. A large-diameter magnifying glass or a salvaged projector lens will work. This lens focuses the image onto the screen.
- Mirror: A flat mirror mounted at a 45-degree angle above the objective lens to redirect the image horizontally onto the wall. A first-surface mirror (where the reflective coating is on the front rather than behind glass) gives a sharper image with no ghosting, but a regular mirror works for casual use.
- Enclosure: A wooden or MDF box to house the lamp and Fresnel lens.
- Heatsink and fan: High-power LEDs generate significant heat. A CPU cooler or aluminum heatsink with a small fan keeps the LED from burning out.
Building the Base Enclosure
The base is a box that holds your light source at the bottom and the Fresnel lens and glass stage at the top. Construct it from plywood, MDF, or even thick cardboard for a prototype. The interior dimensions should be large enough that the Fresnel lens sits flat across the entire top opening, with the glass stage resting on top of or just above it.
Paint the entire interior matte black. This single step makes a noticeable difference in image contrast. Any light that bounces off unpainted interior walls creates stray light that washes out your projected image. Matte black paint absorbs most of it, with only about 4% reflected back. If you want to go further, line the interior with black flocking paper or black velvet, which absorbs even more stray light.
Cut ventilation holes near the bottom of the box and mount a small fan to pull air across the LED’s heatsink. Without airflow, the temperature inside the enclosure will climb quickly. Keep the ventilation openings positioned so light doesn’t leak out. An L-shaped baffle over each vent hole, also painted matte black, lets air pass through while trapping escaping light. The principle is simple: any light that enters the baffle has to bounce off two or three black surfaces before it could escape, and each bounce absorbs most of the remaining light.
Mounting the Optics
Position the LED at the bottom center of the box, facing upward, secured to its heatsink. Place the Fresnel lens flat across the top of the box, with the short-focal-length side (the side with more tightly spaced rings) facing down toward the lamp. The glass stage sits directly on top of, or just above, the Fresnel lens.
The objective lens mounts on an adjustable arm or post above the stage. This is your focusing mechanism. You need to be able to slide the lens up and down to adjust the focus based on how far the screen is from the projector. A simple approach is a vertical metal or PVC pipe with a clamp that slides along it, holding the lens at whatever height produces a sharp image.
The relationship between the lens, the image on the stage, and the projected image on the wall follows a basic formula: 1/u + 1/v = 1/f, where u is the distance from the transparency to the lens, v is the distance from the lens to the screen, and f is the focal length of your objective lens. In practice, you don’t need to calculate this precisely. Just slide the lens up and down until the image sharpens on the wall. A shorter focal length lens produces a larger image at a given distance but requires more precise focusing.
Mount the 45-degree mirror above the objective lens on the same arm. This mirror catches the image coming straight up and bounces it forward onto the wall. The mirror needs to be at least as large as the objective lens, or it will crop the edges of your image.
The Shoebox Smartphone Version
If you want a quick weekend project rather than a full optical build, a smartphone projector is dramatically simpler. You need a shoebox, a magnifying glass (the larger the better, ideally 3 to 5 inches in diameter), tape, and a dark room.
Cut a circular hole in one short end of the shoebox, sized to fit the magnifying glass snugly. Tape or glue the lens into the hole. Place your smartphone at the opposite end of the box, screen facing the lens. The phone’s screen is your light source and image source in one. Lock your phone’s screen rotation and flip the display image upside down (the lens inverts the image, so this corrects it). Seal the box shut to block stray light.
Slide the phone closer to or farther from the lens to focus the image on the wall. The projected image will be dim compared to even a budget commercial projector, since a phone screen only puts out a few hundred nits of brightness. Crank your phone’s brightness to maximum and project in a completely dark room for the best result. The image will be small and soft, but it works as a proof of concept or a fun science demonstration.
Getting a Sharp, Even Image
The most common problem with DIY projectors is blurry edges while the center looks fine. This happens when the projector isn’t aimed straight at the screen. If the projector is tilted upward or downward, the top and bottom of the image sit at different distances from the lens, so they can’t all be in focus at the same time. Even commercial projectors costing several hundred dollars struggle with this. The fix is simple: position the projector so the lens is directly centered on the screen, both horizontally and vertically, with the beam hitting the wall at a perfect 90-degree angle.
If you can’t avoid some tilt, you’ll see a keystone effect where the image looks like a trapezoid instead of a rectangle. Commercial projectors correct this digitally, but in a DIY build, the only real solution is physical alignment. Mount the projector on a level surface at the same height as the center of your screen, and angle the 45-degree mirror precisely.
Uneven brightness across the image usually means the Fresnel lens is mounted incorrectly (wrong side facing the lamp) or the light source isn’t centered beneath it. Double-check that the short-focal-length side of the Fresnel lens faces the bulb, and that the LED sits directly in the center of the base, not offset to one side.
Transparency and Heat Considerations
Standard transparency film is made from PET plastic, which starts losing its clarity around 150°C (300°F). With an LED light source, heat at the stage is generally modest since LEDs produce far less radiant heat directed upward compared to old halogen bulbs. Still, if you’re using a very high-wattage LED in a small enclosure, monitor the glass stage temperature during your first few uses. If the transparencies warp or curl, you need better ventilation or a lower-wattage source.
For the image itself, you can print on inkjet or laser transparency sheets, or simply write on them with dry-erase or permanent markers. Laser transparency film is designed to handle higher temperatures from the printer’s fuser, making it slightly more heat-tolerant than inkjet film if your projector runs warm.