A DIY teleprompter is a box, a sheet of half-mirrored glass at forty-five degrees, and a screen lying flat beneath it — that is the whole machine, and building one is a satisfying weekend project that teaches you more about how prompters work than any purchase would. This guide covers the principles that make a build succeed, the mistakes that make one unusable, and the half that most build tutorials skip entirely: what to run on the screen once the box is made.
How the trick works
The glass is the product. Beam-splitter glass — sometimes sold as teleprompter glass or two-way mirror acrylic — transmits part of the light that hits it and reflects the rest. Mount it at forty-five degrees in front of your camera lens, lay a screen face-up below it, and the screen's image reflects into your eye line while the camera shoots straight through the glass at you. You read words that appear to float exactly where the lens is, which is why beam-splitter rigs solve eye contact outright rather than approximately.
Everything else in the build exists to serve that piece of glass: a frame to hold it at the right angle, a shroud of black material behind and around the lens so the camera does not photograph reflections of the room, and a platform for the screen. People have made working prompters from picture frames, foam board and a phone; the material matters far less than the geometry.
The three things that decide whether the build works
Kill the light leaks. The camera looks through the glass, so anything bright behind the glass — a window, a lamp, a pale wall — ghosts into your footage. The shroud is not optional trim; it is half the build. Matte black card or cloth, enclosing the path from lens to glass, and the ghosting disappears.
Respect what the glass costs you. Beam-splitter glass passes roughly a third to a half of the light that hits it, in both directions. Your camera loses some exposure, which it can compensate for — but your eyes lose the same fraction of the screen, which only the screen can compensate for. A build that seems fine at a desk fails at reading distance because the text is now dim, small and behind darkened glass. The answer is not a brighter build; it is bigger, bolder text than feels reasonable, and strong contrast with no subtle tonal steps carrying anything important.
Get the reflection the right way round. The mirror flips the image horizontally. Reading mirrored text is not a skill worth acquiring — the software must flip it for you, and it needs to be a proper mirror mode that flips the whole surface, not a font trick. Check this before you build, because it is the single most common reason a finished DIY prompter goes unused.
The half the build guides skip
Most DIY tutorials end at the box, as if the words will take care of themselves. But a prompter is a system: the glass gets your eyes to the right place, and the software decides whether reading there actually works.
Three things matter on the screen. Mirror mode, as above — non-negotiable. Text sized for your real reading distance, which through glass at a couple of metres is larger than any default. And the scroll itself, which is where a home build can quietly outperform bought hardware: the traditional weakness of every prompter, built or bought, is that it asks you to guess a scroll speed and then keep up with it. TellyPrompter runs in the browser on whatever screen your build holds — a phone, an old tablet, a spare monitor — with mirror mode, sizing for distance, and a scroll that follows your voice instead of a preset speed: pause, ad-lib or back up, and the highlight waits for you. Speech recognition runs on the device and the audio is never uploaded. One honest limit: voice-following relies on the browser's speech recognition, which Firefox does not provide — the constant and sound-level modes work everywhere. There is a longer piece on why following beats scrolling if you want the argument in full.
Because it is browser-based, the screen in your build needs nothing installed — open the site, load the script, set mirror mode, and the oldest tablet in the drawer becomes the display. That is also the cheapest possible test of whether the whole idea suits you: prop a phone beside your lens and try reading this way before you cut any board.
Build it, or buy the glass in a box?
An honest closing note. The DIY route wins on cost, on fitting odd cameras and odd spaces, and on the understanding you gain. Bought rigs win on rigidity, on optical-grade glass with fewer double-image artefacts, and on your evening. If your build wobbles every time you touch the focus ring, or the glass shows a faint second image of every word, those are the two things the commercial versions are actually charging for — and there is no shame in building the prototype and buying the tool. The full setup roundup covers the bought options honestly, and the beam-splitter setup guide applies to your build exactly as it does to a purchased rig, because from the software's point of view they are the same machine.