Hallway, front view · two players · no AI

Invisible Gate

Two printed posts stretch infrared beams across the hallway. One player aims them, the other reads the angles of ten holders and crawls through for a plushie against the clock.

Age
Whole family, practice mode from 6
Players
Builder and agent, then swap
Cost
700–1,100 CZK (about €30–45)
AI
None. Electronics and printed parts
Inside
ESP32, five 940 nm beams, OLED, piezo, two buttons, power bank
3

The toy

Two printed posts stand on either side of the hallway, and up to five infrared beams run between them where nobody can see. The builder slides and turns ten holders, then presses Set. The gate works out which beams actually connect and shows how many.

The agent reads the angles, guesses where the beams go and crawls to the far side for a plushie and back, on the clock. Every broken beam beeps and costs five seconds. Then the roles swap.

There’s no AI in this one. An ESP32 with a small display and a piezo runs the game, and about 800 g of filament does the rest. The same idea scales up to pairs of posts all over the house and a mission to collect seals; that version is at the end of the roadmap.

How a round goes

  1. The builder slides and turns the ten holders on the posts and presses Set. The gate checks which beams hold and shows the count.
  2. The agent looks at the holders and guesses where the beams run.
  3. Start. The agent crawls to the far side for the plushie and back. Every broken beam beeps and adds five seconds.
  4. A second press stops the clock. The display shows the time and the best of the day.
  5. The builder ties a ribbon along the beams the display lists as broken, the agent sees the mistakes, and the roles swap.
  6. My 6-year-old daughter plays practice mode: instead of a penalty, the gate shows the number of the beam she hit.

Play a round

a simulation with made-up traps

LEDs sit on the left post, receivers on the right. A beam holds only where an LED points at a receiver and that receiver faces back. The display tells the agent how many beams hold, and the angles have to tell the rest.

You are
Whose trap
Front view · the gate
Plan

Left foot: display, Set, Start

1 · Read the angles

Tap a holder, or its chip, to sight along it.

2 · Pick your gap
3 · Run

Press Start on the gate. You cross twice, there and back. When you’re back with the bear, press it again to stop the clock.

1 · Set the trap

Pick a holder in the drawing or here, then slide and tilt it. Dashed lines show where the holders point.

2 · Press Set

The gate counts the beams that hold. That number is all the agent gets.

3 · Hand over

    The wow moment

    The ribbon goes up after the run. The builder ties it along every beam the display listed, and the agent finally sees the line they were sure wasn’t there.

    Then comes the argument about whether that receiver was really turned that way. It was. The click joint doesn’t move by itself, and the gate counted the beam before the run started. I expect the grown-ups to take this worse than the kids.

    One round on the display

    SetBEAMS 2
    Start0.0 s
    3.1 sbeep · +5 s
    8.4 sbeep · +5 s
    12.6 sback with the bear
    StopTIME 22.6 s
    BEST 14.2 s
    HIT 3
    Practicebeep · BEAM 3

    Made up for this page. Beam 3 got hit on the way there and again on the way back.

    Why a tenth time

    • An LED aimed between two receivers, or a receiver turned a few clicks away, looks exactly like a beam. Building decoys like that is the builder’s half of the game, and the number on the display is the only hint the agent gets.
    • Roles swap every round, so the trick that just cost you five seconds goes straight into your next trap.
    • Best of the day stays on the display until someone unplugs the power bank, so there’s always a time to beat.
    • Once straight beams are easy to read, the traps get meaner, with diagonals crossing right where a kid wants to crawl and a decoy next to every real line.
    • Practice mode keeps my 6-year-old daughter in the game. She hears the beep, reads the beam number off the holder and learns the angles instead of collecting penalties.

    What the electronics add

    No AI in this toy

    Nothing in it listens or goes online. The work is done by an ESP32, five pairs of infrared parts and the printer.

    • Invisible beams. With string the trap would be on display. With 940 nm light the agent has to work it out from the angles.
    • A gate that checks itself. After Set it knows which LED reaches which receiver, so “that beam wasn’t even there” stops being an argument.
    • A referee for every break. Beep, five seconds on the clock, and nobody has to stand there watching.
    • Numbers for the youngest. In practice mode the display names the beam she broke.

    What the printer adds

    • Holders that click into an angle and stay there, so the angle the agent reads is the angle the beam takes.
    • Weighted feet that keep the posts upright when someone crawls into them.

    How it’s built

    Printed parts

    • Two feet with ballast sealed inside, each holding a wooden dowel. The left one is bigger and carries the ESP32, the display, the piezo and both buttons.
    • Ten holders that slide on the dowels and turn on a click joint. Five carry an IR LED in a tube, five carry a TSSP4038 receiver.
    • The LED holders get their numbers printed in a second color on the MMU3, so practice mode has something to point at.
    • About 800 g of filament for the feet and holders, PLA or PETG.

    Electronics

    • The ESP32 flashes the five LEDs one at a time in 38 kHz bursts and reads all five receivers after each burst. That’s how the gate knows which LED reaches which receiver.
    • One cable runs from the left foot to the right post, taped flat to the floor.
    • OLED for the count, the clock and the best time; a piezo for the beep; two buttons, Set and Start.
    • A power bank over USB. No network needed.

    Parts list

    PartQtyCZKWhere it goes
    Already in the drawer
    ESP32 board10left foot
    OLED display10left foot
    To buy or print
    Filament, about 800 g–700–1,100
    for this group    		2 weighted feet, 10 holders
    Wooden dowel2the posts
    IR LED, 940 nm5LED holders, in tubes
    TSSP4038 IR receiver5receiver holders
    Piezo1left foot
    Push button2Set, Start
    Cable and floor tape1between the posts
    Power bank1next to the left foot
    Ribbon1for after the run
    Total700–1,100 CZK
    (about €30–45)

    The estimate covers the whole build. Filament is the biggest item.

    Architecture

    LEFT FOOT ESP32 game, clock, rules OLEDcount, time, best Piezo · beep 2 buttonsSet, Start Power bank · USB 5 × IR LED 940 nmleft post, in tubes 5 × TSSP4038right post 38 kHz, one at a time cable on the floor beams no Wi‑Fi · no account · no AI service

    Where code ends and the model begins

    Code does

    1. Fires the LEDs one at a time and reads all five receivers, so it knows which LED reaches which receiver.
    2. On Set, stores the beams that hold and shows how many.
    3. Starts the clock on the first press of Start and stops it on the second.
    4. When a stored beam goes dark: beep and five seconds. The same beam counts again only after it comes back.
    5. Keeps the best time of the day and the list of broken beams for the ribbon.
    6. In practice mode, shows the beam number instead of adding seconds.

    The model does

    Nothing

    There is no model in this toy and no network to reach one. Reading the angles, building the traps and arguing about the ribbon are left to the players.

    Safety and offline

    Nothing listens

    There’s no microphone or camera. The only sound is the piezo beep, so the voice rules have nothing to cover.

    Fully offline

    The ESP32 never joins Wi‑Fi and needs no account. Its code holds the whole game: beams, clock, penalties and the best time.

    Power

    A USB power bank. No coin cells anywhere in the toy.

    The light

    940 nm LEDs, the same kind as in a TV remote, each one inside a tube in its holder.

    Trips and tips

    The cable between the posts is taped flat to the floor. Weighted feet keep a post standing when someone bumps into it.

    Small parts

    The ballast is sealed inside the printed feet and the electronics are closed in the left foot. Nothing small enough to swallow sits on the outside.

    Roadmap

    1. A weekend

      First playable prototype

      Three beams from LEDs and receivers taped to both sides of a door frame, an ESP32 and a piezo. Posts, holders, the display and penalty seconds wait for later. A phone stopwatch keeps time, and you count the beeps.

    2. Then

      Full version

      Two posts on weighted feet, ten click holders, five beams, a display with the count, the clock and the best of the day, and practice mode for the youngest. 700–1,100 CZK (about €30–45).

    3. Later

      A mission across the house

      Four pairs of printed posts in doorways and under furniture, each beam at one of three heights, marked only roughly on the post. The agent collects three printed seals and comes back. A base by the front door shows that all beams hold, beeps at every break and adds ten seconds; the posts report to it over ESP‑NOW and run on AA batteries. Two players race for the best time, or play together, split the route and share five breaks. 900–1,600 CZK (about €35–65), mostly five ESP32 boards and filament for eight posts. The weekend version uses two pairs of cardboard-tube posts with a buzzer on the receiving one, a phone timer and paper seals.

    More ideas to try

    Status: an idea. Nothing on this page has been built or tested yet, and the demo is a simulation with pre-written answers. Prices are estimates from Czech shops in October 2026.