Treasure Swap
A heist from an adventure film on your living room shelf. Lift the idol off a guarded pedestal and put down something just as heavy. You get one heft and two seconds.
- Age
- Whole family. The 6-year-old plays on the widest tolerance, adults on white.
- Players
- 2 or more: a guard and a thief, roles swap every round
- Cost
- 500–850 CZK (about €20–35)
- AI
- None. A load cell, an ESP32 and printed parts.
- Network
- Not needed. It runs offline from a USB charger.
- First version
- One evening, two boards and a kilo of sugar
The pedestal is a scale. The guard secretly fills a hollow idol with rice and weighs it at the press of a button. The thief hefts it once, goes hunting around the house, and has to swap it within two seconds. If the pedestal stays empty too long or holds both things at once, the column flashes red and honks.
Get within tolerance and the LED strip turns green and shows how far off you were and in which direction. Every success makes your tolerance a third narrower.
Play a round
the hand shows no numbersYou are the thief. On a screen your palm can’t feel anything, so the gauge with the gloved hand sinks as the weight grows. It shows no numbers, just like your arm. All weights in this demo are made up.
1 · The guard hides a weight
The heist plan
roles swap every round- The guard secretly fills the hollow idol with rice through a funnel, or picks a decoy somewhere in the house, and weighs it on the pedestal with the button.
- The strip color shows the thief’s tolerance: yellow ±25%, blue ±15%, white ±5% for adults.
- The thief hefts the treasure once, puts it back and has two minutes to find a replacement. That means at most two things from around the house. The rice and the funnel belong to the guard, and breakables are off limits.
- The swap has to happen within two seconds. If the pedestal stays empty longer, or both things stand on it, the column flashes red and honks.
- If the replacement sits within tolerance for three seconds, the strip turns green and shows how far off the thief was and in which direction.
- A success makes that thief’s tolerance a third narrower. Roles swap, and the longest streak of heists wins.
The wow moment
My 9-year-old son lifts the idol with one hand and drops a can of chickpeas and a spoon onto the plate with the other. The whole thing takes about a second. Then nobody breathes for three seconds, and the strip goes green with a single light just under the middle, which means a little light and still a clean heist.
The swap gets planned in the hallway, with the can and the spoon in one hand and the other hand free for the idol.
What the strip says
- Yellow, wide band: the thief plays at ±25%.
- Blue, narrower band: ±15%.
- Green, one light under the middle: a clean heist, a little light.
- Red and blinking, with a honk: outside tolerance, an empty plate for too long, or both things at once.
On the column the strip is vertical. Here the left end is the bottom (lighter) and the right end the top (heavier).
What the ESP32 prints to its serial port
An example of the log I would want on the laptop while testing. The numbers are invented.
guard tare ok guard treasure 612 g tolerance ±15% (blue) thief heft 1/1 thief plate empty ... 1.3 s thief plate 579 g judge stable 3.0 s −5.4% GREEN judge streak 1 new tolerance ±10.0% --- guard treasure 455 g tolerance ±25% (yellow) thief plate empty ... 2.0 s judge ALARM empty too long
Why a tenth time
- The tolerance shrinks by a third with every success: ±25%, then about ±17%, ±11%, ±7%. By the fourth heist in a row, a banana that worked on Monday is no longer close enough.
- The thief is limited to two things from around the house, so every round has a different right answer. Rice can’t be poured until it fits.
- The guard’s job is just as fun. A small heavy decoy, or an idol with very little rice in it, fools the hand on purpose.
- Roles swap every round. My 6-year-old daughter gets to guard against Dad, and Dad plays on white.
What the electronics and the printer add
This toy has no language model and no voice. Everything here is a load cell, a few timers in code and some LEDs. Without the pedestal, the same game is guessing on a kitchen scale with a grown-up as the referee.
- A referee nobody argues with. The load cell weighs both the treasure and the replacement, and the code decides. Nobody has to squint at a kitchen scale.
- The two-second rule. A child can’t count it fairly in the middle of a heist. The ESP32 can, and it also notices when both things are on the plate.
- An exact miss. The strip shows how far off you were and in which direction, so the next guess is better. Without it, the only feedback is yes or no.
- An idol whose look tells you nothing. It is hollow, printed in several colors on the MMU3, and has a wide screw-on bottom. It looks the same with a spoonful of rice inside as with a full one.
How it’s built
about 0.6 kg of PETGPrinted parts
- Column, two parts. PETG. It holds the boards, the strip runs down the front, the button and the buzzer sit in the base.
- Plate, Ø 12 cm. Sits on the load cell, so everything on it gets weighed.
- The idol. Hollow, multicolor from the MMU3, with a wide screw-on bottom so the guard can fill it with rice through the funnel.
The column and the idol take most of the filament, about 0.6 kg of PETG together.
Electronics
- ESP32. Runs the whole game.
- 5 kg load cell with an HX711. Under the printed plate.
- WS2812 LED strip. Tolerance color, the result and the alarm.
- Buzzer. The honk.
- Arcade button. The guard weighs the treasure with it.
- USB charger. It powers the pedestal, which needs no network.
Parts list
| Part | What it does | Price |
|---|---|---|
| 5 kg load cell + HX711 | Weighs whatever is on the plate | ~150 CZK (about €5) |
| ESP32 | Game logic, timers, strip and buzzer | 350–700 CZK (about €15–30) together |
| PETG, about 0.6 kg | Column in two parts and the idol, the largest item | |
| WS2812 LED strip | Tolerance, result, alarm | |
| Buzzer | Honks on the alarm | |
| Arcade button | The guard weighs the treasure | |
| Funnel, bag of rice | Filling the idol | |
| USB charger | Power | |
| Total | 500–850 CZK (about €20–35) | |
Only the load cell with the HX711 has its own price estimate. The other parts share the rest of the total, and the PETG is the biggest share.
Architecture
Where code ends and the model begins
Code does
- tares the empty plate and stores the treasure weight when the guard presses the button
- keeps the tolerance and makes it a third narrower after a success
- sounds the alarm when the plate is empty for more than two seconds or holds both things
- waits for three seconds of stable weight, then works out the miss in percent and its direction
- drives the strip and the buzzer
The model does
Nothing. There is no model in this toy, and no microphone either.
Hands do
- the guard hides the weight with rice or a decoy
- the thief hefts once and hunts around the house
- the swap itself, in under two seconds
Safety and offline
- 1Voices stay home. The toy has no microphone and no speech at all, so there is nothing to send anywhere.
- 2Nothing listens. The only button weighs the treasure.
- 3Code holds the rules. Tolerance, timers and the alarm are plain ESP32 code. There is no servo, lock or printer for anything to control.
- 4Nothing small or loose. Power comes from a USB charger, so there are no coin cells, and there are no magnets. The boards sit inside the printed column. The idol’s screw-on bottom is wide. Breakables are not allowed as replacements, and rice is plain kitchen rice.
- 5No AI service. The toy uses no online service and needs no account, so it works the same with the router switched off.
Roadmap
First playable prototype
one evening
A load cell, an HX711 and an ESP32 between two boards, calibrated with a kilo of sugar. The treasures come from around the house. No column, no LED strip, no idol. The two-second rule and the tolerance stay, and the buzzer announces the result.
Full version
500–850 CZK
The two-part PETG column with the WS2812 strip, the Ø 12 cm plate, the hollow multicolor idol with its screw-on bottom, the funnel and the rice, the arcade button and the honk. Colors for the tolerances, the strip shows the miss, and the longest streak wins.
More ideas to try
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Lying artifacts
Eight printed artifacts whose size lies about their weight: a big cup with sparse infill is light, a small egg with steel sealed inside is heavy. The thief replaces the treasure with an artifact of a different shape, or two of them, against the old illusion that the smaller of two equally heavy things feels heavier.
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Job cards
Before each heist the thief draws a card that limits the replacement: only things from the bathroom, something bigger than the treasure, or hefting with the left hand behind your back. The cards come in yellow, blue and white like the tolerances, so the 6-year-old draws from yellow and adults from white.
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The weight safe
A printed safe that opens only when the right weight lies on the dish on its roof. The kids heft the secret key once in an opaque bag, then build the combination together from things around the house, with seven gold LEDs for seven tries and blue for “add”, red for “take away”. At ±10% a servo slides the latch; it costs 450–750 CZK (about €20–30) and can be the last lock on a treasure hunt.
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.