3D print + ESP32 + BLE + Peltier · for ages 5–10
A stone in your hand gets warm as you head toward the treasure and cold when you go the wrong way. The riddles come from photos of your own home.
Idea stageNothing is printed or soldered yet.Play a round ↓
The chests are hidden. Tap the map to walk somewhere, drag the stone, or click the map and use the arrow keys. The stone knows only what a real one would: a jumpy Bluetooth signal, averaged into warmer or colder. Every square you cross keeps the color the stone felt there.
When you get within arm's reach, the chest shows up, the way your hand finds it under a cushion. Then tap it to the stone. Walls cost the signal about 5 dB each, so the room next door can feel farther away than it is. Real homes do that too.
…
The stone saysPress a key or tap the map. I'll tell you if you're getting warmer.
A parent hides three little chests, each with a small BLE beacon inside, and takes a phone photo of every spot.
The Gateway turns the photos into riddles that fit the child's age.
The child presses the button, and the stone reads the riddle aloud.
Walking closer changes the sound and the buzz right away. The warmth follows slowly, for atmosphere.
Touching a chest to the stone (NFC) means it's found, and the next riddle plays. Each game has three hints.
My 6-year-old daughter walks into the kitchen with the stone in her hand. It gives a soft, slow buzz and turns cool. She turns toward the living room door and her palm starts to warm up. By the couch the buzzing speeds up, and the stone is as warm as a mug of tea that has cooled enough to hold. Under a cushion sits the chest. She holds it up to the stone and a fanfare plays.
The voice lines are MP3 files the Gateway prepared before the game. During play the stone picks which one to play. It never has to think on its feet, which is good, because it's a rock.
Riddle number one. Where does a giant rest his head when the giant isn't asleep?
Brrr… there's a north wind blowing in here.
You're burning up! Hot, hot! Can you feel it?
Treasure! The giant is going to be furious. Next riddle: where do teeth take their morning bath?
Hide the chest, take a photo of the spot, and a couple of minutes later there's a riddle about it, pitched at your kid's age. A vision model looks at each photo and writes down where the chest is ("under the blue cushion on the couch"). Claude turns that into a riddle a 6-year-old can crack, or a trickier one for the 9-year-old, plus three hints that get less subtle.
A home treasure hunt used to mean a parent inventing clues on scraps of paper, and after a few rounds they were always about the same three places. Here the warm palm leads to an actual cushion in your actual living room, and the riddle knows the cushion is blue. The temperature points somewhere real. Pick a photo below to see what the Gateway would make of it.
Every game happens in a different corner of the home, and fresh photos turn into new riddles in about two minutes.
One kid hides the chests and takes the photos on a parent's phone, with a grown-up around. The other one searches. Then they switch, and the hider gets to watch someone walk straight past the couch.
The garden works the same way on a bigger scale. The beacons don't care about walls they don't have to go through.
Offline, the stone plays plain hot and cold with the chests it already knows. You only miss the new riddles.
Two halves joined by M2 screws in brass heat-set inserts, so the battery stays behind a screw and the stone can still be opened for repairs. The outside gets a rocky texture in gray MMU shades; swapping grays every few millimeters of height gives you rock strata without any painting. The aluminum palm plate sits in one side, the vent grille for the little fan on the bottom.
The lid is print-in-place, so it comes off the printer already hinged (about 0.3 mm clearance). Inside is a cavity for the BLE beacon. The NTAG213 goes in during a print pause and ends up under 1–2 mm of plastic: invisible, but the stone can read it.
Everything is sized for a Prusa MK4S with the MMU3, and all three chests fit on one plate. Put something worth finding in the last one.
Indoors, BLE signal strength (RSSI) jumps around by 5–10 dB, so it can't tell you how far away a chest is. It can tell whether the number is going up or down. The stone averages the last 1–2 seconds and sorts the result into rough zones: far, close, burning.
The last few centimeters are the NFC tap, so nobody needs precision. UWB (a DW3000 module) would get to about 10 cm, but the modules cost several hundred CZK each (estimate) and need calibrating. For hot and cold that's overkill.
Ready-made BLE beacons on a coin cell, about 100 CZK each (estimate), which last for months. Or an ESP32-C3 SuperMini on 2× AAA, also around 100 CZK (estimate).
| Part | CZK | In Lite | What it does | Source |
|---|---|---|---|---|
| ESP32-S3-DEVKit (or XIAO ESP32S3, 269 CZK) | 448 | yes | BLE scanning, game logic | LaskaKit |
| Peltier element 20 × 20 mm | 150 | – | heating and cooling the plate | estimate |
| TB6612FNG | 94 | – | H-bridge for the Peltier | LaskaKit |
| NTC thermistor + 45 °C bimetal cutoff | 30 | – | temperature limits | estimate |
| Mini heatsink + fan | 80 | – | moving the heat away | estimate |
| Vibration motor | 49 | yes | instant feedback | GME |
| PN532 | 158 | yes | reading the chest's NFC tag | LaskaKit |
| MAX98357 + small speaker | 140 | yes | the voice | LaskaKit + estimate |
| Protected 18650 + TP4056 | 170 | yes | battery and charging | estimate + GME |
| 3× BLE beacon + 3× NTAG213 | 336 | yes | the treasures | estimate + GME |
| 1× RGB LED (WS2812) | (10) | yes | Lite only: color instead of warmth | estimate |
| Full build · about €65 | ~1,650 | |||
| Lite build · about €50 | ~1,300 | the "yes" rows plus the LED; with the XIAO board instead, ~1,150 CZK (about €45) | ||
Prices are from a Czech parts catalog (LaskaKit, GME) checked in September 2026, VAT included; "estimate" means nobody has looked up a real listing yet. Filament, screws, wires and the button aren't counted.
Never: a model doesn't touch the Peltier, doesn't decide whether you found the chest, and isn't running at all while the kids play.
The code keeps the plate in that range and changes it by 1 °C per second at most, reading an NTC thermistor glued to the plate.
A bimetal switch on the palm plate cuts the Peltier's power on its own. No software involved, so there's nothing to crash.
A pulldown resistor on the H-bridge's enable pin. If the ESP32 freezes or reboots, the pin drops low and the Peltier switches off.
The big Peltier in the catalog, a TEC1-12706 for 88 CZK, draws 6 A and is 40 × 40 mm. That's a lot of heat for a child's palm. The stone uses a small 20 × 20 mm element on a TB6612FNG H-bridge at about 1 A, with a small heatsink and fan inside. A protected 3,000 mAh 18650 at an average draw of ~1.2 A lasts roughly 2 hours, and a game takes 30 minutes.
The battery sits behind a screw. The only camera involved is the parent's phone, and the photos go only to the Gateway. The whole game runs offline. Without the Gateway you don't get new riddles, and the stone plays plain old hot and cold.
Honestly Temperature changes over seconds, so it's no good for steering. Sound and vibration do the steering, and the warmth sets the mood. That's why I'd build the Lite version first (no Peltier: vibration, LED color and voice) and add the Peltier only once the game has caught on at home.
BLE trend, vibration and voice. No Peltier yet, riddles typed by hand if need be.
Nobody has checked yet how readable the RSSI trend is in a real home. If a 6-year-old can't follow it, that gets fixed first and the Peltier waits.
The Peltier with temperature control and all three safety layers, plus riddles from photos through the Gateway.
Scores from the idea round. The reasoning: realistic positioning from the BLE trend rather than distance, a safe Peltier with three layers of protection, and a Lite version that lowers the build risk.
StatusAn idea with a parts list. Nothing has been printed, soldered or measured, and no child has held the stone yet.
Burning. The firmware stops at 40, so this is where the page ends.
One of sixteen finalist toy ideas in the Toy Workshop, generated by AI agents, picked by a dad.
Made by Ondra Malacka with Claude agents · how the agents worked · malacka.cz