Forecast for your home Station: stone no. 1 Palm range 18–40 °C Max change 1 °C/s

TOY CONCEPT · ESP32 + BLE + PELTIER

Hot and Cold for Real

A stone in your palm that warms up as you get closer to the treasure and turns cold when you head the wrong way. The riddles come from photos of your own home.

Ages 5–10~1,650 CZK (about €65) · Lite ~800 CZK (about €30)30-min game · ~2 h battery

Exploded cross-section · two printed halves held by M2 screws in heat-set inserts, a “rocky” texture in gray MMU shades.

01 · LIVE MAP

Guide the stone through the home

22.0 °C

The palm temperature rises slowly (max 1 °C/s). Sound and vibration react right away.

LOOKING FOR: Treasure 1 of 3
ZONE: far
TREND: —
VIBRATION: slow

“Riddle number one. Where does a giant rest his head when the giant isn’t asleep?”

Click a chest on the map to see the photo a parent took and the riddle the Gateway made from it.

Move the stone with the mouse, a finger or the arrow keys (click the map first). The chests show on the map only for this demo; in the real game a parent hides them.

02 · WOW MOMENT

A warm palm by the couch

My 6-year-old daughter walks into the kitchen with the stone in her hand; it buzzes quietly and turns cold. She turns toward the living-room door and her palm starts to feel pleasantly warm. By the couch the stone buzzes faster and it’s as warm as a mug of tea. Under a cushion lies the chest. She holds it up to the stone and a fanfare plays.

Stone“Riddle number one. Where does a giant rest his head when the giant isn’t asleep?”
The 6-year-old heads for the door, the stone cools down
Stone“Brrr… there’s a north wind blowing in here.”
by the couch, the stone warms up
Stone“You’re burning up! Hot, hot! Can you feel it?”
NFC tap
Stone“Treasure! The giant is going to be furious. Next riddle: where do teeth take their morning bath?”

03 · HOW TO PLAY

Five steps, three hints

01HideA parent hides 3 little chests with tiny BLE beacons inside and takes a phone photo of each spot.

02RiddlesThe Gateway turns the photos into riddles that match the child’s age.

03ButtonThe child presses the button and the stone reads the riddle aloud.

04TrendGetting closer means instant sound and vibration; the warmth follows slowly, as atmosphere.

05TapAn NFC tap on the chest means it’s found; on to the next riddle.

04 · WHY A TENTH TIME

A different home every evening

Every game happens in a different corner of the home, with new riddles made from fresh photos in two minutes. The kids can take turns: one hides the chests (and takes the photos on a parent’s phone, with a grown-up around), the other one searches. The same thing works in the backyard on a bigger scale.

WHAT AI MAKES POSSIBLE

A riddle from any corner in a few minutes

A vision model describes the photo (“under the blue cushion on the couch”) and Claude turns that into a riddle a 6-year-old can crack. Here, temperature is the navigation, not a metaphor.

05 · HOW IT’S BUILT

Printed parts and electronics

Stone ~80 × 60 × 45 mmTwo halves held by M2 screws in heat-set inserts, a “rocky” texture in gray MMU shades, an aluminum palm plate set into the side, a vent grille on the bottom.

Chests 50 × 35 × 30 mmA print-in-place lid, a cavity for the BLE beacon and an NTAG tag embedded during a print pause.

Measuring distanceBLE RSSI is noisy indoors (± 5–10 dB). A trend from a 1–2 s moving average and rough zones (far / close / burning) are enough. The final step is the NFC tap.

Part	What for	Price CZK	Source
ESP32-S3-DEVKit (or a XIAO ESP32S3 for 269)	BLE scanning, control	448	platform notes
Peltier 20×20 mm	heating/cooling	150	estimate
TB6612FNG	H-bridge for the Peltier	94	electronics, note #60
NTC thermistor + 45 °C bimetal cutoff	temperature limits	30	estimate
mini heatsink + fan	heat removal	80	estimate
vibration motor	instant feedback	49	#61
PN532	chest tap	158	#32
MAX98357 + small speaker	voice	140	#48 + estimate
protected 18650 + TP4056	battery	170	estimate + GME
3× BLE beacon + 3× NTAG213	treasures	336	estimate + #31
Total, about €65 (Lite without the Peltier: ~800 CZK, about €30)		~1,650

Why not UWB: a DW3000 would get you to ~10 cm, but the modules cost several hundred CZK apiece (estimate) and need calibration. Overkill for a game of hot and cold.

Architecture

Game prep · Gateway on the Mac

parent: photos→Claude vision: describe the spot→Claude: age-matched riddle + 3 hints→CODE: treasure order, beacon UIDs→Piper: MP3→stone

Play · stone offline

BLE RSSI→CODE: moving average, trend, zone→vibration + sound right away+target temperature → regulator with limits→Peltier

Found it

NFC tap→CODE checks the treasure→next riddle

codemodelhardware / person

The code handles

position, trend and zones
temperature and Peltier control
game flow and treasure checks

The model does

describes the spot from a photo
writes age-matched riddles and hints
never controls the Peltier

06 · SAFETY AND OFFLINE

Three independent heat safeguards

LAYER 1 · FIRMWARE18–40 °CThe code keeps the surface within range and changes it by 1 °C/s at most, using an NTC thermistor on the plate.

LAYER 2 · BIMETAL45 °CA hardware bimetal cutoff disconnects power, independently of the firmware.

LAYER 3 · FAIL-SAFEpulldownA pulldown on the enable pin switches the element off if the ESP32 freezes.

The TEC1-12706 from the catalog (88 CZK, about €5) draws 6 A and is too big for a palm. So it’s a small 20 × 20 mm element driven by a TB6612FNG H-bridge at ~1 A, with a small heatsink and fan inside. Battery life: a 3,000 mAh 18650 at an average draw of ~1.2 A lasts roughly 2 h, and a game takes 30 minutes.

The battery sits behind a screw. The only camera is in the parent’s phone. The whole game runs offline; without the Gateway you just don’t get new riddles, and the stone plays plain old hot and cold.

Honestly: temperature changes slowly (over seconds), so sound and vibration carry the quick navigation and the warmth is the “mood”. That’s why the Lite version comes first (no Peltier: vibration + LED color + voice, ~800 CZK, about €30), and the Peltier only once the game catches on.

07 · ROADMAP AND SCORES

From a weekend to a month

First playable prototype · a weekend

Lite version

BLE trend + vibration + voice.

Full version · a month

Warm palm

Peltier with temperature control, riddles from photos.

NOVELTY

ENGAGEMENT

FEASIBILITY

Realistic positioning from the BLE trend (not distance), a safe Peltier with three layers of protection, and a Lite version that lowers the build risk. Not yet verified: how readable the RSSI trend is in a real home. Measure it on the Lite version before building the Peltier.