tenti.ai

design & schematics

How we'd build it, part by part

The working hardware sketch for the base box, the word boxes and the travel cards, with the parts we would order today and what each costs. Everything runs at 5 volts or less, which cannot shock anyone, and we design the contacts as if it could anyway. Prices are what the parts cost in single units from UK hobbyist suppliers, the "at volume" column is our estimate for a proper production run. This page will change as prototypes teach us things.

Three design rules before any part gets ordered. Boxes connect electrically on the left and right faces only, top and bottom are magnets alone (fewer pins is the single biggest cost save on the box). Every pin is recessed in a chamfered well, and the magnets are polarised so a box can only seat the one way that lands the pads, the magnet does the guiding, never a child's finger. And no pad is live until the handshake completes, a box gets power only after the base knows it is a box.

The base box

Part	What it does	One-off	At volume
Raspberry Pi Zero 2 W	runs the stories, plays the voice, Bluetooth to the app and headphones	£16.80	£14
MAX98357A I2S amp	digital audio straight from the Pi, no analog fuss	£5.50	£1.20 (chip on our board)
Mono enclosed speaker, 3W 4Ω	the pup's voice, mounted behind the face	£4	£1.50
PowerBoost 1000C + 2500mAh LiPo	battery, charging and 5V for the whole chain	£30	£9 (charger IC on our board)
PN532 NFC breakout	reads the travel cards tapped on the lid	£24	£2.50 (chip + etched antenna)
microSD, 16GB A1	the stories live here, works offline forever	£6	£3.50
2 ports of pogo pins + magnets	3 recessed pins and 2 polarised magnets each side	£4	£2
Big soft play button	tap for play and pause, long-press to restart the story	£2.50	£0.60
Rotary encoder + knob	volume, software-capped so a child cannot make it loud	£3.50	£1.10
Recessed pairing button	parent long-press pairs Bluetooth headphones, speaker mutes while they are connected	£0.60	£0.20
Beech shell, finish, print	UK CNC shop, quote needed	£25 (one-off)	£10–14
Our PCB + UK assembly	replaces the breakouts at volume	—	£10
Base box	prototype from breakouts vs production estimate	≈ £122	≈ £57–64

The prototype column is real, you could order those links today and have a working base on a bench in a week. The volume column assumes the breakouts collapse onto one board of ours, which is where most of the saving comes from. Safety testing and packaging sit outside this table and are shared with the word boxes.

The controls stay deliberately few. One big soft button on the top does play and pause with a tap and restarts the story with a long press, the same gestures the app uses. Volume is a real knob with a wooden cap, read by the Pi and capped in software so the maximum is always conversational, a child can turn it freely and never make Tenti shout. And a small recessed button on the underside pairs Bluetooth headphones, a parent gesture by design, press and hold until Tenti says "wuf" in your ears. The speaker stays silent while headphones are connected and comes back the moment they disconnect.

The word box

Part	What it does	One-off	At volume
CH32V003 MCU	unique ID, light bar, both ports, the whole brain	£0.80	£0.18
7 LEDs + diffuser strip	the progress bar in the pack colour	£1	£0.30
Regulator + protection	3.3V for the MCU, ESD on every pad	£0.60	£0.20
6 pogo pins	3 per side, left and right only	£1.20	£0.55
8 small neodymium magnets	2 polarised per electrical face, 2 plain per stacking face	£1.20	£0.50
PCB + assembly	one small board, UK assembly	—	£1.60
Beech shell, finish, print	two halves, the picture face	£8 (one-off)	£3–4.50
Per word box		≈ £13	≈ £7–8.50

Dropping the top and bottom pins saves about £1.50 a box against the four-sided version, six pins and two fewer machined wells. The trade is honest, a box above or below is felt by magnets but electrically silent, so side adventures over vertical joints wait for a later revision or arrive over the cards instead. We think that's the right trade for version one and we're not fully sure, it's first on the list to test with families.

The travel cards

Part	What it does	One-off	At volume
NTAG213 tag	the card's identity, powered by the reader's field	£1.60	£0.10
Printed card, rounded, thick stock	the picture and the words, child-proof lamination	—	£0.45
Per card	a ten-word pack lands near £6 with packaging	≈ £2	≈ £0.55–0.70

The cards are where the expansion economics live, a ten-word pack costs around £6 to make and packs flat into an envelope. NFC reads a handful of tags at once but never their order, so the toy asks for taps one at a time and order comes from time, the same trick the box chain uses for arrival.

How it all talks

The base polls its two ports a few times a second. Each box answers with its ID and whether its far port has a neighbour, answers hop inward, and the base ends up with the left chain and the right chain in order. A connect is a new ID appearing, a disconnect is one vanishing, and no pad carries power until the box on it has answered the handshake. There is no radio in the boxes, no microphone anywhere, and the whole bus runs at 5 volts, which is less than a USB phone charger.

Still open, and we'd rather say so here than pretend. Whether 7 LEDs read as a progress bar from across a room. Whether the recessed pin wells collect biscuit crumbs (a real risk with this audience) and how they clean. And whether side adventures belong on a second row of pads in version two or on the cards. Prototypes will answer all three.