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The TK85 at an angle: a black board with a gold edge, eight right-angle pins along the top edge for the keypad, the 16-pin PCF8574 chip below them with two small capacitors to its right, two round LEGO holes, a keypad-and-finger drawing, two small resistors at the lower right, lonely binary up the left edge, and four right-angle pins along the bottom edge for GND, VCC, SDA and SCL.
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This board opens in a VR headset: life-size, or big enough to stand beside, with every pin and every part named where it sits. Open this address in the browser on a headset such as Apple Vision Pro or Meta Quest.

learn.lonelybinary.com/vr/tk85
You have this

Matrix Keypad (I2C)

A 4×4 keypad on two I²C wires instead of eight GPIO pins. The TK85 adapter at a fixed 0x20, the compact adapter in the keypad packs with eight addresses, the soft and heavy-duty keypads, and the two keymaps they need.

Signal wires
2SDA + SCL
Address
0x20fixed on the TK85
VCC
3V3 or 5Vyour board's logic voltage
Pull-ups
10 kΩon SDA and SCL, on as shipped
Start here · 01 Sixteen keys, eight wiresOpen the board in 3D

Turn it over and see both headers: GND, VCC, SDA and SCL at the bottom, the keypad's eight wires at the top, straight to the PCF8574. The chip's address pins are wired to GND, which is why the TK85 is always 0x20; the cut pad on the back ships joined.

Product questions & answers ↓

The whole handbook12 articles · 55 minQuick referencePDF · 405 KBKeypad Adapter model (GLB)GLB · 1.6 MBKeypad Adapter CAD (STEP)ZIP · 1.6 MBDatasheetPDF · ti.comPinout and specsThe pin table, and the questions people ask first
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Matrix Keypad (I2C)

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Not on general sale yet. Reply to your order and we will sort out a spare — quote tk85.

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Email us or reply to your order, and quote tk85 — that identifies the product. If something is missing from the bag, your order is the record of what shipped.

Take it with you

Point a phone camera at the code and this page opens there. It is the same address as the one printed on your box, so it is also what to show someone you are passing the board to.