The four pins

What is on the board

Three parts. A blue LED with a 10 mm lens, a 150 Ω resistor beside it, and a four-pin header along the bottom edge. The header is right-angled, so the pins point out past the edge of the board rather than down through it, and the board lies flat when it is plugged into a breadboard or a cable.

The four names

GND is the return path, on the square pad. Current leaves your board on SIGNAL, goes through the LED and the resistor, and comes back here.

NC means not connected, and on this board it means it literally. The net list, which is the list of every electrical connection on the board, does not mention either NC pin at all. They are there so that every TinkerBlock block has the same four-pin header, and one cable fits all of them.

SIGNAL goes straight to the LED's long leg, the anode. Nothing sits between the pin and the LED. Your sketch sets the pin HIGH and the LED lights; LOW and it goes out. The silkscreen says this twice: ACTIVE HIGH on the front, GLOWS WHEN HIGH on the back.

There is no supply pin

Most blocks have a VCC pin that powers the part, and a signal pin that tells it what to do. This one has no VCC. The pin you drive is the power, so the current the LED draws comes out of your microcontroller's own pin.

That has two consequences, and the rest of this book is about them. The pin has a limit on how much current it may supply, which is what the resistor is for. And the LED's brightness depends on your board's logic voltage, which is why the same sketch looks different on an Uno and on an ESP32.

The back of the board

Turned over, the order reverses. The square pad is on the right now, and it is still GND. The back is where the part number is printed, TK01, which is the number on the box and the address of this block's page.