The four pins

What is on the board

A coin motor, the parts that switch and feed 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 and it lies flat in a breadboard or on a cable.

The motor is 10 mm across and 2.7 mm thick, lying flat on double-sided tape. Inside it a small weight sits off the centre of the shaft, and when it spins the whole disc shakes. That shake is the product. Everything else on the board is there to switch it and feed it.

The four names

GND is on the square pad. The motor's current returns through it, so it has to be the same ground as the board driving SIGNAL.

VCC goes to one place: the input of a 3.3 V regulator. The motor runs from the regulator's output, not from VCC, and nothing on the board connects VCC to SIGNAL. That is why this book wires VCC to 5V on every board, even a 3.3 V one. Why VCC can be 5V is the whole argument.

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 it at all.

SIGNAL is the gate of a MOSFET, a transistor used as a switch, with a 10 kΩ resistor to GND. HIGH turns the motor on, LOW turns it off, and left unconnected it stays off. It carries none of the motor's current.

The light

The small LED in the middle lights while SIGNAL is HIGH and the switch is closed. It is lit by the same current path as the motor, from the board's 3.3 V, so it is a quick check that power and SIGNAL both arrived. It is not proof that the motor turned: when it does not buzz uses exactly that difference.

The back of the board

Turned over, the order reverses. The square pad is on the right, and it is still GND. The back prints TK30, the number on the box, and the whole of the logic in two lines: vibrates when the input is high, stops when it is low. It prints no pin names.