The four pins
Four pins, three of them wired: GND on the square pad, then 3V3, NC and SIGNAL. The pin printed 3V3 is the board's VCC, and it powers nothing: it is the level a closed sensor puts on SIGNAL. NC is connected to nothing. A closed sensor reads HIGH, which is what ACTIVE HIGH on the front says.
Three wires and a spare
The header is four right-angle pins, 2.54 mm apart, printed GND, 3V3, NC and SIGNAL from left to right with the parts facing you. GND is on the square pad, and that pad is the only way to tell four identical holes apart. Count from it.
GND is where the light ends. 3V3 goes to one screw of the terminal and to nothing else on the board. SIGNAL goes to the other screw, and to the light. NC is in no net at all.
What HIGH means
The front prints ACTIVE HIGH up its right edge. The sensor you screw into the terminal is a switch between the 3V3 pin and SIGNAL. When the switch closes, SIGNAL is joined straight to that pin and reads HIGH. When it is open, SIGNAL is joined to nothing.
That second sentence is the part the other TinkerBlock sensors do not share:
there is no resistor on this board that gives SIGNAL a level while the
switch is open. No pull-down on the
board is about
that, and every sketch in this book deals with it in setup().
3V3 is a label, not a requirement
The pin is printed 3V3 because that is the level most TinkerBlock hosts run at. On this board it is the VCC net, and a closed sensor passes whatever you put on it to SIGNAL. So it must be your board's own logic voltage: 3V3 on an ESP32, an ESP32-S3 or a Pico, 5V on an Uno. VCC is your logic voltage covers why.
When it does not work
The square pad. From the front, parts up and pins pointing down, it is the left-hand pin, and the order is GND, 3V3, NC, SIGNAL. The back prints no pin names; turned over, the square pad is on the right and it is still GND.
No. The label names the level the pin was drawn for, not a requirement. This pin is VCC: it goes to your board's logic voltage, which is 5V on an Uno and 3V3 on an ESP32 or a Pico. Nothing on the board runs from it.
No. It is soldered to a pad that no trace reaches, so it makes no difference either way. It is there so every TinkerBlock block has the same four-pin header and one cable fits all of them.
A screw terminal, a resistor and a light, and what each one is for.
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