The four pins
Four pins, three of them wired: GND on the square pad, VCC to the whole block, and SIGNAL from its comparator. NC is connected to nothing. Something in the slot reads HIGH; a clear slot reads LOW and lights the red LED. VCC is your board's logic voltage, because the block pulls SIGNAL up to it.
Three wires and a spare
The header is four right-angle pins, 2.54 mm apart, printed GND, VCC, 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.
Pick a pin to see where it goes. GND is the return for every part on the board. VCC powers all of them, including an infrared LED that is on the whole time. SIGNAL is the output of a comparator chip, with a 10 kΩ pull-up to VCC. NC is in no net at all.
What HIGH means
The black part at the top is a slot with a beam of infrared across it. With the slot clear, SIGNAL is LOW and the red LED beside the header is lit. Push a piece of card into the slot and SIGNAL goes HIGH and the LED goes out. So HIGH means something is in the way. A slotted disc on a shaft turns that into one pulse for every bar that passes, which is how the block measures speed.
VCC is not a free choice
SIGNAL's HIGH comes from a 10 kΩ resistor to VCC, so VCC decides what voltage your board's pin sees. Wire it to the voltage your board's pins run at: 3V3 on an ESP32, an ESP32-S3 or a Pico, 5V on an Uno. VCC is your logic voltage shows what the wrong one does.
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, VCC, NC, SIGNAL. The back prints no pin names; turned over, the square pad is on the right and it is still GND.
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.
No, that is the normal state. The LED lights while the beam is clear and goes out while something blocks it. It shows the block's answer, not whether it has power.
The slot sensor, the comparator, and the resistors that set how it decides.
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