The four pins
Four pins, three of them wired: GND on the square pad, VCC to every other loop of the pattern, and SIGNAL, an analog voltage that sits at 0 V dry and rises as water bridges the loops. NC is connected to nothing. There is no HIGH or LOW here: SIGNAL goes to an analog pin.
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 loops 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 where the 1 MΩ and the 100 nF end. VCC reaches four of the seven loops of the pattern and nothing else. SIGNAL is the voltage the pattern makes, through a 150 Ω. NC is in no net at all.
ANALOG, not HIGH or LOW
The front prints ANALOG up its right edge, and it means it. Dry, SIGNAL is
0 V. Water across the loops lifts it, a little for a drop, most of the way to
VCC for a fogged or splashed board. Read it with analogRead, on an analog
pin. The page this book replaces said SIGNAL went HIGH when it detected
steam, and wired it to a digital pin; nothing on the board makes a HIGH or
a LOW.
VCC is not a free choice
A soaked board can put VCC itself on SIGNAL, so VCC decides the highest voltage your analog 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. A divider made of water shows why.
When it does not work
The square pad. From the front, loops 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.
It will read something, but not what you want. SIGNAL is a voltage anywhere from 0 V to VCC, and a drop of condensate may lift it only a few per cent. A digital pin calls all of that LOW. Use an analog pin: A0 on an Uno, GP26 on a Pico.
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.
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