The light on the board

Two paths to ground

With the button down, SIGNAL is at VCC and current has two ways back to ground. One is the 10 kΩ pull-down. The other is R3, 1 kΩ, and the red LED after it. Both paths only carry current while the button is down, so the LED is a direct picture of SIGNAL: lit is HIGH, dark is LOW.

The current for both comes from the block's VCC pin, through the switch. Your microcontroller's input pin supplies none of it.

How bright

The LED is red, and its exact part is not recorded, so the arithmetic below takes the typical drop of a red 0805 LED: about 2.0 V, somewhere between 1.8 and 2.2 V. It is worked out rather than measured, and the currents are approximate:

from 5 V:    (5.0 V - 2.0 V) / 1 kΩ = about 3 mA
from 3.3 V:  (3.3 V - 2.0 V) / 1 kΩ = about 1.3 mA

From 3.3 V the LED is a little dimmer. The reason is the same as for the TK01's blue LED, and why it is dimmer on 3.3 V works it through.

A test that needs no code

The LED lights from the wiring alone, so it splits every fault in two before you open the sketch:

• Press and the LED lights. VCC, GND and the switch are fine, and SIGNAL went HIGH. If the sketch still sees nothing, the fault is between SIGNAL and your code: the pin number, or the SIGNAL wire.
• Press and the LED stays dark. The block has no supply. Check VCC, then GND, then that the cable is not one pin over.

When a press does nothing starts from exactly this split.