Active low, and the light

Why LOW

The chip's output has two positions. No magnet, it connects SIGNAL to VCC. A magnet, it connects SIGNAL to ground. So the magnet is the LOW state, and a line that does its job by going LOW is called active low. The front of the board says ACTIVE LOW for that reason, and the schematic's own note says it in words: the output goes low when a magnetic field is detected.

Where the light is

The LED and its 1 kΩ resistor run from VCC to SIGNAL, not from SIGNAL to ground. With no magnet, both ends sit at VCC, there is no voltage across it, and it is dark. With a magnet, SIGNAL drops to near 0 V, the LED has the whole of VCC across it and its resistor, and it lights.

So the light means the opposite of what a first-time reader expects: lit is LOW, and LOW is the magnet. The LED and the pin agree; they just agree on LOW.

How much current

The LED is red, and a red 0805 LED drops about 2.0 V at these currents. The arithmetic below takes that figure. It is a model rather than a measurement:

LED,     from 5 V:    (5.0 V - 2.0 V) / 1 kΩ   = about 3.0 mA
LED,     from 3.3 V:  (3.3 V - 2.0 V) / 1 kΩ   = about 1.3 mA
pull-up, from 5 V:     5.0 V / 10 kΩ           =       0.5 mA

The chip's output sinks all of it while a magnet is near, about 3.5 mA from 5 V. The datasheet only states the output's low level at 0.5 mA, where it is under 0.2 V, and gives no figure at 3.5 mA. That is a gap on paper rather than a problem you are likely to see, and the check is simple: if the LED is lit and the pin reads LOW, the level is low enough.

None of that current comes from your microcontroller's pin. It flows from the block's VCC pin, through the LED, into the chip, and back out of GND. Your pin only measures the voltage.

A test that needs no code

The LED lights from the chip's output alone, so it splits a fault in two before you open the sketch:

• Magnet near, LED lights. Power, GND and the chip are fine, and SIGNAL went LOW. If the sketch sees nothing, the fault is between SIGNAL and your code.
• Magnet near, LED stays dark. Either the chip has no power, or the magnet is not close enough to the chip, or not face on to it.

When a magnet does nothing starts from exactly this split.