Hall Effect Sensor

What it is

A Hall effect chip that switches when a magnet comes near, a 10 kΩ pull-up, and a red LED that lights while it does. No magnet, SIGNAL sits at VCC and your board reads HIGH. Bring either pole of a magnet to the chip and it pulls SIGNAL LOW, and your board reads LOW. The front of the board says ACTIVE LOW; the back says OUTPUTS LOW VOLTAGE WHEN MAGNETIC FIELD DETECTED.

Nothing touches and nothing wears out, and a magnetic field passes straight through plastic, wood and glass. That is why door and lid sensors are built this way. The chip is a CrossChip CC6201, which responds to either pole, lets go when the magnet leaves, and saves power by looking at the field only once every 22 ms or so. That last part decides what it is for: a door or a lid that stays put for seconds, not a magnet flicking past on a fast wheel.

The page this replaces said it responds to one pole only, might latch, and reads HIGH for a magnet. All three were wrong.

VCC is the voltage your pin sees

With no magnet near, the chip drives SIGNAL up to VCC, so whatever is on VCC is what your pin sits at most of the time:

Never 5V beside a 3.3 V board. The LED currents are worked out from its datasheet's forward voltage rather than measured.

Which pin is which

Parts up, header at the bottom, reading left to right:

The back prints TK18 HALL EFFECT SENSOR instead of pin names. Turned over, the square pad is on the right, and it is still GND.

Wiring, in three lines

1. GND to your board's GND.
2. VCC to 5V on an Uno, 3V3 on an ESP32, ESP32-S3 or Pico.
3. SIGNAL to a digital pin: D2 on an Uno, GPIO 25 on an ESP32, GPIO 4 on an ESP32-S3, GP15 on a Pico.

Leave NC unconnected. Set the pin to INPUT: the chip drives SIGNAL both ways and the pull-up is already on the board.

Example

// The GPIO number SIGNAL is wired to.
// Uno: 2. ESP32: 25. ESP32-S3: 4. Pico: 15.
const int HALL_PIN = 4;

void setup() {
  Serial.begin(115200);
  pinMode(HALL_PIN, INPUT);   // the chip drives SIGNAL both ways
}

void loop() {
  if (digitalRead(HALL_PIN) == LOW) {   // active low: LOW is a magnet
    Serial.println("magnet");
  } else {
    Serial.println("no magnet");
  }
  delay(200);
}

from machine import Pin
import time

# The GPIO number SIGNAL is wired to. ESP32: 25. ESP32-S3: 4. Pico: 15.
hall = Pin(4, Pin.IN)         # the chip drives SIGNAL both ways

while True:
    if hall.value() == 0:     # active low: 0 is a magnet
        print("magnet")
    else:
        print("no magnet")
    time.sleep_ms(200)

Where to start

The handbook below is ten short articles, each with a working figure. The first read is the whole build in three wires and a few lines.

If nothing happens when the magnet comes near, how far a magnet reaches is usually the answer. For a door or a lid, a door alarm is the project; for a wheel, read once every 22 ms first.

And before plugging it into an ESP32, VCC sets the level is the one page that protects the board.