Reed Switch: ESP32 Magnetic Input and Debouncing

Before you start

Board and software

Classic ESP32-WROOM-32/32E DevKit with these GPIOs exposed and unused. Arduino IDE 2.x; esp32 by Espressif Systems 3.x (API review baseline 3.3.2, not a latest-version claim). Select the exact board or ESP32 Dev Module for a generic WROOM board. Other ESP32 families need their own pin map. Serial Monitor: 115200 baud.

Hardware assumption

A two-terminal, normally open dry-contact reed switch plus a magnet. Exact model/contact rating and operating distance are unspecified; verify your own part. Normally closed and changeover types need different assumptions.

Prerequisites and parts

First complete Arduino IDE setup and Blink. Reed switch, magnet, breadboard and short wires; a continuity meter is useful for identifying contact behavior.

Expected result

After a stable 30 ms sample window, Serial prints CLOSED when the contact closes and OPEN when it opens. Magnet distance is not specified.

Verification status

Reviewed against the technical references below. Not compiled for an ESP32 or tested on hardware in this update. The approved product-style illustration identifies the family; follow documented terminal labels and the logical wiring diagram, not its rendered pin positions.

Overview

02Technical Specifications

Arduino library: No additional library; Arduino-ESP32 GPIO and millis

03Pinout

• Terminal A Dry contact GPIO33 Either lead of an unpolarized bare contact can be A; do not infer behavior from artwork.
• Terminal B Dry contact return ESP32 GND Closing the contact grounds the input.
• Pull-up Defined open state INPUT_PULLUP on GPIO33 For long/noisy cables use a documented external pull-up and suitable input protection; do not connect a 5 V pull-up.

04Wiring Diagram

Normally open reed contact between GPIO33 and GND; INPUT_PULLUP defines HIGH when open. No 5 V connection or additional sensor power pin.

Open wiring diagram at full size (new tab)

1. 1

   Disconnect USB; check continuity with and without the magnet to confirm a normally open contact.

2. 2

   Connect the two terminals as shown, without bending against the glass seal.

3. 3

   Upload the sketch and move/align the magnet slowly; observe contact state rather than assuming a detection distance.

4. 4

   Mount only after deciding which physical lid position should mean open or closed.

Wiring and matching Arduino code

Stable magnetic-contact state

Normally open reed contact between GPIO33 and GND; INPUT_PULLUP defines HIGH when open. No 5 V connection or additional sensor power pin.

#include <Arduino.h>
constexpr uint8_t CONTACT_PIN = 33;
constexpr uint32_t DEBOUNCE_MS = 30; // Starting value; adjust for actual contacts.
int lastRaw = HIGH;
int stableState = -1;
uint32_t changedAt = 0;
void setup() {
  Serial.begin(115200);
  pinMode(CONTACT_PIN, INPUT_PULLUP);
  lastRaw = digitalRead(CONTACT_PIN);
  changedAt = millis();
}
void loop() {
  int raw = digitalRead(CONTACT_PIN);
  uint32_t now = millis();
  if (raw != lastRaw) { lastRaw = raw; changedAt = now; }
  if (now - changedAt >= DEBOUNCE_MS && raw != stableState) {
    stableState = raw;
    Serial.println(stableState == LOW ? "CLOSED" : "OPEN");
  }
  delay(1);
}

The first reported state also waits for the stable window. Unsigned millis subtraction handles rollover. This is a polling demonstration, not the mailbox deep-sleep wake algorithm.

How it works

Technical references

• Littelfuse 59165 reed-switch datasheet — Example normally open contact type; ratings belong to that product, not the unidentified illustration.
• Espressif GPIO API — Pull-up mode and digital input reads.