NB Magnetic Door Switch

In daily life, we often see fire doors in the stairwells of shopping malls, office buildings, or residential complexes. These doors may seem ordinary under normal circumstances, but they play a critical role in blocking fire and smoke during a fire. The ability of fire doors to close automatically and provide status feedback relies on a small yet crucial component—the door contact switch. Today, let’s explore how this "little device" works.

1. Basic Components of a Door Contact Switch

A door contact switch consists of two parts: a magnet and a reed switch. The magnet is typically installed on the door leaf, while the reed switch is mounted on the corresponding position of the door frame. When the door is closed, the distance between them is reduced to within a few millimeters. Inside the reed switch, there are two reeds made of a special alloy that come into contact or separate under the influence of a magnetic field.

Here’s an interesting fact: the reed switch is actually a "lazy invention." It operates without requiring an external power supply, relying entirely on the magnetic field generated by the magnet. This design ensures that the door contact switch remains functional even during a power outage, which is particularly important for fire doors.

2. Detailed Working Principle

When the fire door is closed, the magnetic field generated by the magnet causes the reeds inside the reed switch to attract and form a closed circuit. At this point, the monitoring system receives a "door closed" signal. In the event of a fire, if the door is accidentally opened, the magnet moves away from the reed switch, causing the reeds to separate due to their own elasticity. This breaks the circuit, and the system immediately triggers an alarm.

It is important to note that fire door contact switches have a key difference from ordinary door contacts: their reed materials are specially treated to maintain stable performance in high-temperature environments. While ordinary door contacts may fail at temperatures above 80°C, those designed for fire doors can withstand temperatures exceeding 200°C.

3. Signal Transmission Methods

LoRaWAN wireless door contacts are intelligent security devices based on Low-Power Wide-Area Network (LPWAN) technology. By detecting the open/close status of doors and windows and leveraging LoRaWAN long-range communication technology, they enable real-time monitoring and alarm functions. This section details their hardware composition, communication mechanisms, workflow, and core advantages.

I. System Composition
1. Door Contact Sensor Module
Magnetic Sensor: Composed of a permanent magnet and a reed switch. When a door or window is opened or closed, changes in the magnetic field trigger the reed switch to turn on or off, generating a switching signal.

MCU (Microcontroller Unit): Responsible for signal processing, such as detecting open/close status and triggering alarm logic (e.g., if the door remains open beyond a set threshold).

Power Management: Designed for low power consumption, typically using a CR2032 coin cell battery, providing a battery life of 2-5 years.

2. LoRaWAN Communication Module
LoRa RF Chip: Such as the Semtech SX1276, supporting spread spectrum modulation technology to enhance anti-interference capabilities.

Communication Protocol: Complies with LoRaWAN Class A/B/C standards (door contacts commonly use Class A for low-power mode).

Data Encryption: Supports AES-128 end-to-end encryption to ensure transmission security.

3. Network and Cloud Platform
LoRaWAN Gateway: Receives sensor data and forwards it to the network server.

Cloud Server: Parses data and triggers alarms (via SMS/app push notifications), while storing historical records.

User Terminal: Mobile app or web platform for real-time status monitoring and remote control.

II. Working Principle and Communication Process
1. Status Detection and Triggering
Normal Closed State: The magnet aligns with the reed switch, closing the circuit. The MCU detects a low-level signal.

Door/Window Open: The magnet moves away from the reed switch, breaking the circuit. The MCU detects a high-level signal and identifies an abnormal event.

2. Data Transmission Process
Event Trigger: When the door contact status changes, the MCU wakes up the LoRa module and packages the data (including device ID, status code, and timestamp).

LoRaWAN Uplink Transmission:

Data is modulated by the LoRa RF chip into a spread spectrum signal and transmitted to the gateway via the ISM band (e.g., EU 868MHz).

The gateway forwards the data to the network server for decryption and verification.

Cloud Processing:

The server determines the event type (e.g., "open timeout") and triggers alarm rules.

Notifications are pushed to the user’s app, and the event is recorded in the database.

3. Low-Power Optimization Strategies
Sleep Mode: When no event occurs, the MCU and LoRa module enter deep sleep, reducing current consumption to as low as 1μA.

Adaptive Data Rate (ADR): Dynamically adjusts transmission rate and power consumption based on signal strength.

Heartbeat Mechanism: Periodically sends status packets (e.g., every 24 hours) to confirm device connectivity.

III. Core Advantages
1. Ultra-Long-Range Communication
LoRaWAN offers a coverage radius of 2-15 km (depending on the environment), enabling connectivity across multi-story buildings or industrial areas without repeaters.

2. Low Power Consumption and Long Battery Life
Event-driven operation combined with sleep mechanisms allows battery life of up to 5 years, reducing maintenance costs.

3. High Network Capacity
A single gateway can support thousands of door contact devices, making it suitable for large-scale deployments (e.g., smart campuses, chain stores).

4. Strong Anti-Interference Capability
Spread spectrum modulation technology effectively resists multipath fading and noise interference, with a bit error rate below 0.1% in complex urban environments.

IV. Typical Application Scenarios
1. Smart Home Security
Real-time monitoring of door and window status, integrated with camera snapshots or audio-visual alarms.

2. Commercial Asset Management
Prevents unauthorized entry in warehouses, server rooms, and other settings, while recording abnormal opening times.

3. Industrial IoT (IIoT)
Monitors the status of equipment cabinet doors to ensure production safety and compliance (e.g., in chemical plants).

6. Common Troubleshooting

If the door contact switch is malfunctioning, follow these steps to check:

- First, confirm whether the door is fully closed.

- Check if the magnet and reed switch are properly aligned.