A 3 x 20 mm Reed Switch is a specialized, hermetically sealed electrical switch that is operated by a magnetic field. Unlike standard switches that require physical pressure, this device acts as a proximity sensor, triggering when a magnet is brought nearby.

The "3 x 20 mm" designation refers to the dimensions of the glass capsule: 3 mm in diameter and 20 mm in length.

Key Features

• Hermetically Sealed: The internal contacts are encased in a glass tube filled with inert gas. This makes the switch waterproof, dustproof, and corrosion-resistant, allowing it to work in harsh or explosive environments where a spark might be dangerous.

• Passive Operation: It requires no power to stay in its "waiting" state, making it ideal for battery-powered devices.

• High Sensitivity: It can detect magnetic fields through non-ferrous materials like plastic, wood, or glass.

Common Applications

• Security Systems: Used in door and window sensors (the magnet is on the door, the reed switch is on the frame).

• Fluid Level Sensing: Attached to a float with a magnet to detect when a tank is full.

• Speed Sensing: Used on bicycle computers or fans to count rotations as a magnet passes the sensor.

• Laptops/Clamshells: Detecting when the lid is closed to put the device into sleep mode.

⚠️ Handling & Usage Note

Fragility: The glass body is very delicate. When bending the metal leads for mounting, always use needle-nose pliers to hold the lead near the glass. If you apply stress directly to the glass seal while bending, it will crack, and the switch will fail.