This is the Magnetic Door Switch Set, a small reed switch assembly specifically designed to alert you when doors, drawers, or any other aperture opens. 

Technical Specification:

• Operating Supply Current: 3.5 mA
• Maximum Output Current: 10 mA
• Operating Supply Voltage: 3 V to 6.5 V
• Minimum Operating Temperature: – 40 C
• Maximum Operating Temperature: + 85 C
• Mounting Style: Through Hole
• Package/Case: TO-92S-3
• Output Voltage: 2.5 V
• Sensitivity: 1.6 mV/GS
• Termination Style: Through Hole
• Supply Voltage: Max: 6.5 V
• Supply Voltage: Min: 3 V

Features:

• Easy to install
• Strong concealment
• Magnetic sensor alarm, control the switch of the circuit via the built-in magnet

Applications:

• Can be used in places like apartment, hotel, office, etc.
• Can be used as home security systems