Door contact sensor BLOW 4926# (reed switch) — magnetic door/window sensor, normally closed

BLOW 4926# is a compact magnetic door and window contact sensor with a reed switch configured as normally closed. Designed for basic alarm and security systems, access control setups and DIY monitoring, the sensor detects changes in state when its magnet moves away from the contact. Its small footprint and simple two-part design make it suitable for discreet mounting on frames and sashes where reliable open/closed detection is needed.

Advantages

The normally closed reed contact offers stable closed-state monitoring and clear state change when the magnet is separated, reducing the chance of false triggers caused by contact bounce. The sensor’s compact dimensions and low-profile housing allow for unobtrusive installation on doors and windows. Because it uses a passive magnetic reed mechanism, it requires no internal power source and provides long-term reliability with minimal maintenance.

Suitable for

• Home and small-business alarm systems for door and window monitoring
• Access control installations that need simple state detection
• DIY security and monitoring projects
• Retrofit installations where space for sensors is limited

Key specifications

• Sensor type: magnetic contact with reed switch (normally closed)
• Mounting: surface mount with separate magnet and contact units
• Operating principle: magnetic field closes the reed; separation opens the circuit
• Power requirements: passive operation (no internal power required)
• Housing: compact, low-profile design suitable for door and window frames
• Environmental suitability: indoor use; designed for typical residential and commercial climates

How to use

Position the contact unit on the fixed part of the frame and align the magnet on the moving sash or door so that the two parts sit close together when closed. Secure both units using screws or adhesive appropriate to the mounting surface. Connect the sensor leads to the alarm control panel or access controller input, observing the normally closed wiring scheme expected by your system. Test the installation by opening and closing the door or window to confirm the contact opens when the magnet is moved away and closes when it returns.

Usage recommendations

For reliable operation, mount the contact and magnet on clean, flat surfaces and maintain the recommended alignment and gap specified by your installer guide. Avoid placing the sensor near strong external magnetic fields or metal obstructions that could affect the reed operation. Periodically inspect mounting and wiring for corrosion or loosening in environments subject to moisture or vibration.