Some basics on Reed Switches:

WHAT IS A REED SWITCH?
A glass enclosed, magnetically operated contact using reeds as the contacting members.

Reed switches and sensors come in three basic types:

1. Single Pole Single Throw (SPST) Form A, Normally Open. In the form A the reed contacts CLOSE on the presence of magnetic field.

2. Single Pole Single Throw (SPST) Form B, Normally Closed. In the form B the reed contacts OPEN on the presence of magnetic field.

3. Single Pole Single Throw (SPDT) Form C, also referred to as a changeover reed switch. The form C has two contacts, one that opens and one that closes in the presence of a magnetic field.

HOW DOES A REED SWITCH OPERATE?
The reed blades act as magnetic flux conductors when exposed to an external magnetic field from either a permanent magnet or an electromagnetic coil. Contacts in a Form A switch, like shown here, close when the magnetic force exceeds the spring force of the reed blades. As the external magnetic force is reduced, the contacts spring back to the open position.

WHAT ADVANTAGES DO REED SWITCHES OFFER?
• Low cost and long life
• Small size
• High signal isolation
• Low contact resistance
• Millisecond contact operation
• Requires no power to operate
• Relay and switching functions in one unit
• Converts mechanical motion to electrical signals
• High voltage holdoff and switching
• No-touch switching
• Operation is not affected by external contaminants
• Safe operation in a hazardous environment
• Can be customized to a specific sensitivity and lead configuration

REED SWITCH APPLICATION CONSIDERATIONS
As a coil is energized or as a magnet approaches a reed switch, it operates. However, the interaction between the reed switch, the magnet (or coil) and their surroundings is affected by various factors. For a complete outline of these factors, do's ands dont's,
click here.

(for complete, detailed Application Notes on Reed Switches, click here)