10 Pcs MKA-10110 10-15AT Magnetic Sensor N/O SPST Contact Reed Switches The reed switch contains a pair (or more) of magnetizable, flexible, metal reeds whose end portions are separated by a small gap when the switch is open. The reeds are hermetically sealed in opposite ends of a tubular glass envelope. The reed switch's application is to detect the opening of a door, when used as a proximity switch for a burglar alarm.