Reed sensors have four function types. These consist of two flexible, magnetizable Reeds. Under the influence of a magnetic field the contact surfaces touch each other. Thus current flows through the switch.
Normally, normally closed contacts can be realized in two ways: Either a change-over contact is used, but only the normally closed contact is soldered, or an external magnet is attached to a normally open contact, which keeps the Reed contact closed. The reed contact opens when an external magnet with a different polarity approaches the Reed contact.
The tongue of the changer touches a normally closed contact without magnetic field and a normally open contact in active position.