Reed switch is a commonly used electromagnetic switch, whose contacts are composed of reed switches. A reed switch is made by winding two hardened steel strips (or wires) of opposite polarity, with a thin insulation layer filled between them, and sealed in an airtight glass tube. The internal structure of the reed switch allows the contacts to produce a suction or release action under the action of an external magnetic field.

Specifically, the contacts of a reed switch are hardened steel strips or wires at both ends, which maintain a certain distance and do not come into contact when there is no external magnetic field acting on them. But once an external magnetic field approaches, the effect of the magnetic field will attract the contact between the steel strip or wire together, forming a closed state, allowing current to pass through. When the external magnetic field disappears, the contact will return to an open state.

The advantage of a reed switch is that its contacts do not move mechanically, resulting in a longer lifespan, and the opening and closing actions respond quickly without arc generation. Therefore, it is suitable for applications that require high-frequency switching or high stability, such as electronic devices, computers, communication equipment, and safety systems.

The contact types of reed switches can be classified according to their connection methods and structural characteristics. The following are several common types of reed switch contacts:

Single pole single throw (SPST): This is the simplest type, with only one contact and a switch between two states, open or closed.

Single pole double throw (SPDT): This type has one contact but two possible output states. There is a normally closed contact in one position and a normally open contact in another position.

Double pole double throw (DPDT): This type has two independent contacts that can control two circuits simultaneously. It has two output states, and the states of the two contacts may be different in each state.

Single pole four throw (SP4T): This type has one contact but four possible output states. It can be connected to one of four different circuits.

Double Knife Four Throw (DP4T): This type has two independent contacts and can control four circuits simultaneously. It has four possible output states, and the connection method of the two contacts may be different in each state.

These contact types enable the reed switch to flexibly meet different control and connection requirements in various application scenarios. For example, SPST is commonly used for simple switch control, SPDT and DPDT can be used to switch and select different circuits, while SP4T and DP4T are commonly used for more complex signal routing and switching applications.