Introduction to Industrial Automation

2.3.4   Reed  Relays

Despite their widely used name “relay”, reed relays are switches that are simple in structure and 

magnetically actuated. As shown in Figure 2.62a, where a reed relay is depicted, it has a quite dif-

ferent form from the relay. Reed relays or switches are manufactured with two ferromagnetic reeds 

that actually are electric contact blades, partially enclosed in a hermetically sealed glass capsule. 

Reed blade

Reed blade

Glass

capsule

Contact point

Inert gas

(a)

Contact gap

N

S

Magnet

N pole

S pole

S pole

N pole

(b)

(c)

Figure 2.62  A simple no-cased form of a reed switch (a), NO status of the reed switch contact 

(b), and the permanent magnet that closes the contact (c).

---

Hardware Components for Automation and Process Control 

◾

 

71

The two opposing blades are overlapped at their free ends inside a capsule with a gap between 

them, as shown in Figure 2.62b, while the other two ends are the terminals of the reed switch that 

are able to be soldered. The contact area of the two nickel-iron blades is coated with special met-

als, such as ruthenium for protection from arcs. The glass capsule either has an internal vacuum 

(for high voltage switching) or is filled with an inert gas to exclude contaminants and to prevent 

oxidation of the contacts.

When the magnetic field from a permanent magnet or a wire coil is close to the reed relay, 

then poles of opposite polarity are created among the blades and the contact closes, as shown in 

Figure 2.62c. Obviously, the field strength must be suitable so that the magnetic force exceeds 

the self-spring force of the reed blades. When the permanent magnet is moved away and the force 

between the blades is less than the restoring force, the contact opens. An electromagnetic coil 

for the creation of the required magnetic field (instead of a permanent magnet) may be part of a 

unified construction together with the reed switch. From the above description, it is advisable to 

distinguish the reed switch from the reed relay in order to remove any confusion. A reed switch is 

only the glass encapsulated two-blade contact, while a reed relay is a reed switch with an embed-

ded electromagnetic coil.

Reed relays and switches have several advantages, for example their low cost, long life (billions 

of switching operations), simple construction, fast switching time, absence of moving parts except 

for the elementary motion of blades, etc. But they also have some disadvantages; for example, 

the low current rate, sensitivity to other magnetic fields, sensitivity to vibrations, etc. However, 

both devices are ideally used for switching and sensing applications in industry, instrumentation, 

security installations, vehicle manufacturing, and many others. In fact, a reed switch is the sim-

plest sensor of a magnetic field and, in combination with a permanent magnet, it can be used for 

liquid level detection, position detection, and counting. For example, a double-action pneumatic 

cylinder usually has two piston position states (in or out) that may be equipped with an internal 

permanent magnet for reed switch activation, as shown in Figure 2.63. Then, by installing a num-

ber of reed switches outside the cylinder, it is possible to stop the piston in intermediate positions 

corresponding with the reed switches.