Reed switch

Magnetic reed contacts are glass-encapsulated magnetically operated reed switches used on doors and windows. The contacts can be overcome by cutting the door, leaving the magnet undisturbed. 

Ruthenium, iridium and osmium Baths based on the complex anion (NRu2Clg(H20)2) are best for ruthenium electrodeposition. Being strongly acid, however, they attack the Ni-Fe or Co-Fe-V alloys used in reed switches. Reacting the complex with oxalic acid gives a solution from which ruthenium can be deposited at neutral pH. To maintain stability, it is necessary to operate the bath with an ion-selective membrane between the electrodes . 

The trend towards miniaturization continues. The original reed switch is getting continually smaller and therefore can be integrated at minimal space. Its magnetic sensitivity has been improved and stabilized. New materials allow the use of even smaller magnets without reducing the distance between reed switch and magnet. 

The reed switch was first invented by Bell Labs in the late 1930s. Most of the manufacturers of reed switches today produce very high quality and very reliable switches. This has given rise to unprecedented growth. 

Fig. 5.5 The basic hermetically sealed form 1 A (normally open) reed switch and its component makeup.

Fig. 5.6 The basic operation of a reed switch under the influence of the magnetic field of a permanent magnet. The polarization of the reed blades occurs in such a manner to offer an attractive force at the reed contacts.

A perpendicular approach with the magnet parallel to the reed switch gives one operation at max. operation distance (Fig. 5.7). This method of operation offers medium sensitivity to stray magnetic fields, large switching courses and hysteresis and high switching position tolerance. 

A parallel motion with the magnet perpendicular to the reed switch allows two switching operations (Fig. 5.9). The distance over which the switch opens is very... 

Handling of Reed Switches in Various Sensor Applications 5.2.4.1 Cutting and Bending of a Reed Switch... 

Many users of reed switches for sensor applications try to make their own sensors. Often, however, they do not observe some basic precautions and preventive measures to ensure reliable operation of the switch. Below we try to cover the key areas that users and manufacturers must be aware of. 

Fig. 5.14 Presentation of the proper and improper way of bending a reed switch. (Supporting the switch lead while bending is a must.)...

Fig. 5.15 Properly supported the switch lead while cutting is required, otherwise damage can occur to the reed switch.

Most reed switch suppliers can perform value-added cutting and shaping of the leads in a stress-free environment, using proper tools and equipment This is often the most economical approach for users, although it may not seem so at the time. 

Often, users will choose to make their own modifications, and only when experiencing manufacturing and quality problems with the product will they go back to the reed switch manufacturer and let him carry out the desired modifications. In Figs 5.14 and 5.15, the proper approach for cutting and/or bending the reed switch is shown. The effect on the pull-in and drop-out characteristics of cutting and bending the reed switch will be explained in more detail later. 

Often, a reed switch has to be soldered or welded. Reed switches are usually plated with suitable solderable plating. Welding is also quite easy on the nickel/iron leads of the reed switch. However, both, if not carried out properly, can cause stress, cracking, chipping or a rupture in the reed switch. When soldering or welding, it is best to keep as far away from the glass seal as possible. This may,... 

Fig. 5.16 Soldering and welding can generate a heat front to the glass to metal seal of the reed switch causing potential damage.

Fig. 5.17 U se of heat sinking or preheating reed switches for soldering or welding can prevent heat stress damage.

Most commercial wave soldering machines have a preheating section before the PCB or assembly is immersed into the solder wave, the thermal shock is reduced As the ambient temperature has already been raised before the solder wave, the thermal gradient in the reed switch seal is reduced. 

Dropping the reed switch or a reed sensor on a hard object, typically on the floor of a manufacturing facility, can induce a damaging shock to the reed switch. Shocks above 200 Gs should be avoided at all cost (see Fig. 5.18). Dropping any of the above on a hard floor from 20 cm or more (more than one foot) can and will... 

Fig. 5.18 Dropping the reed switch on a hard surface can induce several 100 Gs to the contacts many times altering the switch characteristics.

Temperature cycling and temperature shock that occur naturally in a reed switch application must be taken into consideration. Again, temperature changes creating movement with various materials due to their linear coefficients of thermal expansion will induce stress to the reed switch if not properly dealt with. 

In this application, the sensor recognizes the movement of water. The reed switch, in going from an open to a closed state, produces a fast response to the initiation of water flow in turn, an action sequence is initiated. 

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