TechArchive

This article was originally written in the period 1995-2000

By Colin Everett, Director and General Manager, Gentech International Ltd.

The most intriguing aspect of the glass reed switch is its extreme simplicity, which over the years has perhaps been one of its major strengths. In its basic form, the device has only three component parts – two magnetic armatures and a glass tube. It is simply operated by the application of a magnetic field.

Introduced by Dr W B Ellwood of Bell Telephone Laboratories in the USA back in 1956, the reed switch was originally designed to provide a sealed electro-mechanical device for telecommunications switching, which would be free from the noise problems associated with open style relay contacts of the day.

It quickly became apparent that this simple but effective device had many potential applications outside of telecommunications, and a whole new industry was born, ranging from proximity sensing and counting devices to sophisticated data switching applications where the benefits of a reliable sealed contact could be exploited.

During the past 40 years, three distinct phases can be identified, the first geared around the telecommunications industry which continued into the 1980’s and saw the development of large scale manufacturing capacity in the UK to satisfy the requirements of the TXE4 family of telephone exchanges which required a reed switch for each speech path. With the introduction of the new digital exchanges, this phase has now become all but extinct, with speech switching being handled digitally by solid state technology.

The second phase, overlapping the first, saw the development of a general commercial market for the reed switch, taking advantage of the broad application potential of the device coupled with its ability to offer a competitively priced solution. Gentech International became one of the market leaders during this period, specialising in providing a new product custom design service.

During the 70’s and 80’s custom relays were designed, covering almost every sector of the OEM market, which varied from non-standard operate and switching characteristics to high frequency operation and low thermal EMF characteristics. Parallel with this, the ability of the reed switch to operate as a proximity sensor was also being exploited. Perhaps the best known market application is security sensors, where the reed still offers the most reliable and cost-effective solution today. Industrially, applications have been too numerous to mention, almost any proximity sensing requirement being satisfied by reed switch technology. As an example, the latest programmable pneumatic systems continue to make extensive use of the reed switch to sense cylinder position.

The third and most recent phase in the development of the reed switch has been associated with the explosion in demand for sensing devices in recent years. Ironically, the very technology which was predicted would replace all electro-mechanical devices by the 1990’s has actually been responsible for resurgence in demand, due to the need to have sensing devices communicating information to a central processor bout the real world. The reed switch continues to offer the simplest, most cost-effective solution in many of these applications, often avoiding the need for any associated slave circuit components.

Although this development has taken place right across the industrial spectrum, perhaps the best and most easily recognisable example is in the automotive industry, where the explosion in sensing requirements has become apparent to all of us who life the bonnet of our car and wonder what has happened to the engine we used to recognise so well. Almost every function of the vehicle is now subject to some form of sensing control, with the reed switch featuring heavily in the monitoring of all fluids and also as the main switching element in the airbag safing sensor.

The latter is an excellent example of how the flexibility of the reed switch has enabled its adaptation to new technology. Remembering that its original purpose in life was to offer good, stable, low signal performance with a typical life expectancy of 100 million operations, the safing sensor with its requirement to reliably handle in excess of 5 amps for very few operations, is at the opposite end of the application spectrum.

Following the successful development of contact materials to handle directly the switching load required to actuate the airbag, the reed switch has developed a successful partnership with micro-machined sensors also used to monitor the acceleration waveform at the point of impact. Only when both devices register a crash situation will the airbag fire.

Design of the sensor package which makes use of moving magnet technology has necessitated the use of sophisticated mathematical modelling techniques to predict the response of the sensor in live crash situations. Given the variable size, weight, crush zone design and sensor position, every vehicle model will exhibit a different acceleration profile, requiring different sensor design characteristics.

Given the safety-critical nature of the airbag sensor, control of design parameters and the monitoring of sensor performance are key automotive requirements, involving 100% test of operation using simulated acceleration waveforms on long stroke shaker equipment.

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