Electrical & Electronic

11-2001  Notebook

Switches reach through solid walls

New switching technology looks especially suitable for wet, corrosive and hazardous environments. Tom Shelley reports

It is now possible operate low-cost switching devices on the other side of solid walls, 50 to 100mm thick, without needing to drill any holes.

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Based on GMR or 'Giant' Magnetostrictive Resistance, they are immune to electrical interference or changes in environment, and expect to find uses ranging from operating bathroom and kitchen appliances, to applications on offshore oil rigs.

The Top Switch is the brainchild of Bury St Edmunds-based Herga Electric. The company says it originally looked at capacitance and induction as ways of making non-contact connection. It rejected capacitance, because of nuisance tripping problems, which can be accentuated by variations in humidity, and rejected inductance because of relatively high component counts.

GMR only requires a magnet in the controlling knob or switch and a small sensor in the switching device. Turning round or flipping a magnet in the switch operator leads to a change in sensor resistance sufficient to produce a reliable, measurable response.

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At present, switch operators are only available as knobs, with a 90 deg angular rotation between the on and off positions. The knob is moulded in two parts – the visible rotating part which contains the magnet and a hidden fixed part which is secured to the label and then stuck to he work top or panel surface. Sensor electronics are encapsulated surface mount, with four external wire connections. The electronics require a low voltage supply, typically 6 to 12V DC. Output is a voltage-free, normally open contact rated at 0.5A. A microprocessor reads the signal from the GMR device and works out where to set the thresholds.

Herga makes large numbers of pneumatic switch operators for whirlpool paths, sink units and showers. The new devices could allow these to be operated by a knob instead of push button, as at present. Anticipated selling price is expected to be around 20 to 40 depending on quantities and options, with substantial discounts possible at very large numbers. One prospective customer is said to be interested in purchasing in excess of 100,000 per year.

Other potential markets include small yachts and cabin cruisers, because of a reluctance among builders of such products to drill holes in fibreglass hulls, and the offshore business. The latter always has an interest in devices which allow intrinsically safe device operation, remote from electrical switching devices which can be sealed away or placed in less hazardous zones. (More information at www.herga.com )

Design Pointers

Switching devices require no mechanical connection between the operator, above the worktop, and the electronic sensor mounted beneath

The devices will operate through most materials from 10 to 50mm thick