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Philips switches

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Philips switches, formerly known as “PED” switches, and before that as “Type 3” BBC Micro switches (per Chris Richardson’s nomenclature), are one of the four types of keyswitch used in the Acorn BBC Microcomputer. They are the rarest of the four types. They are also the only tactile switches used in the BBC Micro, although Philips’s definition of tactility differs from that of other manufacturers.

During my research into Hi-Tek Corporation, I encountered the patent for these switches, and dismissed it as irrelevant, believing that I had never seen the switch depicted. Having since bought two of these switches from 4D (part of CJE Micro’s) and dismantled them fully, I subsequently re-encountered the patent and recognised the design instantly.

The switch is recorded as being invented by Laurence Harry Finlayson, from Philips Electronics Uk Limited/Philips Electronics N.V., as European patent EP0043618. This patent was filed on the 24th of June 1981, and published on the 13th of January 1982. The switch would not appear in the BBC Micro until 1984. The patent itself depicts an illuminated switch, but appears to cover the entire switch design including the contact system. The BBC Micro Type 3 switch is a simpler design without support for illumination; the patent does however suggest why the keycap mount takes the form that it does.

Hi-Tek’s own 1972 patent, for their High Profile switches, is referenced by the patent; this awareness of Hi-Tek’s design might be why the separator bar has a totally different purpose in Philips switches than it does in Hi-Tek switches.

Overview

The switches were taller than I expected: there is almost as much switch below the mounting plate as there is above it. Overall height is around 16.3 mm. Height above the mounting plate is around 9.3 mm, with around 6 mm below the plate. The switch is around 15.6 mm square at its widest point. Total travel is difficult to measure, but it is somewhere around 3.2 to 3.5 mm.

The switch is plate mount, but it also features two fixing pins, in opposite corners of the base.

Because the contacts are pressed together by the slider, the feel is distinctly non-linear, but neither is it tactile. The travel starts out linear, up to where the slider starts to press the contacts together; from here on, the switch is noticeably stiffer. The patent specifically describes how the switch design generates tactility, but the feel is not what one would normally classify as a tactile feel, and the effect is comparatively weak.

The return spring is around 13.7 mm long with five inner turns, 9.4 mm outer diameter, 8.6 mm inner diameter, and 0.4 mm gauge.

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“PED”

The name “PED” itself is from a contemporary parts catalogue, part of which was at one stage transcribed by Wouter Scholten; the mailing list post is no longer available online. I cannot yet account for the name “PED”. Other patents include US4385219 (to U.S. Philips Corporation) and DE8119605 (to Naamloze Vennootschap Philips' Gloeilampenfabrieken), neither of which provide an expansion for “PED”.

Disassembly

To open the switch, press a flat blade screwdriver against one of the retention tabs and press down hard. The top of the switch will release sharply, so be careful to avoid letting the screwdriver enter the insides of the switch too hard.

View full-size image Screwdriver placement
View full-size image Top partially removed; note the now exposed return spring
View full-size image Switch opened

Base plug

Curiously, the contacts are not securely attached to anything. The contacts are held in place by a base plug, which is secured by four lugs. It’s not clear whether the lugs are melted or whether they are intentionally a press fit, but unlike most examples, the base plug can be prised out with a knife and reinserted without damaging the lugs. In fact, I removed the base plug first, before I determined how to correctly open the switch. The contacts rest on the base plug, and protrusions from the base of the switch sandwich them between the base and the base plug; as such, the legs are free to move a small amount, and do so when the contacts are closed.

View full-size image Base plug partially removed
View full-size image Base plug removed
View full-size image Contact assembly
View full-size image Symmetrical contacts

Contact operation

Hi-Tek’s much older design used sprung phosphor bronze contacts held apart by a separator bar in the slider. Pressing the slider lowered this bar and allowed the contacts to close together. Releasing the slider would separate the contacts again. The tip of one contact was split into four “fingers” to improve reliability. Later switch designs such as those from Stackpole and SMK would also feature a contact leaf tip divided into fingers.

Philips switches also have a pair of opposing contacts, but the design and operation is quite different.

View full-size image Side view of the contacts
View full-size image Top view of the contacts

Philips switch contacts are identical, simplifying manufacturing. Instead of having flat contacts with fingers, the contacts bear gold-plated prisms. To allow for the contacts to meet at 90° to each other, the prism is placed at 45°, giving a total angle of 90° between them without needing two different parts.

View full-size image Identical phosphor bronze contacts with prisms

A separator bar exists in the Philips design, but for a different purpose. According to the patent, it is to stop the contacts from touching due to external impact. The sliders are instead closed under pressure from a recess within the slider: the slider bears an asymmetric guide funnel that drives one contact sideways into the other. This gives the switch its decidedly non-linear feel.

View full-size image Inverted slider, showing funnel
View full-size image Slider engaging with contacts
View full-size image Demonstration of slider–contact engagement
View full-size image Second slider added for illustrative purposes

While playing with the switch, I noticed that the contacts are forced sideways by the slider. The patent indicates that the reason for this is to induce a wiping motion, so that the contacts keep themselves clean. Wiping action, incidentally, was also a feature cited by GRI of the KBM switches.

Components

As always, full disassembly of the switch. By my way of reckoning, there are seven distinct components in total, but nine when you consider the prisms.

View full-size image Top views of components
View full-size image Bottom views of components

Revisions

  1. 2015-09-14: original document
  2. 2015-09-18: rewritten in light of the switch being successfully identified as Philips, and knowledge gained from the patent