Micro Switch KB
- General features
- Reed switch subfamily
- Models and part numbers
KB was a series of pushbutton switches from Micro Switch, that also contained the associated indicator units, buttons, spacers, mounting bars, interlock assemblies and wiring connector blocks. It sat in a family alongside Series DS miniature display switches and Series 3 power and reed switches, all of which assemble into strips by way of mounting bars. A photograph of a Series 3 switch in Electronic Design Vol. 21 No. 10 (10th of May 1973; page 27, PDF page 31) shows how similar KB and Series 3 are.
The series provided both mechanical and keyboard reed switches. The reed switches from this series were briefly used in keyboards, including RW Series from Micro Switch. KB switches in general provide alternate action, tactile feedback and mechanical interlock. The mechanical switches also support output encoding and illumination, and there are models suitable for power switching. The mechanical switches are documented as being single, double or quad pole; the reed switches are capable of quad pole, but only single and double pole models are known.
In the 1973 catalogue, the name “Series KB” is used in the table of contents, but the section for that series calls them only “KB”, as does all other literature discovered to date.
KB is also one of only two types found to have symbol codes stamped onto some switches. These appear to indicate the manufacture date of the internal reed capsule, according to an unclear remark given in the charts regarding the nature of the markings on the switches.
The earliest mention of the series is from Information Display, January/February 1966 (page 14, PDF page 12), where they are described as “NEW MICRO SWITCH KB”. The reed switches are not mentioned in the advertisement. Promotional material from 1966 to 1969 (see under Documentation below) also omits the reed switches, even though they are confirmed to have been in production since the end of 1966, based on manufacture dates stamped onto the switches. Dates on KB switches in general have been found ranging from 1966 to 1982.
The series was present in the 1973 manual switches catalogue (Catalog 51 issue C), but was absent from the 1988 manual switches catalogue, Micro Switch Catalog 30 Issue 8. This latter document includes Series 2, 3, 4 and 6, but lacks both Series KB and Series DS, suggesting that KB was no longer considered viable by that time (although manufacturing may have continued past that point for specific customers).
Keyboards using these switches have so far only been found in the 1966–1970 timeframe; KB reed switches were swiftly replaced with SW Series solid state switches for keyboard purposes, which former Solid State Keyboards staff member Larry Bishop confirms. Although reed switches offer high longevity and short contact bounce time, Hall effect surpassed reed in both performance and lifetime.
KB switches are mounted together in rows into pairs of metal mounting bars. The top edge of each mounting bar has regularly spaced notches; a small ridge around 2.25 mm wide on one side of the switch—called the “index tab”—locates it within a corresponding slot in the mounting bar. These slots are spaced at half-unit intervals. The top of the switch has overhanging ridges that sit around the mounting bar, and each switch has two threaded rubber pieces with corresponding ridges that secure the switch to the mounting bar from below. The lower grips are pressed against the mounting bar by turning the screws on which they are placed. Each row in turn appears to be attached to the keyboard by way of narrow modules that attach to both the mounting bar and the top case.
The illustration below is only approximate, as dimensions were taken only from a single loose switch.
Series 3 mounting bars come under subseries 3M. Catalogues only depict 3M1 full-height bars; the short bars for direct wiring is 3M2. KB shares the mounting bar system from Series 3. Mounting bars also exist in KB, under subseries 7E.
Mounting bars are used regardless of whether the switches are connected using flying leads or by a printed circuit board.
KB switches can be connected to the circuitry using connector blocks. These are plastic bases that receive the switch terminals, with one receptacle for each terminal on the switch. All the terminals of the switch can be connected simultaneously as a result, which is useful for switch models with many terminals, such as those with both lamps and multiple poles. The reed switches have at two terminal types: quick connect (for wire wrap solder, tag attachment and connector blocks) and solder pins.
KB switches can be grouped together to provide mutual operating control. Momentary lockout prevents more than one button from being pushed simultaneously, and all buttons are momentary. Bail and lockout with key-down memory functions like the buttons on a radio: one button at a time is held depressed, and pressing a different button releases the one currently held. These functions are provided by an extra row of modules below the buttons, which are worked by trombone slide–shaped extensions to the plunger that protrude below the switch.
At least three KB subfamilies exist. This page focuses on the reed switches found in keyboards; brief details of all three subfamilies are given below.
Illuminated panel buttons
These are mechanical switches that have space for as many as four incandescent bulbs. Travel is 0.250″, and the switch contact surfaces are silver. They are rated for 115 and 230 V AC for 5 A resistive loads, and 5 A resistive and 3 A inductive loads at 28 V DC. These are targeted towards control panels.
Although four lamps may be fitted, the switches only support addressing two sources of illumination. A switch can have one or two redundant pairs of lamps.
The reed switches were a specialist subfamily not included in the promotional material, and were only mentioned briefly in the catalogue. Travel distances of of both 0.250″ (as per the illuminated panel buttons) and 0.180″ (better suited to keyboards) was provided. Keyboards made using these switches were placed into RW Series.
Encoding switches generate their identifying codes through an 11-way gang of contacts that includes one contact for each bit of the switch ID. Although considerable detail is given in the promotional material for encoding switches, and surplus parts are available for sale dated from 1966 to 1969 (indicating that they were in production), no details on specific models are given in the known catalogues.
Each switch is assigned an 8-bit identity, which is enough to cover not just 7-bit ASCII but any other special keys required. When pressed, the key station generates this numeric ID directly without the need for any additional circuitry. These switches have a total of 12 terminals:
|3–10||Switch ID (0–255)|
The final two terminals are joined (and only have a single pair of contacts between them) and are used to determine when two switches are pushed simultaneously. As all switches share a common bus, there is no rollover, and the monitor wires detect collisions between buttons. To avoid problems with contact bounce, the strobe contacts close last, ensuring that all the other contacts are made before power is applied to the switch.
Encoding switches remove the need for separate diode arrays to define the ID of each key. Instead, the key’s ID is set by clipping the tabs that connect to the switch terminals, to determine which bits on the connecting bus will be pulled low. Alternatively, the switch terminals themselves can be clipped. A photograph in the promotional material (see Documentation below) shows a complete QWERTY keyboard with numeric keypad and function key row assembled using encoding switches, although such a keyboard has yet to be observed in the wild. Other companies provided self-encoding keyboards (Datanetics with the elastic diaphragm array and Mechanical Enterprises with a variant of their Mercutronic switches) but the idea seems not to have lasted long in keyboards.
Encoding switches use trifurcated contacts. They appear to provide the same keycap mount as the reed switches. Model 7A1AH only has two terminals; no details are known for this or any other specific encoding switch models.
Reed switch subfamily
The reed switch models are omitted from known literature. The reed switches physically support single, double, triple and quad pole, although only single and double-pole types are known from the charts obtained to date. The double-pole types can have the two reed capsules in opposite corners, driven by two magnets (one magnet per reed), or the reed capsules can be on the same side of the switch, sharing a single magnet.
Typically the magnet is made of a smooth, bright, shiny metal, and is a cylinder 11.3 mm tall and 2.3 mm in diameter. Certain types instead have cuboid magnets, according to the charts.
Two shell colours are known: black and grey. The black shell is single-pole only, while the grey shell supports quad pole arrangement, the through plunger wire that operates multi-key lockout, and the inserts for alternate action and click feedback.
The magnets sit in a magnet frame placed atop the return spring. The magnet frame may be a complete plunger, or it may bear a separate shaft that protrudes from the switch for attaching keycaps. This extension shaft can be black or grey; the reason for the separate colours is not known. Keycaps cannot fit directly onto this shaft: a flat metal adapter is also required, of suitable dimensions to mount the keycap family selected.
Travel can be 0.180″ or 0.250″ (4.57 mm or 6.35 mm). Keyboards typically use the shorter 0.180″ travel, although the longer travel types are occasionally seen.
There are at least three keycap families used with the reed switches. Two types are associated directly with the series: the 7B stepped types (such as those found on the Burroughs D8565 keyboard), and slot-mount “truncated” keycaps similar to those found on IBM beam spring switches, found so far in a single keyboard.
KB reed switches were also used with specially-modified 2SW Series keycaps.
Keycaps cannot be attached directly to KB reed switches. The switch itself has a slot mount, that is a combination of two overlaid slots around 1.1 × 2.2 mm, one at 0° and one at 90°. A flat metal adapter is placed into this slot, bearing the desired keycap mount. It appears that at least two such adapters exist: a narrower one that fits older keycaps, and a wider one that takes specially-modified 2SW Series keycaps. The illustration below shows the design of the narrow adapter, with approximate dimensions based on photos:
The diagram below shows how 2SW keycaps were were revised to support KB switches. To retain compatibility with SW, SN and SD switches, the slots for the adapter (at 0° and 90°) had to be made longer than those in 7B keycaps—extending past the existing slot—requiring a wider adapter.
The diagram below shows the approximate dimensions of the slot within the plunger (or plunger extension) for the adapter:
Many switches appear to have a simple hole in the keystem, without the slots. Examination of 7A1H-X55 shows that there are small ridges on the inside that serve to define the slots. The shape is something similar to this; the image is only a suggestion as the details are too small to photograph or measure:
In other examples, these do not appear to exist at all, but there are no photographs clear enough to be certain, and there is likely to still be something within the hole to ensure correct orientation of the keystem adapter.
Within the switch there is a flat plastic frame which sits above the return spring and holds one or two cylindrical magnets. This frame may extend out to form the intermediate keystem. The intermediate keystem can also be a separate part that presses onto the magnet frame. The intermediate keystem fits onto a plain circular post on the magnet frame; correct orientation is set during manufacture to a tolerance of 0°30′ and is retained purely through a very tight fit. The intermediate keystem is hard to remove, and just as hard to orientate correctly when reattached.
The same general style of adapter system appears to be used with 1KS switches, being observed in 1KS1-T and 1KS3-T (which look identical).
Two terminal types are provided for reed switches: quick connect and solder. Switches with quick connect terminals do not have stand-offs. The solder terminals are wider just where they protrude from the switch, with a step in width; this short wide area may serve as a stand-off.
Alternate action reed switches have a flat metal leaf spring attached to the plunger. A small pin is pressed into the hole in the side of the plunger at the top, and this provides a pivot point for the leaf spring. A standard heart cam is used, and this is provided by way of a special insert that is placed into the switch and heat-staked using the upward-facing pin in grey switch shells.
A single model—keyboard type 7A1HW with 0.180″ travel—is known to offer audible feedback. This uses a similar arrangement to alternate action, with a special insert added into the switch that is engaged by a leaf spring that pivots from the plunger. The insert used for alternate action is depicted in the charts, but not the insert for audible feedback. The design may be similar to that of Hi-Tek Series 725 which uses a cam track to generate a click sound. (In theory there should be a corresponding model with solder terminals.)
Models and part numbers
KB part numbers are anomalous. All the catalogue part numbers begin “7”, as though it were Series 7. The part number pattern follows those of actual numeric series, such as Series 2 and the closely related Series 3. The internal components are mostly from series KB, but there are exceptions. The reed capsules also have CS series part numbers, one of the spring types has both KB- and 10- prefixes (the remainder of the digits are the same), and some switches have 09- part numbers for the magnets (these part numbers differ entirely).
|7C*||Indicators (lamp-only units)|
|7E*||Mouting bars, mounting blocks|
|7G*||Spacers, lugs, screws|
The fourth character in the part number indicates the switch grouping. Non-reed types include:
|7A1A*||Encoding switches||Various||1 unit|
|7A1C*||Illuminated non-reed||Double||1 unit|
|7A1H*||Reed switch with quick connect terminals (unconfirmed)||Single, double||1 unit|
|7A1M*||Reed switch with solder terminals (unconfirmed)||Single, double||1 unit|
Numerous reed types have been discovered to date, either in keyboards or in Micro Switch charts. The charts only cover a portion of the total range of part numbers. (For the charts, see under Documentation below.)
Operating force given in the charts is somewhat confusing. Typically, two figures are given: the initial operating force (“INITIAL OPR”) and the force at basic travel (often “BASIC OT POS”). The term “basic travel” is not explained, and it is slightly lower than full travel. For a switch of 0.250″ travel, the basic travel is typically given as 0.230″, while a keyboard type with 0.180″ travel has a basic travel of 0.160″. Initial operating force is given as a minimum, and basic travel force is given as a maximum.
In the Arrangement column, the reed positions are given where known per the charts. However, all SPST switches are assumed to be reed X unless stated otherwise, for clarity. The reed positions are illustrated with the diagram below:
(Micro Switch note that the index tab should be at the front in horizontal rows, and on the left for vertical columns of keys.)
The discovered types are as follows:
|Model||Type||Arrangement||Preload||End force||Travel||Terminals||Source||Observed dates||Notes|
|7A1HA||Momentary||SPST||1 oz||5 oz||0.250″||Quick connect||Charts||6832||NSN 5930-00-488-9958|
|7A1HB||Alternate||SPST||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1HC||Momentary, through plunger||SPST||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1HD||Alternate, through plunger||SPST||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1HF||Momentary||DPST (X+Y)||1 oz||5 oz||0.250″||Quick connect||Charts|
|7A1HG||Alternate||DPST (X+Y)||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1HH||Momentary, through plunger||DPST (X+Y)||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1HL||Momentary||DPST (W+X)||1 oz||5 oz||0.250″||Quick connect||Charts|
|7A1HM||Alternate||DPST (W+X)||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1HN||Momentary, through plunger||DPST (W+X)||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1HP||Alternate, through plunger||DPST (W+X)||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1HS||Momentary||SPST||1 oz||5 oz||0.180″||Quick connect||Charts||6949|
|7A1HW||Momentary, audible feedback||SPST||1 oz||5 oz||0.180″||Quick connect||Charts|
|7A1H-X1||Momentary||DPST (X+Y)||1 oz||5 oz||Charts|
|7A1H-X4||Support||—||None||Keyboards||6752||Dark grey plunger|
|7A1H-X17||Momentary||SPST||Quick connect||Keyboards||6649, 7019|
|7A1H-X18||Momentary||SPST||Quick connect||Keyboards||6651||Used for space bar|
|7A1H-X19||Alternate||SPST||1 oz||8 oz||0.250″||Solder||Charts|
|7A1H-X20||Alternate||SPST||1 oz||8 oz||0.250″||Quick connect||Charts|
|7A1H-X27||SPST||Quick connect||Keyboards||6738, 6826|
|7A1H-X39||Momentary||SPST||0.180″||Quick connect||Own collection||6916||Gold-plated terminals?|
|7A1H-X50||Momentary||SPST||1 oz||0.250″||Quick connect||Charts|
|7A1H-X52||Momentary, through plunger||SPST||1 oz||8 oz||0.250″||Quick connect||Charts||No screws|
|7A1H-X55||Momentary||SPST||0.180″||Quick connect||Own collection||8215||Heat treated terminals; black plunger|
|7A1MS||Momentary||SPST||1 oz||5 oz||0.180″||Solder||Charts||7030|
|7A1MT||Momentary||SPST||1 oz||5 oz||0.180″||Solder||Charts||6950, 7001|
|7A1MW||Alternate||SPST||1 oz||11–13 oz||0.180″||Solder||Charts|
|7A1MY||Momentary||SPST||1 oz||8 oz||0.250″||Solder||Charts|
|7A1M-X11||Momentary||SPST||4 oz||26 oz||0.180″||Solder||Charts|
|7A1M-X12||Momentary||SPST||1 oz||5 oz||0.250″||Solder||Charts||No screws|
|7A1M-X13||Support||—||1 oz||5 oz||0.180″||None||Charts|
|7A1M-X14||Momentary||SPST||1 oz||5 oz||0.180″||Solder||Charts|
|7A1M-X19||Momentary||SPST||1 oz||5 oz||0.180″||Solder||Charts|
7A1H-X39 and 7A1X-55 are largely the same. The terminals on 7A1H-X39 are a bright orange-yellow colour, as though they are plated with some kind of gold alloy. Those of 7A1X-55 have a dark grey surface finish with multi-coloured patches (similar to blued steel), suggestive of some kind of heat treatment. Both return springs are of wire gauge 0.008–0.009″ (around 0.22 mm) and are 3.45 mm diameter; the spring of 7A1H-X39 has 7.5 free turns and a length of 16.9 mm, while that of 7A1X-55 has 7 free turns and is 14.7 mm long. Both appear to have the same operating force, give or take tolerance.
Illuminated panel buttons
The following models are known (with prices in US dollars):
|Catalogue listing||Action||Poles||Size||Feedback||Features||Price (1967)||Price (1973)|
|7A1CE||Momentary||Double||1 unit||Tactile||Extended plunger for momentary lockout||$5.20|
|7A1CG||Momentary||Double||1 unit||None||Extended plunger for bail and lockout||$4.90|
|7A1CJ||Alternate||Double||1 unit||Tactile||Extended plunger for momentary lockout||$5.55|
The following types are not in the catalogue, and have not been inspected closely.
|7A1AA||Six sets of terminals; found dated 6952 (eBay)|
|7A1AH||One set of terminals, with long trombone protrusion from base (eBay, 6631)|
The following table lists keyboards made by Micro Switch customers using KB switches. For Micro Switch’s own keyboards, see RW Series.
|Burroughs D8565 keyboard||7A1HA, 7A1H-X27, 7A1H-X4||ca. 1968|
|Mohawk Data Sciences keyboard||7A1H-X17, 7A1H-X18||ca. 1966?|
|Unidentified keyboard||7A1HS, 7A1CA||ca. 1970?|
The Micro Switch Catalog 51 excerpt was provided by the National Museum of American History Library (part of the Smithsonian Institution). The promotional material was scanned in by the Computer History Museum. No literature has been found that covers the reed keyboard switches, but Honeywell were able to locate a subset of the A charts for them.
- Micro Switch Bulletin 70a: KB — Lighted pushbutton switches and matching indicators (lower-resolution scan with OCR)
- Introducing the Micro Switch KB Switch/Display Matrix (December 1966)
- Micro Switch List Price and Discount Schedule D-70d (January 1967)
- Micro Switch Catalog 51 issue C section D (March 1973), covering Series DS, Series 3 and Series KB
- 7A1H and 7A1M A charts from Honeywell (partially redacted)