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Micro Switch SW Series



SW and SN Series are Micro Switch’s original series of Hall effect pushbutton switches. Solid State Keyboards use SW Series switches, which are mounted into special frames within the keyboard, and cannot be used separately. By comparison, SN Series switches are fully-self contained and can be snapped into a panel or attached directly to a PCB, depending on the model.

SW Series keyboards are comparatively uncommon. By 1976, Micro Switch had introduced a replacement series of keyboards—SD Series—and these are far more commonly encountered. SW Series keyboards continued to be produced until at least 1995, and the final update to the switch drawings was in 1999.

Via his son Scott, Everett Vorthmann from Micro Switch has indicated that “SW” denotes wired solid-state keyboards, i.e. fully-assembled keyboard units with reed switches.


SW Series switches are all open at the base. The return spring protrudes out from the bottom, and the Hall sensor IC (“lead frame package”) is a slide-in fit. These switches clip into mounting frames placed onto the keyboard matrix PCB. These frames have locating nubs for each return spring, loops to secure each switch into position, and holes to allow the terminals of the Hall sensors to pass through to the PCB. Micro Switch give the reason for the mounting frame arrangement as being a way to prevent keystroke impact from reaching the solder joints on the PCB, although this seems to be contradicted by the fact that the frames are riveted to the PCB.

The design rationale for having open-base switches (instead of fully-enclosed switches) is not known, but one explanation would be serviceability. Since the switches are contactless, they would typically only need servicing to replace a fouled housing or broken plunger. In such cases, there would be no need to supply or replace an expensive Hall sensor: all that is needed to replace a broken switch is to swap out the housing assembly, without the need for any desoldering. Replacement switches bought singly do come with a fresh Hall sensor, as well as an instruction leaflet (except in later years) and a pair of pry tools, suggesting that service technicians would not buy boxed replacement parts as they would soon have a mound of unused sensors, tools and paper.

The return spring is not part of the bill of materials for the switch: the switch models cover only the physical design and choice of Hall sensor. When sold separately, the spring is included, and these parts have separate numbers indicating that they are replacement modules and—for models with a spring weight other than the default of 3 ounces—the weight of the included spring.

Switches are marked with an arrow on the top, which points towards the back of the switch. Confusingly, the corresponding arrow in SD Series switches points towards the front.

The diagram below depicts the general construction of an SW Series keyboard, starting from the printed circuit board (PCB). The PCB is often dual-layer, and consequently it is not uncommon to see some form of insulating material between the PCB and the mounting frames. In one keyboard this seems be to pieces of PCB substrate, while in others it is more like self-adhesive paper.

Metal mounting frames are placed above the PCB and secured with rivets. Each row of keys has a separate mounting frame, and these frames hold the switches in place. Replacement switches on eBay can be found both with the Hall sensor fitted and left separate. It is more likely that the switches were placed into keyboards with the sensor already fitted, but once the IC is soldered in place, the switch module can be extracted without removing the Hall sensor. This allows for easy switch repair.


SW Series keyboards were designed to use two-of-N encoding. Each of the sensor’s dual redundant outputs is fed separately into an encoding grid, which in turn feeds the encoder circuit with the identity of the switch in two-of-N code. As a result, SW Series keyboards typically have every switch wired independently, rather than as positions within a matrix. There are multiple encoder options, chosen according to factors such as cost. The encoder can be TTL-based (medium-scale integration), using an assortment of separate chips. Alternatively, the encoder can be MOS-based (large-scale integration), for a reduced part count and significantly greater flexibility in terms of output, such as permitting the shifted output from a key to be unrelated to the unshifted output.

TTL encoding uses decimal encoder chips, shown in the Beehive Mini-Bee keyboard circuit diagram (in the Mini Bee Computer Terminal Service Manual) to be custom 8-to-4 and 7-to-4 encoders. Each encoder handles half the input values (1–8, 9–F) for each of the high and low nybble. In theory only the values of 0–7 are required for the high nybble (since ASCII only ranged from 0x00–0x7F) but the Mini-Bee keyboard circuit nonetheless was capable of full 8-bit output codes, possibly due to being an off-the-shelf encoder circuit (even with dedicated modifier key wiring). Separate circuitry on the PCB is resposible for collision detection and modifier key processing.

With a MOS encoder, the circuitry becomes much simpler.


N-key rollover

The use of two-of-N encoding means that only one regular key can be active at once. If two keys were to be held simultaneously, the encoder would receive incorrect or invalid input: the output from the two keys would collide, with the encoder seeing the bitwise OR of the two scancodes (this would likely be an illegal two-of-N code). Thus, N-key rollover in SW Series keyboards carries a different meaning to how it is otherwise understood. SW Series keyboards with N-key rollover do not allow unlimited keys to be held: they simply avoid clashes from keys being struck in rapid succession.

SW Series keyboards with N-key rollover use sink pulse Hall sensors, whose outputs are only active for around 10–100 µs from the point that the key is struck regardless of how long the key is held. This prevents clashes arising, with a trade-off that key release cannot be detected. Before the second key is pressed, the first has already been detected and its sensor has shut itself off. Modifier keys still use current level sensors, and these must be wired separately.

Two-key rollover

Even two-key rollover would be subject to clashes with current level switches. Micro Switch define two-key rollover as an electrical interlock that disables keyboard output while two or more keys are pressed at once. Presumably (as this is not stated) once the number of active keys has reduced back to one, the remaining active key will be reported. This implies that a continuous sequence of keys each pressed with the preceding key still active will not be understood, but occasional instances of a single key pressed too early would be recoverable and not result in a missed keystroke.

The Beehive Mini-Bee terminal keyboard uses TTL two-of-N encoding combined with an electrical monitor circuit. This circuit appears to detect when any two keys are held simultaneously, suggesting a two-key rollover arrangement. This may indicate how two-key-rollover was implemented, as details remain difficult to obtain.

Product Brochure SW reported that Honeywell’s Systems and Research Division found that use of N-key rollover reduced operator error by up to 30% over 2-key rollover. It may help to understand this claim in context: as late as 1967 (shortly before SW Series was introduced) and possibly later, Invac were still advertising their photoelectric keyboards with a mechanical interlock that made depressing two or more keys simultaneously physically impossible. Purely electronic keyboards (with no mechanical contrivances such as solenoids or motors) were still relatively new in the late 1960s.

Key repeat

Key repeat in SW keyboards was achieved by the keyboard repeating the strobe signal. The strobe signal indicates when a key has been pressed and tells the host equipment that there is a key code present on the data bus ready to be collected. There were at least three repeat options:

Bi-level (double action)

Bi-level switches—found in 1SW200 Series—have a larger sensor IC, with two separate Hall elements, positioned at the top and the bottom. The plunger has two metal pins protruding from the bottom, each connected to a stiff spring. As the plunger is depressed, these pins make contact with the switch mounting frame. Pressing the plunger further requires these extra springs to be compressed at the same time.

Bi-level switches still have four terminals on the sensor; they will thus not have redundant outputs. Because these switches are used for auto-repeat, they must be level instead of pulse switches. It is not known whether they are current sourcing or current sinking.

Lock keys

Lock keys are achieved using one of the following methods:


SW illuminated switches are centre-lit using a small bi-pin incandescent lamp, the same as those used in Clare and Pendar switches. These have an ivory plunger. The plunger is tubular and rises up around the lamp. Special keycaps are required for illuminated switches.

Illuminated SW Series switches are rarely encounted; they can be seen however in a 101SW1 keyboard (possibly 101SW1-3-H or 101SW1-4-H).

Tactile feedback

Apparent tactile feedback is demonstrated in the switch model 1SW12-BL force curve. This switch is known from the IBM 3277 keyboard. The only characteristic visible in the photos that is different from normal switches is a pair of horizontal ridges across the back of the plunger, that look like they are designed to catch on something. The force curve also has the double step characteristic of Alps tactile switches.

No mention has ever been made of 1SW12-BL switches being tactile or clicky.

Product groupings

Product and part numbers are divided into two main groupings. Components use catalogue listings of the form “SW-” followed by five digits. This grouping includes magnets, shells, plungers, “spacers” (pry tools), PCBs, sensors etc. Switches, keycaps and complete keyboards have catalogue listings where “SW” is prefixed by a number.

Grouping Usage
1SW… Switches
2SW… Keycaps
500SW… Uncertain; includes 500SW90-3, which is a sealed keyboard plunger
Other nSW… Keyboards, where n denotes the number of keys
SW-nnnnn Components



Replacement (-R) switches are supplied together with the spring and a pair of pry tools for removing the failed switch. Typically, the package also included an instruction leaflet for switch replacement; see under Documentation for a copy of the leaflet. Older switches were wrapped in thin brown paper and placed into a cardboard carton, while more recent switches shipped in a clear plastic packet, and often lack the instruction leaflet.

Before the switch can be placed into the keyboard, the return spring needs to be attached to the plunger. This is achieved by placing the spring over the boss on the plunger and rotating it anti-clockwise. It will then lightly grip the plunger and allow the replacement switch to be lowered into position.

The following photos represent neither a real unboxing nor the precise state in which the package contents were supplied. The sequence is a recreation using a previously-unboxed switch, where the box contents were returned to the box in perhaps a less suitable and less expert way than how they were placed originally. However, the instruction leaflet does unfortunately arrive just as creased as depicted.

View full-size image Micro Switch PK-44600 standard shipping carton for smaller components; this was later replaced by clear plastic packets
View full-size image The leaflet is placed around the inside of the box, and the switch is placed wrapped into the centre
View full-size image Box contents: one switch, one return spring (to be fitted separately), two pry tools and the instruction leaflet
View full-size image Unwrapped box contents


The part number schema for SW Series switches is as follows:


Prefix for keyswitch modules
Series name
Model number, in a range of at least 1–301
Denotes a replacement part
Operating force in ounces; where omitted, the operating force is the default of 3 oz


Switches in SW Series fall into at least two subseries:

Subseries Range Type Numbering
1SW Series 1SW1–1SW99 Non-illuminated Odd sloped, even stepped
1SW100 Series 1SW100–1SW199 Illuminated; non-illuminated accepting illuminated keycaps Unknown
1SW200 Series? 1SW200–1SW299? Includes double action (bi-level) Unknown
1SW300 Series? 1SW300–1SW399? Unknown

1SW1 Series switches appear to exist in pairs, with each odd-numbered model having a straight keystem for sloped keyboards, and each even-numbered model having an angled keystem for stepped keyboards.

1SW100 Series switches are not fitted with lamps: unlike 201SN Series, there are not pairs of part numbers depending on whether a lamp is fitted.

No useful data remains on 1SW300-R and 1SW301-R.

The allocation of part numbers to the secretarial shift types is not known.

Plunger colour

SW Series was advertised in Information Display, Volume 5 Number 6 from November/December 1968. In this advertisement, the plungers were all shown to be grey. Leaflet PK 8503 2 (see Documentation, below) describes two plunger types: one grey (with a “hat shaped boss” for the spring) and one black (with a “plunger spring seat”). This document was drawn up in November 1971, but black plungers go back at least as far as 1970 (see under discovered keyboard examples). At this stage, all plungers of the same age appear to have been the same colour, either grey or black depending on age.

At some point in the mid-1970s, switches became colour-coded, primarily according to the sensor type. The oldest SW Series switch on eBay with a colour-coded plunger is from 1977, while the most recent one with a black plunger (of a type that would change to colour) is from 1974. Inspection of the SW and SN charts has allowed a reconstruction of most of the colour code.

Note that illuminated switches appear to all have ivory sliders, regardless of age and sensor type (just as they do in SN Series). This can be seen in a 101SW1 keyboard (possibly 101SW1-3-H or 101SW1-4-H).

White plungers in non-illuminated switches can be seen in an unidentified custom keyboard; frustratingly there is no photo that clearly shows the Micro Switch labels that would give the chassis model and manufacture date.

Plunger colour Era Colour usage
Grey ca. 1968–1970 Possibly all switches
White Uncertain Uncertain
Black Early to mid-1970s Possibly all switches
Mid-1970s onwards Alternate action and dummy switches; bi-level; additional types
Red Mid-1970s onwards Sink level
Pale red
Blue Mid-1970s onwards Sink pulse
Green Mid-1970s onwards Source level

Model interpretation

Precise details on the various switch types are rare, in particular the output type of each model. However, some of these details can be derived from existing information.

Keyboard 54SW11-14 uses 1SW17 switches for most keys and 1SW11 for the modifier keys, which indicates that 1SW17 (sloped) and 1SW18 (stepped) must be sink pulse, with 1SW11 being a level type (sink level or source level). According to the SW and SN charts, 101SN1B1 and 1SW17 both used sensor SD-10023 (replacing SW-11504), and 101SN1B1 is demonstrated to be pulse type by Ed Nisley, as expected from the “B” designation shared with SD Series switch part numbers. This confirms the expectation of 1SW17 and 1SW18 being sink pulse. 1SW Chart 12 indicates that 1SW17 and 1SW37 are both sink pulse (by way of using the same sensor), and implies that both types have blue plungers (much of the data was previously deleted as parts went obsolete). Switch 1SW18 is confirmed to be blue, and the sensor inside is marked “B” as expected.

Keyboard 59SW9-1 has primarily blue switches (which will be sink pulse) with red switches for the modifier and repeat keys, and a few unexplained black switches. The red switches will be a level type. Switches 1SW51 and 1SW52 are both red types, and each have sensors marked “A”. This again corresponds with SD Series designation and would make these sink level.

This leaves green plungers, and source level switches, with the suggestion that these two correspond. 1SW41 is documented as being green, and as having sensor SW-11897. A number of types also have this sensor, including 201SN1C1, 201SN1C2, 201SN4C1 and 201SN4C1. “C” in SD Series denotes source level, which appears to confirm that indeed the green switches are source level as anticipated.

Although it is not possible to be completely certain about the above details, the various details all appear to fit in a way that makes sense.


Switch model numbers appear to have started from 1SW1. 1SW1 uses a grey plunger, as shown in advertisements from 1968. The top of the plunger contains a circular hole, as also seen in some older Licon and Cortron switches, something seen in some later models. By around 1970, the plunger was redesigned (as illustrated in installation guide PK 8503 2), and it appears that the replacement models were given new numbers starting from 1SW11. 1SW1 is presently the only known original switch model.

The “R” suffix indicates a replacement switch sold separately in a box along with an instruction leaflet, return spring and two “spacers” (pry tools). The digits that follow the “R” indicate the weight in ounces of the spring supplied in the box and are only seen on “R” models. The exact spring weights are not yet determined, but taken at face value, “1.5” would be a half force switch (used together with a support switch on wide keys); “2” would be 2 oz semi-light, and “8” would be a heavy 8 oz switch. Testing with an R8 switch demonstrates that it is in the region of twice the weight of a normal switch, but proper testing is impossible without a spare mounting frame to hold the switches together.

Clearer details of the switches will be forthcoming, pending completion of collection and analysis of charts. The details below are a combination of the various charts together with the results of analysis and interpretation. The plunger colours listed are a combination of the documented colours (per the charts) and the observed colours.

A source of “↓” indicates that the switch is given in the keyboards table that follows the switches table.

Catalogue listing Action Output Plunger colour Plunger style Source NSN
1SW1 Momentary Grey Sloped Unspecified keyboard
1SW1-R Parts listings
1SW11 Momentary Source level Black, green Sloped 5930-00-524-0338
1SW11-R eBay (date 7409), 1SW Chart 1 5930-00-524-0338
1SW11-R1.5 1SW Chart 1
1SW12-R Stepped 1SW Chart 1, eBay (date 7703) 5930-01-141-2002
1SW12-BL Momentary tactile Black Stepped Deskthority
1SW13-R Alternate Black Sloped 1SW Chart 1, eBay (date 8121) 5930-01-039-3169
1SW14 NSN 5930-01-174-7525
1SW15-R Momentary Black Sloped 1SW Chart 1
1SW17 Momentary Sink pulse Black, blue Sloped 5930-01-046-3393
1SW17-R 1SW Chart 1 5930-01-046-3393
1SW17-R1.5 1SW Chart 1
1SW17-R2 eBay (date 9541)
1SW17-R8 eBay (date 8444)
1SW18 Stepped NSN 5930-01-337-6279
1SW18-R eBay 5930-01-155-5558
1SW19-R Momentary Blue Sloped 1SW Chart 1
1SW21-R Momentary Black Sloped 1SW Chart 1
1SW23-R Parts listings
1SW31-R Momentary Green Sloped 1SW Chart 1
1SW37-R Momentary Sink pulse Blue Sloped 1SW Chart 12
1SW41-R Momentary Green Sloped 1SW Chart 1
1SW43-R Momentary Red Sloped 1SW Chart 1
1SW45-R Momentary Green Sloped 1SW Chart 1
1SW51 Momentary Sink level Black, red/pale red Sloped NSN 5930-01-046-0767
1SW51-R 1SW Chart 1, eBay (date 8446), eBay (9511) 5930-01-036-5181
1SW51-R1.5 1SW Chart 1
1SW52 Stepped NSN 5930-01-337-6280
1SW52-R eBay (date 8742)
1SW52-R8 1SW Chart 1
1SW53-R Alternate Black Sloped 1SW Chart 1 5930-01-175-8352
1SW54-R Stepped 1SW Chart 1
1SW55-R R&J Components Corp
1SW71 Black Sloped stemless eBay (date 9412)
1SW101-R Momentary illuminated White Sloped 1SW100 Chart 1 5930-01-039-3167
1SW107 1SW100 Chart 3
1SW107-R 1SW100 Chart 3
1SW108-R Momentary, no lamp terminals 1SW100 Chart 3 5930-01-039-3168
1SW111-R Parts listings
1SW111-HR Momentary illuminated Logic scan White Stepped 1SW100 Chart 34
1SW113 Alternate action illuminated Sink level Sloped 1SW100 Chart 6
1SW117 Momentary, illuminated Sink pulse White Sloped 1SW100 Chart 5
1SW121 Momentary illuminated Logic scan White Sloped 1SW100 Chart 7 5930-01-354-7030
1SW151-R Indicator None 1SW100 Chart 4
1SW152-R 5930-01-039-3170
1SW181 Momentary illuminated Sink level White Sloped 1SW100 Chart 1
1SW181-R Momentary 1SW100 Chart 1, eBay
1SW201-R Double action (bi-level) Black Sloped Own collection
1SW204-R Double action (bi-level) Black Stepped Own collection
1SW300-R “Momentary” Black Stepped 1SW Chart 1
1SW301-R “Momentary” Black Sloped 1SW Chart 1



The model number schema for SW Series keyboards is as follows:


Number of keys (at least three, as “1” in this position denotes a single switch and “2” denotes a keycap)
Series name


There are at least two known subseries:

12SW Series
Current sinking non-encoded 12-station keypads (can be paired as a 24-station assembly)
16SW Series
Current sinking non-encoded 16-station keypads (can be paired as a 32-station assembly)


For more details, see Documentation below.

Model Description Output Rollover Lock Keycaps Source
50SW11-50 Keypunch Keyboard System 3 Code NKRO Sloped Product Brochure SW (373)
51SW5-1 Key-to-Tape/Disc Keyboard EBCDIC 2KRO Sloped Product Brochure SW (373)
51SW12-1 Typewriter Keyboard 6-bit address code NKRO Secretary shift lock Sculptured Product Brochure SW (373)
53SW1-2 Teleprinter Keyboard US ASCII 2KRO Sloped Product Brochure SW (373)
61SW12-1 Communications Keyboard Full US ASCII NKRO Electronic with LED Sculptured Product Brochure SW (373)
63SW5-4 Teleprinter Keyboard Full US ASCII 2KRO Alternate action Sloped Product Brochure SW (373)
70SW12-1 Remote Batch Keyboard US ASCII NKRO ? Sculptured Product Brochure SW (373)
75SW12-2 Communications Keyboard US ASCII NKRO Alternate action Sculptured Product Brochure SW (373)


The following is not a complete list of all known SW keyboards. It only lists keyboards where at least one switch model is shown, or where it is particularly early or late production. In almost all instances of SW keyboards found online, the switch part numbers are not all shown, or are not shown at all (except for where it seems the switches were not marked in the factory).

Keyboard Micro Switch model Switches Date code Notes Reference
Bare assembly K50326-62SW3 1SW1; possibly others 6933 Sketchfab
Bare assembly 64SW1-4 7012 Earliest example of black plungers Flickr
Hitachi keyboard 60SW5230-173 1SW11, …? 3-74 Manufactured by Micro Switch Yamatake-Honeywell, Tokyo Deskthority
Decision Data 8010 54SW11-14 1SW17 (sink pulse), 1SW11 (source level) 74/34 Flickr
Texas Instruments 914 keyboard 102SW11-1 75/02 A rare example of SW illuminated within a keyboard Deskthority
Custom keyboard Unreadable 1SW17 (sink pulse), 1SW51, … ca. 1980 Deskthority
Honeywell TDC keyboard assembly 101SW1-4-H Sink pulse, illuminated, … 9546 NRI Industrial Sales


Keycaps for SW, SN and SD switches all come under 2SW. Part Target list 326 separate entries for keycaps, and this list appears to be very far from complete, as presumably every combination of legend and colour has its own part number and, where necessary, its own NATO Stock Number (NSN). There is no single catalogue listing format for keycaps; the catalogue listing numbers are quite varied in their format, and at times very long and complicated (e.g. “2SW1-4C-M-4-2-1598C”). In Part Target’s search result, 2SW1 is associated with around 26 different NSNs.

Standard 2SW keycaps are 0.475″ tall.

The catalogue listing numbers were largely not depicted and varied in formatting; thus, they have been normalised according to the Part Target listings for consistency.

Cat. listing Type Size Mount Colour Legend NSN Reference
2SW1-M-4-2-4928 Double-shot 1 unit SW Green PAGE UP 5930-01-213-1690 eBay
2SW1-M-5-2-95 Double-shot 1 unit SW Black N / SO 5930-01-212-5854 eBay
2SW1-M-7-2-282 Double-shot 1 unit SW Black 4 / $ 5930-01-212-2998 eBay
2SW3-9 Blank 1 unit ? ? eBay
2SW603-11 Relegendable 1 unit ? Beige eBay
2SW701-04D-N Illuminated 1 unit SW illuminated Green eBay
2SW79105 Blank 2 unit SW Black Deskthority

Other components

Cat. listing Component
500SW90-3 Sealed keyboard plunger (eBay); these can be seen in a 26SW3-3-S keypad
500SW90-5 Uncertain (NSN 5930-01-368-9087), allegedly a pushbutton switch
SW-10485 Keycap puller; mentioned in the Fairlight CMI System Service Manual (for the CMI IIx)
SW-11485 Keycap puller; depicted in PK 8919 2 for use with SD Series


When fitting a replacement switch module, or returning a module to the keyboard, the return spring needs to be attached or reattached to the plunger. PK 8503 2 instructs repair technicians to “place the spring on the spring boss and compress and rotate it until the last coil of the spring expands around the boss”. For those who instinctively try to rotate the spring clockwise, note that in fact the spring must be rotated anti-clockwise. The leaflet recommends a pencil eraser for the task, but bare fingers are sufficient. Only a small amount of rotation is required for the spring to remain attached to the plunger, although this process can be a bit finicky.


Honeywell have kindly provided a collection of SW series switch charts. Only a subset of the charts appear to have survived. The charts themselves were extensively redacted by Honeywell as switches went end-of-live, but they do have some older versions of the charts (as is the case with 1SW Series Chart 1), and with luck, they may find some more older revisions with more switch details.

Product Brochure SW “Solid State Keyboards” and Product Sheet 51SW12-1 “Typewriter Keyboard” are in item SILNMAHTL_31166 at the National Museum of American History Library (part of the Smithsonian Institution), who kindly provided scanned copies of both documents. The dates for these documents are derived from the three-digit codes placed at the bottom of the last page of each. Product Brochure SW—seemingly from March 1973—is mentioned on Electronic Design 17, page 118, from the 16th of August, 1973.

See also