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Micro Switch SW and SN Series



SW and SN are Micro Switch’s original series of Hall effect keyboards and switches.

SW Series switches are open at the bottom, and snap into special mounting rails fitted to the keyboard. The basic switches do not provide a spring; when sold as replacement parts, the spring is included.

SN Series switches are divided into two types: PCB mount and snap-in panel mount. Both types are fitted with a base and include a suitable spring. 11SN Series snap-in panel mount switches have an integrated panel mount assembly. 101SN and 201SN PCB-mount switches have a pair of self-adhesive strips on the bottom, which allow them to be affixed to the PCB prior to soldering.

All three designs share the same shell, and most components are within SW Series. SN Series contains the SN switches along with all components specific to SN switches, such as the quick-connect terminals, bases and panel mount frames.

View full-size image 1SW (left) with open base, 101SN (centre) with self-adhesive strips and 11SN (right) with panel mounting assembly and quick-connect terminals

The only difference between 1SW switches and 101SN and 201SN switches is the addition of a base. The base attaches with a pair of clips; following this, it is ultrasonically welded.

View full-size image 1SW (left) with open base and 201SN (illuminated, right) with base removed; white colouration shows where the ultrasonic welds were severed when the base was removed (note that the base is warped from shipping damage)

In most instances (including all of SN Series) each switch is stamped with its catalogue listing (part number). Some switches (likely those made in the 1980s onwards) are also stamped with the manufacture date. According to the Micro Switch charts, there is a suggestion that the internal-use SW switches (for assembly in-house into keyboards) were not marked with the catalogue listing, which may have only been performed on replacement parts. (The charts list various exclusions for catalogue listing stamping, but not the reason, and the extensive level of deletion of data from the charts obscures the specifics of this process.)


In late 1968, Micro Switch introduced SSK, the world’s first solid-state keyboard, and reportedly also the first practical use of Hall effect, which had gone unused since its discovery decades prior. A variety of illustrations of the SSK keyboards and switches can be seen in Information Display, Volume 5 Number 6 from November/December 1968 (pages 35–38, PDF pages 18 and 19). The product range was also announced in Computers and Automation, November 1968 (page 54). Everett Vorthmann of Micro Switch and Joseph Maupin of Honeywell are given as the inventors of the sensor chip, as per US patent 3596114 filed on the 25th of November 1969. The expected cost of a keyboard was $100 (over $700 in 2019).

In Computerworld, May 10, 1972 on page 28, Micro Switch announced that they would be demonstrating the keyboards and switches at the Spring Joint Computer Conference in Atlantic City, New Jersey (15th to 19th of May 1972). This is the earliest mention to date of 101SN and 201SN switches. As such, it is not clear whether SN was conceived and introduced at the same time as SW, or shortly afterwards. SW must have been marginally earlier, as SN uses SW part numbers for all parts that it shares with SW, including the sensors, plungers, springs and shell. A single keyboard model, 75SW12, is also mentioned in the magazine, as well as SS series general purpose solid-state switches including the then-new 15SS current sink type.

The SW design was only prominent for a few years. In 1975, Micro Switch had superseded it in keyboard applications by SD Series. SD series took the more common form of self-contained modules that sit in a punched metal mounting plate. SD required all new keyboard designs, but continued to use SW keycaps.

The last known existence of SN Series is 1999. On the 22nd of February 1999, Honeywell’s Hall-Effect Keyboards page on their website still advertised SN and SD Series. Customer drawings provided by Honeywell show revision dates of 15th of December 1999 for 1SW Series switches and 14th of April 1999 for 201SN Series switches. Honeywell appear to have stopped advertising SW Series keyboards by this time, presumably has they had been long supplanted by SD Series.

No data exists on either series beyond 1999; Honeywell themselves report that all these switches were made obsolete in the 1990s.


SW Series was previously referred to in the keyboard enthusiast community as “First Generation Dual Magnet Honeywell Hall Effect”. As the name indicates, these modules have a pair of magnets, instead of the single magnet normally associated with Hall effect and ferrite core switches. One magnet passes in front of the sensor, and one passes behind. The magnets are made from barium ferrite–filled PVC, and are fairly large, and much larger than the tiny magnets in RAFI full-travel switches from the mid-70s. (SD Series and RAFI Hall effect are around the same age, but Micro Switch continued to use fairly large magnets.)

Switch modules were available in sloped and stepped profiles. Stepped switches use a 13° plunger angle. Momentary and alternate action were provided, as well as illumination.

The range of output options for 201SN includes sink level, sink pulse, source level and logic scan according to the charts. The corresponding details for 1SW are not yet confirmed. Testing by Ed Nisley shows that type B sink pulse sensors generate 50 µs output pulses upon detecting a magnet, and accept either north or south magnetic poles; the stated range in SN Series specifications is 10–100 µs.

The later SD Series introduced its own Hall sensors, which were smaller than the SW sensors, and consisted of a bare board. Many models of SW and SN were swapped over to using SD sensors. The terminal spacing of these two families is not quite the same.

11SN Series snap-in switches feature quick connect terminals for the Hall sensor and (in illuminated models) the lamp. All other models have PCB terminals. When various models changed from using SW sensors to SD sensors, the drawings were updated, and all affected types were redrawn with quick connect terminals. This appears to be an error on Micro Switch’s part, and the bills of materials for subseries other than 11SN do not include the quick connect terminals. No switches other than 11SN have been observed with quick connect terminals.


The SW sensor die itself is 0.040″ square. The circuit pathways are large enough that they can be made out with a 10× illuminated loupe. Ed Nisley has taken photographs of a 201SN1B1 sensor. The large diamond area in the photographs does not appear on the sensor of a 1SW51-R switch.

Micro Switch SW sensor chips Two 1SW51-R sensors, and sensors from 1SW204-R and 1SW201-R
Micro Switch SW sensor chips SW (left) and SD (right) sensors; SW and SN switches would later be converted to using SD sensors

Sensor identification

The back of each sensor is marked with a code. The codes are not understood, but in some instances the final character is a letter which appears to indicate the type of sensor, using the same mapping as for SD Series sensors.

Micro Switch SW sensor chips 1SW51-R “A” sink level sensor (left) and 201SN1B1 “B” sink pulse sensor (right)

A 1SW17 switch shown on the Deskthority wiki has a sensor marked “N”; this model seems to be sink pulse, meaning that the sensor should be marked “B”. However, there may be more than one type of sink pulse sensor. A 1SW51-R listed on eBay has a sensor marked “J59”, which does not appear to indicate the sensor type at all.


Micro Switch were unusual in that they did not cite a rated lifetime for their solid-state switches. In Product Brochure SW from 1973, they describe how 90 switches had accumulated 20 billion cycles of testing. To achieve 20 billion cycles per switch within five years would require around 125 keystrokes per second (20,000,000,000 ÷ (365.25 × 86400 × 5)), which is possible but seems unlikely. Footage of Cherry’s MX test rig shows their testing to run at somewhere around 10–15 cycles per second. It seems more likely that the 20 billion figure indicates the total number of test keystrokes across all 90 switches.

Keycap mount

SW and SN switches have at least two key mounts. Non-illuminated switches use the standard Micro Switch mount, illustrated below:

Illuminated switches have a hollow, tubular plunger. The lamp is placed into a socket in the centre of the switch, and the two separate halves of the plunger rise up around it. Illuminated keycaps appear to attach to the plunger using a metal fitting that attaches to the sides of the keycap, as seen in an eBay listing for keycap part 2SW701-04D-N.


Illuminated SN switches could be ordered with or without a lamp. In contrast it seems that no SW types came with a lamp fitted, but this is not confirmed at present. The lamp is a 115 mA T-1 bi-pin type, Micro Switch part SW-10569. Micro Switch cited alternative models of Chicago Miniature CM-7-7715 and CIMCO International OL 718BP in the documentation for their illuminated switches.


Indicator switches are fitted with spring SW-10666, with a total travel force cited as 33 oz (around 936 grams). Ramon Cases Jové has measured that it takes around 1 kg to fully press the plunger, thus verifying that indicator switches are not immobile and can be fully depressed albeit with a huge amount of force.


The table below is taken from Product Brochure SW. While it can only be directly associated with SW Series, the parts in SN Series are the same, so the specifications will likewise be the same.

Characteristic Momentary Alternate action
Total travel 0.187″ (4.75 mm) nominal 0.250″ (6.35 mm) nominal
Force at operating point 3 oz (85 g) nominal 4.5 oz (128 g) nominal
Pretravel 0.100″ (2.54 mm) nominal 0.100″ (2.54 mm) nominal (latch at 0.165″/4.19 mm nominal)
Release point 0.040″ (1.02 mm) nominal

Charts for the individual springs give two compression lengths and their corresponding forces. Typically, these appear to be 0.381″ and 0.194″, which is a range of 0.187″, matching the officially-documented switch travel. Taking the operating force to be (0.1 ÷ 0.187) × (end_forcestart_force) + start_force, a standard switch comes out at 2.6 oz, or 74 g, which is distinctly short of the cited force of 3 oz (85 g). However, 2.6 oz is a more practical figure for typing purposes, and examination of a standard SN switch shows it to be around the same force as a 2.5 oz SD Series switch.

In terms of tolerance, the operating point is between 0.060″ (1.5 mm) and 0.150″ (3.8 mm) for momentary switches, and between 0.060″ and 0.120″ (3.0 mm) for alternate action switches; these can also be expressed as 2.65±1.15 and 2.25±0.75 mm respectively. How the tolerance differs so noticeably between the types is rather curious. In both cases, the reset point must be at least 0.040″ (1.0 mm); the Hall sensors provide hysteresis.


Avoid excessive heating of the terminals when soldering and de-soldering switches, as this risks damaging the sensor. For SW and SN Series, Micro Switch recommended a 1/8″ diameter thermostatically-controlled tip set to 500 °F (260 °C), with heat applied for no more than 10 seconds.

See also