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Micro Switch SW and SN Series

Contents

Overview

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.)

History

In late 1968, Micro Switch introduced SSK, the world’s first solid-state keyboard. The switches in this keyboard use what Honeywell report in Hall Effect Sensing and Application to be the first instance of a complete Hall sensor on a single chip. 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).

It’s not clear whether SN was conceived and introduced at the same time as SW, or shortly afterwards. What is known is that SN uses SW part numbers for all parts that it shares with SW, including the sensors, plungers, springs and shell. A 1SN product sheet was advertised in Electronics magazine in March 1970, but not seen; 1SN is described as “developed for keyboards.” This mention of the product sheet is presently the earliest-known mention of SN Serires.

The SW design was only prominent for a few years. By 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.

Reliability

Hall effect keyboards are highly regarded for their robustness and longevity. However, when they were first introduced, the reputation of Micro Switch Hall effect keyboards across the industry faltered, with numerous customers reporting problems their purchases. These problems were mentioned in a news item “Hall keyboards having problems” in the Electronics Newsletter section of Electronics magazine, 13th October 1969 (see Documentation below). Alleged problems include temperature sensitivity, random failure of the strobe signal, intermittent or short circuits, the linking cables between the switch and encoding boards and even the PCBs themselves. Honeywell at the time denied all of these allegations. In the 10th November issue, Micro Switch product manager E C Leibig pointed the blame at the way that customers were using the keyboards, including improper supply voltage (see Users’ choice is name of keyboard game). The article goes on to note, “However, the consensus throughout the industry is that the Hall-effect keyboards do not live up to all the hoopla that preceded them. In fact, several large computer console manufacturers, including Honeywell's computer division, are sticking with the older Micro Switch reed keyboard.”

Of all the complaints levelled at the keyboards, the only one relating specifically to the Hall sensing process was temperature sensitivity. Micro Switch’s rise to the top of the keyboard industry in the early 70s suggests that these problems were resolved in time. In Focus on Keyboards (Electronic Design, 9th November 1972) Micro Switch was described as the “largest supplier of keyboards”, a position that appears to have shifted in time to Key Tronic.

Characteristics

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.

Sensor

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.

Pinout

The pinout for the SW Series A, B and C sensors (sink level, sink pulse and source level respectively) is, from the perspective of the legend side (with the IC window facing away from you), VCC, output 1, output 2 and GND. SN Series switches come with the terminals labelled, as “+V”, “1”, “2” and “GND” on the base of the switch, but SW Series switches have no base and the terminals are thus not marked anywhere.

Longevity

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.

Switch output

SW and SN switches were available with a number of output behaviours.

The sink level, sink pulse and source level switches have dual isolated outputs. Each of the two outputs is connected to a transistor that is controlled by the Hall transceiver circuitry. The separate transistors isolate the outputs: current cannot flow between the output terminals of an inactive switch. As with double-pole switches, this arrangement is highly suited to two-of-N encoding, which was Micro Switch’s chosen encoding method for their keyboards. In January 1973, Micro Switch posted the following advertisement in Electronic Design magazine (vol. 21, no. 1, page 136):

The solid-state 12-key (12SW) and 16-key (16SW) keyboards have current-sinking outputs for compatibility with TTL and DTL. The keyboards permit greater fan out, since each key switch offers two isolated outputs rated at 3.2 mA (outputs may be wired in parallel to sink 6.4 mA). Both keyboards are available with either level or pulsed outputs. The small Hall-effect keyboards have been designed for end-to-end mounting, and have standard 3/4-in. key spacing.

All known switch models used the same lead frame dimensions and layout; consequently, all known models have exactly four terminals regardless of behaviour. Double action and logic scan switches did not offer isolated outputs.

SW Series was followed by SD Series, which continued all the same output types as the SW sensors.

The diagrams below are essentially those provided by Micro Switch, and represent the output behaviour of the types indicated.

Sink level

Sink level switches connect the “output” lines to ground via transistors controlled by the Hall transceiver circuit. The term “output” reflects the signalling function rather than the direction of current flow (which is into, rather than out of, the output terminals). The outputs remain active while the switch is held. Encoding clashes are detected via an Electrical Monitor Detector (parallel resistance measurement to ensure only the correct number of encoder lines are active) or via MOS logic.

Sink pulse

Sink pulse is largely the same as sink level, but the outputs are enabled only for a period of between 10 and 100 µs. For keyboard purposes, pulse output prevents encoding clashes: when two keys are struck in rapid succession, the first switch has returned to idle before the second has actuated. This behaviour was also offered with PB switches and appears to serve other purposes outside of keyboards. Pulse output was Micro Switch’s approach to N-key rollover, a term that they are alleged to have coined.

Source level

Source level switches feed the transistors from the +5 V line (VCC) instead of grounding them: the output terminals supply 5 V when the switch is active. Current sourcing sensors are only known to be available in level type.

Logic scan

Logic scan switches are designed for matrix scan keyboards. The single output transistor can be enabled and disabled via the input line; this allows only a single row of switches to be active at a time, without needing to power the switches up and down. Dual isolated outputs are not a requirement for matrix keyboards, and a single output suffices here. It is certainly possible to toggle the power to switches one row at a time, and it’s not clear why Micro Switch opted instead to toggle only the output transistor. However, in the interests of reducing power consumption, their stance would later change.

Logic scan switches are current sinking.

Timed repeat

Timed repeat is extremely rare in SW Series keyboards. Contrary to the name, the sensor itself does not contain any repeat behaviour. Instead, timed repeat is an adaptation of the sink pulse type where one output sits at an intermediate voltage until the key is released. Additional circuitry within the keyboard detects an active repeat key and cycles the strobe signal, based around Micro Switch’s custom SW-10667 chip. This can be seen in an 84SW12-2 keyboard and at least one unidentified Diablo terminal keyboard. The use of external timer circuitry allows the trigger interval and repeat period to be customised arbitrarily without increasing the complexity of the sensor chip itself.

Whether there was ever a call for an SN Series timed repeat switch is not presently known.

Bi-level

Bi-level (double-action) switches have two travel distances, for two actuation levels. A high operating force separates the two levels. Each output line is connected to a separate transducer. Double-action switches are almost exclusively found in electronic typewriters for a small number of specific keys—typically space, return and backspace—and thus there is no need to provide dual outputs for each level.

The output from such a sensor has never been measured or found illustrated. The output could be as depicted in the diagram below, although this is by no means certain:

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.

Lamps

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.

Indicators

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.

Specifications

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.

Maintenance

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.

Documentation

See also