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Micro Switch SD Series



SD Series is Micro Switch’s second generation of Hall effect keyboards and keyboard switches, that followed on from SW Series. The series provided a wide range of standard and customer-specific options including multiple keycap mounts, plunger-mounted illumination, alternate action, secretarial shift and tactile feedback, as well as a comprehensive range of Hall effect circuit options. This page covers only the keyswitch modules. Honeywell were kindly able to locate a series of charts that have shed a huge amount of light on the series, so many thanks go to them for making this possible.

Components in SD series have part numbers in the form SD-nnnnn. Switches fall under subseries 1001SD, while keyboards use a prefix indicating the number of keys. For example, 12-key keypads will have a catalogue listing number beginning 12SD, while model 125SD12-1 indicates that it has 125 keys.

1SW switches were clip mounted into metal rails, while 101SN and 201SN switches were affixed using self-adhesive strips prior to soldering. 1001SD switches are primarily plate mount, but a hole in the centre allows them to be secured to the PCB from below using a screw.

Just as with 1SW switches, the Hall sensor module can be pulled out of the switch from below. This allows the plunger assembly to be swapped in the field without needing to desolder the Hall sensor. While SW Series sensors were fully-encosed plastic packages, SD Series sensors are a bare substrate (possibly a printed circuit board) with three or four terminals soldered to it, and a Hall sensor enclosed in some kind of resin. The Hall sensor faces away from the magnet.

Each switch is marked with an arrow; with stepped switches, this arrow faces the front of the switch. This is not to be confused with the arrow on Clare/Pendar reed switches, where the arrow seems to point to the terminals for the primary switch contacts, which can be either at the front (short reed switches) or the back (tall reed switches). The terminals in SD Series switches are on the left-hand side.

While SW keyboards were designed for 2-of-n encoding using dual-output sensors, SD keyboards are more commonly matrix scanned using single-output sensors. SW was designed to avoid the expense and complexity of matrix scanning, but by the time SD was introduced, matrix scanning was a normal approach.


From the drawings provided by Honeywell, SD Series dates back to 1975. The exact date of introduction is not known, but model 125SD12-1 (made for Univac) has been found from 1976, giving us a low water mark. The series is known to have been in production until at least 1999, when the Honeywell website is last demonstrated to have listed them; beyond this point, the Wayback Machine was unable to gather data on them.

Prior to discovery of the series name, these switches were referred to by keyboard enthusiasts as “Honeywell Hall Effect”. Although Micro Switch was already owned by Honeywell before the series was instigated, these switches are all branded “MICRO USA” and all the charts are from Micro Switch.

Options overview

There are over 60 different switch models identified to date, based on charts, discovered keyboards and surplus part listings on the Internet. The number of possible combinations of options exceeds a thousand, but most of the possible variations were likely never produced.


Tactile feedback uses a mechanism very similar to that of the alternate action switch. This was crudely illustrated in Fujitsu magazine, volume 27 number 7, November 1976 on page 1299:

The above illustration has been cleaned up a little and coloured; the truly bizarre keycap has however been left as-is! A small pin suspended on a spring projects outwards from the plunger. As the plunger is depressed, this pin encounters an obstruction; compressing the pin’s spring to clear the obstacle increases the stiffness of the switch momentarily. Curiously, the drawing was published in November 1976, not long after the SD8 chart in September 1976.

Disassembly of model 1001SD11A1A shows that the plastic pin is 3.7 mm long and 1.6 mm in diameter (1.2 mm diameter at the end that engages with the shell), and the spring supporting it is 3.1 mm long and 1.55 mm in diameter. The “A” chart for the alternate action type shows that this uses a sprung follower pin, very similar to that of RAFI full-travel keyswitches, and it may well be that the alternate action provided inspiration for the tactile design. Hi-Tek Series 725 is similar, in that the click mechanism appears to be based on the alternate action design. Tactile SD switches do produce a faint click when released, caused by the pin being pushed outwards rapidly.

1001SD Series Chart 13 A shows that SD18 switches still use the exact same mechanism.

Tactile SD switches make use of the lower return spring weights, using the 1.3 oz spring for normal keys, and the 2.0 oz spring for space bars. Overlaid onto this is the tactile feedback, which provides switches with their normal operating force.

The force curves for standard weight tactile switches are given in 1001SD Series charts 8 and 9. Curiously, stepped switches have a higher tactile force than sloped switches.

As indicated by the force curves, the tactility is pronounced, with a clear lead-in period. Testing with two 11A1A standard weight tactile switches does reveal a significant variation in force, with one being much easier to press than the other; the stiffer of the two feels like it borders on the edge of practicality, while the other feels fairly soft. The tactile feel is not that dissimilar from the sample tactile Datanetics DC-60 switch from Meryl Miller, but not as smooth.

1001SD Chart 10 gives the tactile forces for the soft tactile switches, but does not depict the force curves. While the SD16 catalogue entry shows the force curves for the linear types, the graph is substantially at odds with the published figures. The published figures correspond much more closely with the tactile graphs than the linear graphs do. Thus, the following hypothetical graph for the soft tactile switches assumes that the underlying curve is the same as for standard weight tactile switches, and that the tactile forces are all that differs. Disassembly of the standard weight tactile switches suggests that this is true, because the tactile peak magnitude is set using a separate, perpendicular spring and should thus be entirely separate from the main force.


There are at least two designs of illuminated switch. Older switches have the LED placed directly into the plunger. To make space for this, the normally square section keystem is replaced with a “blade” stem similar to the “straight” stem of Cherry M5/M6/M7 switches. On these switches, the LED moves with the keycap, providing a constant level of illumination. This approach was taken with several other brands including Alps (with KCC series) and Omron (with B2H, B2R and B2C).

The 16B3E switches found in the Sun 32SD38-4-E keypad instead have the LED located in the body of the switch. This approach is cheaper and more robust, but has the disadvantage that the light level differs as the key is pressed and released. The chart for SD16 and SD18 does not depict the LED placement, yet there is nothing about the identification code of 16B3E that would indicate that it is anything but a standard switch per the chart. All switches have a vertical recess in that position, and it is possible that the LED is enclosed in a carrier that is pressed into that space.

This corner illumination can also be seen in Sperry/Univac keyboard. There is again some kind of extra plastic on the far side of the LED, but there are no close-up photos of the LED switch.

Sensor output

German-made Hall effect keyboard switches from RAFI, RFT and possibly also Sasse all used a single Hall sensor model. Older RFT TSH 19 switches used a TESLA MH1SS1 Hall sensor, copied from an SW Series sensor. RFH TSH 19/F switches, and all of RAFI’s RS 74 C and RS 76 C switches, use VEB HFO B 461 G Hall sensors. B 461 G are four-terminal with an enable line for matrix scanning.

By comparison, Micro Switch offered a variety of output options. Details on sensing modes are not at all clear to anyone who does not either understand electronics or have a test bench and parts to play with.

Sink level
Sink level works like a normal pushbutton switch: current is passed while the switch is actuated. The “output” terminal is a current sink: current passes into the output during actuation. The output is held high while the switch is released, and goes low when the key is pressed, allowing current to flow into the sensor as though it were a regular switch. The Hall sensor has dual isolated outputs.
Sink pulse
Sink pulse is similar to sink level, but current is only passed momentarily. Switch actuation is signalled, but release is not. The Hall sensor has dual isolated outputs.
Source level
This appears to be the reverse of sink level, in that current flows out from the sensor while the key is held. The Hall sensor presumably has dual isolated outputs.
Timed repeat
No documentation has been found for these. These have been sighted in keyboards, but at present there are no documented examples.
Logic scan
Logic scan switches provide the ability to use SD switches in a matrix with a scanning encoder. One of the output terminals is replaced with an input terminal; when this terminal is low, the output is active. The separate input and output terminals allow a switch to bridge a matrix row and column, allowing it to be identified within a matrix. Logic scan switches are sink level when active.
Three-terminal switches date to around 1979 and are a special adaptation of the logic scan type that is designed to greatly reduce the power consumption of a keyboard. They function either as source level or logic scan. The ground terminal is renamed “input”, and when the input is high or disconnected, there is no potential difference across the switch and it is inoperative. When the input is pulled low, the switch powers up and the output reflects the key state (low when inactive, high when active). If input is hard-wired to ground, the switch functions as a source level type. When input and output are connected into a matrix, the switches are only powered up when their row is active. A standard logic scan switch requires 3.5 mA current when idle, and for 100 switches there would be a total of 350 mA of idle current. The ability for switches in active matrix rows to be powered off saves a considerable amount of power. There is only one output on these sensors, as they are targeted at matrix keyboards, while the original sensors are designed for 2-of-n wired encoding which requires dual outputs per switch.

The Focus on Keyboards article in Electronic Design magazine from 24th May 1979 notes the following:

“Keyboards featuring a Hall-effect device, magnet and plunger in each keyswitch position (Fig. 1) are available from Honeywell’s Microswitch Division, which recently introduced a three-terminal Hall-effect module that helps reduce power consumption by a reported 60%. The module’s “scan” input shares a pin with the negative power supply, so deselected modules draw no current. An on-board µP activates only eight keys at a time. The modules themselves come individually or in completely built, intelligent keyboards.”

Although figure 1 depicts SW Series, SD Series is implied here (as three-terminal SW is not known), and this gives a rough idea of when the three-terminal types were introduced.

View full-size image Micro Switch sink level (A), sink pulse (B) and three-terminal (E) Hall sensors (the last one has seen better days)

Secretarial shift

Secretarial shift was included in the series. The exact nature of the mechanism is not documented. Micro Switch did not find an “A” chart for SD7 or SD10, and the “M” charts omit much of the mechanism. Deepak Kandepet sent these preliminary photos of 10B3S, showing much more of the mechanism:

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Left side
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The spring-loaded sliding bar appears to be what locks the plunger in position. The linkage to the left shift key is not clear, nor is it clear whether right shift can be connected. (At least one Cherry-made keyboard has only the left shift key connected to shift lock.)

Product groupings

Product and part numbers are divided into two main groupings. Components use catalogue listings of the form “SD-” 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 “SD” is prefixed by a number.

Grouping Usage
1SD… Found on a single switch; unexplained
2SD… Keycaps, possibly SD-specific (normal keycaps are 2SW series)
1001SD… Switches
Other nSD… Keyboards, where n denotes the number of keys
SD-nnnnn Components


Part schema

The part number schema for SD Series switches in most cases is as follows:


Prefix for keyswitch modules
Series name
This prefix letter is found on some switches and it indicates the variant of the switch, including Hall sensor variants. See switch variants below.
Nominally this field denotes the action, but each action has a separate code within each subseries. The choice of linear versus tactile is also encoded in this position. See action below. The options are: momentary (linear, tactile and reduced-force tactile), alternate action, support, secretarial shift and illuminated.
Plunger type: A = sloped, B = stepped, C/D/H = custom blade types for NCR, F = flat top, G = flat top, K = special blade type, N = flat stepped, S = sloped (variant), T = stepped (variant), V = unknown (stepped); see plunger types below.
Nominal operating force (at pretravel); see operating force below.
Output: A = sink level, B = sink pulse, C = source level, D = none (dummy), E = three-terminal, K = timed repeat, S = logic scan

Abbreviated identification codes

Some keyboards have abbreviated identification codes marked on the switches. Normally, the identification code omits only the constant characters (“1001SD”), but in some cases the identification code is the last two characters of the model number, followed by a letter of unspecified meaning. These codes are therefore ambiguous. For example, 1001SD chart 7 gives both 1001SD1C3A (momentary) and 1001SD2C3A (alternate action) as having identification code “3A S”. All the switches in chart 7 are custom types produced for NCR.

Codes 1AS, 1DS, 3AS, 3BS, 6AS and N3AS also appear on a 1976 Univac keyboard (125SD12-1), which are all standard switches (plunger type B or T). The switches in 125SD12-1 all appear to be stepped, while 1001SD chart 7 lists sloped switches; both sets of switches have “S” as the final letter. No meaning is ascribed to that letter.


Model 1001SD1C1A also exists as a replacement part with model 1SD1C1A-R. This part number is in the general format used with SW switches: a prefix of “1” (single key) and a suffix of “-R” for replacement. The year of manufacture is not included. No explanation is known for this anomaly.


The only subseries for which documentation has been recovered, is SD16, which is officially documented as “SD16 Keyswitch Modules” within SD Series. Other variants are named in this pattern here for convenience.

SD1–3, SD7–9

These are standard keyboard switches. The distance between the top of the mounting plate and the top of the plunger body is 10 mm (0.395″). These all appear to have clear plungers. Only a single tactile weighting is known to be offered (SD8).

SD4–6, SD10–13

These switches differ from the above types by having a plunger body that extends an extra 0.05″ in height to 11.3 mm (0.445″). The plunger is black in all observed models. Additionally, there is a lower-weight tactile model (SD13) in addition to standard weight tactile (SD11).

These are seen with additional ridges and bumps on the keystem to afford greater grip on the keycap. It is not clear whether this is specific to these taller versions as insufficient data exists to be sure; photos of SD1–3 are not clear enough to be sure that they do not have this characteristic.

Where SD1 has plunger is type S or T, the shell design is slightly different, with a ridge around the plunger aperture; this ridge seems to be the 0.05″ taller bearing surface described in chart 12 A.

SD16 and SD18

SD16 switches are described in the SD16 Keyswitch Modules catalogue excerpt. These take CT Series keycaps (as found on ST and SC keyboards); this may be a way to reduce the height of SD Series keyboards. SD16 is momentary, and SD18 is the corresponding tactile version. Sadly, the rated lifetime was not included in the specifications. All SD16 and SD18 switches are stepped, but with a reduced tilt angle of 4° (not documented; derived from the drawing) versus the 11° tilt of earlier models.

These switches appear in the Sun 32SD38-4-E keypad, made in 1996; the specific switch used is 1001SD16B3E (marked 16B3E).

SD16 was offered in both three and four-terminal variants, just as were some other models. Chart 13 covers SD16 and SD18, and they are depicted as four-terminal; output type E is not included.

Documentation for the tactile force of SD18 has not been recovered yet.


The charts from Micro Switch cite all of the following possibilities:

Action Plunger type Operating force Output
1, 2, 3 A, B, C, F, G 1, 2, 3, 4, 5, 6, 8 A, B, C, D, K, S
1, 2 K 1, 2, 3, 4, 6, 8 A, B, C, D, K, S
4, 5, 6 A, B, G, H, S, T 1, 2, 3, 4, 5, 6, 8 A, B, C, D, K, S
7, 10 A, B 1, 2, 3, 6, 8 A, C, K, S
8 A, B 1, 2 A, B, C, D, K, S
9, 12 A, B 3 A, B, C, D, K, S
11 A, B, G 1, 2 A, B, C, D, K, S
13 A, B 1, 2 A, B, C, D, K, S
16, 18 B 1, 2, 3, 4, 5, 6, 8 A, B, C, D, K, S

Additional types exist for which documentation has yet to be recovered. Three-terminal types are not included in any of the charts.


Schema position: 1001SD(V)APFO

The following items are arranged in subseries order:

Code Action/type Subseries Colour Chart date
1 Momentary, linear Standard Clear 1975-09-25
2 Alternate action
3 Support (not observed)
7 Secretarial shift 1975-09-24
8 Momentary, tactile 1976-09-20
9 Illuminated Clear 1975-09-23
4 Momentary, linear 0.05″ taller plunger Black 1975-09-25
5 Alternate action
6 Support
10 Secretarial shift 1976-01-13
11 Momentary, tactile 1976-10-07
12 Illuminated 1976-03-11
13 Momentary, tactile, lower tactile force (not observed) 1978-02-03
16 Momentary, linear Slot mount, lower profile Black 1985-12-04
18 Momentary, tactile (not observed)

The charts for SD1–3 and SD4–6 have the same drawn and checked dates. This may explain why the plunger dimensions differ but the shapes are the same (the extra 0.05″ plunger height is not shown), yet this makes it even harder to understand why that extra 0.05″ was offered, since it was clearly not a later revision.

Plunger types

Schema position: 1001SD(V)APFO

Plunger type Name Description
A Sloped This indicates that the standard keycap mount is used (cuboid in most cases, blade for illuminated switches, and slot for SD16 and SD18), and that the mount is upright.
B Stepped This indicates that the standard keycap mount is used, and that it has an 11° tilt. Perhaps to maintain proper clearance for the keycap, the base of the keystem is 1.83 mm (0.72″) higher than on a sloped switch. Strangely, stepped switches with tactile feedback use 5 gf extra tactile force than their sloped counterparts. Note that SD16 and SD18 have only around 4° of tilt, and are slot mount.
C Special (NCR) This looks like the same keystem used on integral illuminated switches, as though NCR wanted to use a single keystem for all switches on a keyboard. This is not confirmed, as the keystem dimensions for illuminated switches are not given.
D Special (NCR) This is either a very tall custom keystem with a hybrid Micro/tee mount, or a strange kind of blade stem. Chart 4 A is not clear which is which; the other will be H.
F Flat top This is a switch with no keystem. It has been observed in the UGC-74 Keyboard. This plunger type is also used—fitted with a pompom—for dummy switches of the support type (SD3 and SD6).
G Flat top This has not been observed.
H Special (NCR) See D above.
K Special blade This has been found in a Bunker Ramo keyboard. It is similar to plunger type C, but narrower.
N Flat-stepped This has been observed in a Honeywell terminal keyboard in switch type 4N1E where it is fitted with a pompom and used alongside a dummy switch with a regular B plunger. This seems to be the stepped equivalent of plunger type F.
S Sloped The plunger appears to be the same as A. The plunger aperture has a ridge around it, and the plunger has a different internal part number. These switches so far have been found (as per the drawing) to lack the vertical channel used by the tactile mechanism.
T Stepped T is to B, as S (above) is to A.
V (unknown) This is not in the charts. It appears to be the same as B. It is found in a Sperry/Univac keyboard.

Operating force

Schema position: 1001SD(V)APFO

Micro Switch used newtons, but the SI figures are represented here as centinewtons to match common industry practice of using centinewtons and grams force (1 cN is approximately equal to 1 gf).

Codes 1 and 2 are used for both linear and tactile switches. For linear switches, they denote “half force” and light switches. For tactile switches, they represent standard weight switches (normal and space bar) as these use lower-weight springs to compensate for the tactile peak.

Code Operating force Tactile force (tactile models only)
US SI SD8, SD11 SD13 SD18 Non-tactile models
1 1.3 oz 36.1 cN
 gf (sloped)
 gf (sloped)
Unknown N/A
 gf (stepped)
 gf (stepped)
2 2.0 oz 55.6 cN 95±15 gf (space bar) 80±15 gf (space bar)
3 2.5 oz 69.5 cN N/A
4 None
5 3.5 oz 97.3 cN
6 6.0 oz 166.8 cN
8 8.0 oz 222.4 cN


Schema position: 1001SD(V)APFO

The letter indicating the sensor type is written on the sensor PCB. If one or more other letters are included in the markings, then the letter indicating the sensor type is written in a different colour, such as a dark grey.

Code Output
A Sink level
B Sink pulse
C Source level
D None (dummy)
E Three-terminal
K Timed repeat
S Logic scan

Switch variant

Schema position: 1001SD(V)APFO

It seems to indicate that the sensor is a different model, as the same letter is written on the sensor in place of the normal letter indicating the sensor type. Known letters are D, Q, R, T and U.

Code Assignment
D Unknown; seems to indicate sensor type
F Category 1 chip package only (1001SDF4B3A, 1001SD Series Chart 13 A)
Q Unknown; has matching sensor type (confirmed with model 1001SDQ4A3A, whose sensor PCB is marked “Q” instead of the expected “A”)
R Unknown; seems to indicate sensor type
T Unknown; seems to indicate sensor type
U Unknown; seems to indicate sensor type
Y SD-10181 stop fitted to the switch (appears to be some kind of damper)


Removing the plunger is possible, but difficult. Larry Bishop (formerly a quality control technician at Micro Switch) notes that inserting a slim blade screwdriver onto the side of the plunger adjacent to the part number and twisting will release the plunger. This is not easy, and the shell will suffer some gouging (unless there is a technique that delivers better results) but the plunger can be removed in this manner.

Avoid excessive heating of the terminals when soldering and de-soldering switches, as this risks damaging the sensor. Micro Switch 1001SD charts instruct the use of a thermostatically-controlled soldering iron with a 1/8″ diameter tip set to 500 °F (260 °C), applied to the terminals for no more than two seconds.

For examples of both problems, see this forum topic.


Of all the SD Series keyboards whose switch types are documented, over half use S and E sensors and are likely use matrix scanning. The other models generally use either sink level or sink pulse switches. The logic scan (S) type goes back at least as far as 1976, and it may have been specifically introduced with SD Series. The low-energy scan (E) type goes back to around 1979.

See the SD Series keyboards page for known keyboard details.


The Interdata Master Part Number List from the 26th of January 1979 lists the following keycaps:

It is unlikely that these are complete part numbers. 2SD339 appears to denote the form (single-level, single-unit with integrated LED) as well as top surface texture, presence of legend, and colour of LED. The additional details (legend text, legend colour and body colour) are not part of the basic catalogue listing and would likely be encoded within a longer catalogue listing of the sorts seen for 2SW Series. The table below is all the types from 2SD Series Chart 30, the chart that contains 2SD339:

Cat. listing Size Legend Finish LED colour
2SD337 1 unit No Red
2SD337-M No Matt top surface
2SD339 Yes
2SD339-M Yes Matt top surface
2SD345 No Green
2SD345-M No Matt top surface
2SD347 Yes
2SD347-M Yes Matt top surface
2SD349 No Yellow
2SD349-M No Matt top surface
2SD351 Yes
2SD351-M Yes Matt top surface

Until further charts are observed, one can only guess what function 2SD Series plays. As SD switches have a number of different keycap mounts, it may be that 2SD Series contains keycaps specific to SD switches. The switch charts list the following:

Chart Switches Keycap type
1 SD1, SD2, SD3 (standard height) Standard truncated SD
2 SD4, SD5, SD6 (extra ¹⁄₂₀″ plunger height) Standard truncated SW
3 SD7 (secretarial shift standard height) Standard truncated SW
4 SD9 (LED standard height) Standard truncated SD LED
5 SD7 (secretarial shift extra ¹⁄₂₀″) Standard truncated SW
6 SD12 (LED extra ¹⁄₂₀″) Standard truncated SD LED
7 Plunger C Not depicted or specified
8 SD8 (tactile standard height) Standard truncated SD
9 SD11 (tactile extra ¹⁄₂₀″) Standard truncated SW
10 SD11 (light tactile extra ¹⁄₂₀″) Standard truncated SW
11 Plunger K Not depicted or specified
13 SD16, SD18 (reduced profile) Standard 3CT103

Other components

Cat. listing Component
SD-10101 Switch module removal tool (unclear if single or a pair; depicted in PK 8919 2)
SD-10181 Stop; possibly some kind of damper (see the “A” charts)



The following official documentation has been retrieved thus far:

See also