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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.


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 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 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. However, this could be entirely incorrect, and no such switches have ever been discovered.


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. The following descriptions are only a very approximate guide and may be incorrect.

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
These provide the same output as sink level, but have an enable line (similar to the Freigabeeingang terminal on HFO B 461 G Hall sensors). However, the enable line must be switched off in order to generate output, which appears to be the exact opposite of B 461 G. These are four-terminal as before, but there is no longer a second output line as the fourth terminal is used instead as the input line.
Unclear; these can function as source level or logic scan, and are designed to significantly reduce power requirements. See the SD16 Keyswitch Modules catalogue entry for an explanation. The official name of this type is not known, as none of the charts cover it, even though it goes back at least as far as 1980. In the SD16 literature the switches are simply given as “four-terminal” and “three-terminal”.

Electronic Design magazine from 24th May 1979 notes the following, from a screenshot included in a video from Chyros:

“… 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.”

The next paragraph mentions Honeywell, so SD Series is implied here, and this gives a rough idea of when the three-terminal types were introduced. Chyros never revealed how he gained access to that magazine.

Secretarial shift

Secretarial shift was included in the series. While a loose switch has been observed for sale on eBay, neither the switch nor the relevant charts indicate how the secretarial shift mechanism works.

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. 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.
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 = custom blade type for NCR, F = flat top, G = flat top, H = flat stepped, K = special blade type, 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 = unspecified (three-terminal type), 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.

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(Q)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 1975-09-23
4 Momentary, linear 0.05″ taller plunger Black 1975-09-2x (unclear)
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 chart for SD4–6 is only in fax resolution, so the date is hard to read, but the drawn and checked dates so closely resemble those of the SD1–3 chart that they appear to have been drawn together. That in itself may explain why the dimensions differ but the shapes are the same (the extra 0.05″ plunger height is not shown). However, this makes it even harder to understand why that extra 0.05″ was offered.

Plunger types

Schema position: 1001SD(Q)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.
F Flat top This is a switch with no keystem. It has been observed in the UGC-74 Keyboard.
G Flat top This has not been observed.
H Flat-stepped This has not been observed; it would seem to be the same as G, but stepped.
K Special blade This has been found in a Bunker Ramo keyboard. It is similar to plunger type C, but narrower.
S Sloped It appears to be the same as A; the difference is not yet discovered.
T Stepped It appears to be the same as B; the difference is not yet discovered.
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(Q)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(Q)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 Unspecified (three-terminal type)
K Timed repeat
S Logic scan

Other components

Cat. listing Component
SD-10101 Switch module removal tool (unclear if single or a pair; depicted in PK 8919 2)


There is at present no known safe way to remove the plunger without risking tearing the top of the housing apart.

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. The equivalent details for SD Series (PK 8919 2) have yet to be recovered.

For examples of both problems, see this forum topic.



The following official documentation has been retrieved thus far:

See also