Sasse Series 25
Series 25 (in German, Baureihe 25) was a long-running series of flat-base PCB-mount keyboard switches manufactured in Germany by Sasse; they are known from 1980s computers and were still listed on the Sasse website in 2003. Mechanical switches are confirmed, and Hall effect appears to have been an option.
The Sasse website noted in 2003 that the switches can have 2.5 or 4 mm travel, can be linear or tactile, and that there were nine different contact arrangements. Hall effect is also depicted using the same style of Hall IC as used by RAFI, but such an option is not mentioned.
Thus far, two distinct sub-styles have been found. An unreferenced claim on the Deskthority wiki (under Keycap mount, RAFI mount) cites an introduction of 1981 for Sasse using RAFI switch mount, but no mention is made of the switch in question. Photos of the mid-80s microcomputer, the Tiki 100, show the same switches as mine, with a combined RAFI-like/Cherry M8–mount slider. I referred to these as “A” type, and these are given in the Tiki 100 technical manual as 200MN (without a brand).
On the 9th of August 1989, Sasse filed German patent DE 3926331, succinctly entitled “Keyboard key with improved interference fit - has recess in one, projection in the other in cross-shape with keys or notches in arms”. Jacob’s switches use this newer keycap mount, although his keycaps are still RAFI-like mount only. I am referring to these as “B” type.
These are the variants that I have seen so far, all SPST-NO:
|Variant||Branding||Mount||Slider colour||Slide block colour|
I was aware of these switches at AliExpress for some time, but for the price they were going for, I didn’t want to take a chance on them, as they did not appear to be keyboard switches. I did later get a chance to obtain a few, but the seller discovered that they were out of stock.
Months later, when another AliExpress seller presented them to me for a reasonable price, I decided to buy a few. Previous gambles had not paid off (not everything that looks like a keyboard switch, or is described as one, is a real keyboard switch), but when these switches arrived, they turned out to be proper medium weight full-travel switches: I am estimating something like 70 cN actuation force and just under 4.5 mm travel.
My switches are unbranded; I was guessing that they were a predecessor of Siemens STB 11, but Jacob Alexander recognised them as being Sasse switches: those in his collection are branded. As his keypad is dated 2000, I figured that my switches are older, being manufactured prior to the introduction of branding.
The switches are 15.8 mm square, and have an 18.9 mm base (not quite large enough for the switches to touch when placed side by side on a standard 19.05 mm keyboard). The terminals are placed very close to each other, with only 2 mm clearance between them. Including the PCB standoffs, the switches are around 8 mm tall.
My “A2” switches are the wide-key version (grey slide block, stabiliser wire clips). Photos of the Tiki 100 show the A1 switch (ironically, for unstabilised wide keys), of which the internal construction has yet to be seen. The overhang on the wide-key switches bears two pairs of holes—on each side—which correspond with blind holes in the base of the switch, and minuscule dots on the bottom of the switch, one of each pair marked “+”. In addition to stabiliser clips, these switches also appear designed to hold LEDs. For this to work though, the holes in the base would need to be proper through holes!
I have yet to find a patent for the switches themselves, although they are depicted in German patent DE 3742711 (“Push-button switch or push-button panel (key panel)”, filed on the 6th July 1989) which describes not the switch, but methods for damping both the small and large Sasse switch modules. European patent EP 0123818 (“Keyboard push button, particularly for a telephone apparatus”, filed on the 7th November 1984) depicts a much taller version of the switch with two sets of contacts.
Comparison with STB 11
There are distinct similarities between Sasse series 25 and Siemens STB 11 switches. Both types have a clip-on top which (with the wide-key Sasse switch) has an obstructed clip. The overall design of the slider is also very similar, as is the style of the diode symbol on the bottom. This is what led me to initially consider that the switches could be a forerunner to STB 11.
Keycaps and keycap mount
Sasse created a unique keycap shape. The shape is difficult to describe; the design is similar to that of Siemens, but the raised area for the legend is a rounded rectangle instead of a circle, with near-vertical sides and a more sloped front and black. Sasse keycaps are in effect spherical.
Although the keycap mount is based on the RAFI design, it is not RAFI mount. The square post mount is very similar to RAFI and Siemens mounts, but the dimensions are not quite right. Sasse keycaps are a loose fit on STB 11 switches, and an even looser fit on RAFI RS 74 M switches.
RS 74 keycaps fit STB 11 switches (although they don’t grip tightly enough), but they do not fit Sasse switches at all. The dual mount requires extra clearance inside the keycap that RS 74 keycaps do not possess. I lack RS 76 keycaps with which to verify if sufficient clearance exists to place them onto Sasse keycaps; photos of RS 76 keycaps suggest that they have similar clearance to RS 74 keycaps and therefore will not fit.
Sasse keycaps are, as expected, a good tight fit on Sasse switches, and require a fair amount of force to remove. Apparently Siemens STB 11 keycaps do fit Sasse switches (as found on the Tiki 100) but sadly I do not possess any Siemens keycaps. Photos of Siemens keycaps used on RAFI switches suggest that they have similar clearance to RAFI keycaps and therefore won’t fit Sasse switches, even though it is claimed that Tiki 100 computers used Siemens keycaps over Sasse switches.
Cherry M8 mount
The slider also features a cross mount. Filco keycaps do not fit at all (in any orientation), but GMK keycaps do fit. It is not possibly to actually place a GMK keycap on a Sasse switch, however. The clearance between the wall of Sasse keycap and the switch shell is just under 0.5 mm, and Sasse keycaps have vertical sides at the base. GMK keycaps have tapered sides and thicker walls; as such, only half the stem on the slider can be pressed into the keycap before it collides with the switch shell, which is why for now I am taking the mount to be M8 and not MX (especially if this dual mount predated Cherry MX). The purpose of a Cherry M8–compatible slider remains a mystery as I have never seen any M8 keycaps that would fit these switches. Worse, it appears that this exercise may have split the stem inside one of my GMK keycaps (though not severely enough that the keycap doesn’t still fit correctly on my Majestouch).
One thing that strikes me about the switches I received is the quality of construction. Whether this is due to cheapness on the part of the manufacturer, or whether I simply received a batch of rejects, I do not know. The edges tend to be rough, and the PCB mounting nub often contains a lump of stray plastic.
Three clips hold down the lid: two at the front on the left and right sides, and one at the back in the centre. There are two basic shell designs: with, and without clips to hold a stabiliser wire. The stabiliser wire clips are located at the back of the switch; possibly for protection, the back of the lid overhangs the stabiliser clips, obstructing access to the rear lid clip.
With care, the switch can be opened using only the front clips. Slide your thumbnail under the front-left clip and ease it apart; the lid will pop up around half a millimetre. Now slide your thumbnail under the front-right clip and ease that apart also. At this point, press the slider all the way down; you should now be able to rotate the lid upwards and slide it back away from the rear clip. Be warned that the switch is likely to fly apart at this point; try to avoid another lengthy game of “find the missing part”.
Once the lid is removed, the switch components can all be removed fairly easily. Despite the second pair of legs being marked with a diode symbol on the bottom of the switch, in then case of both my switches and Jacob’s switches, a jumper is used instead. The jumper appears to be specially shaped so as to fit the diode bay.
The vertical motion of the slider is rotated through 90 degrees using a “slide block”: a thin, wide plastic block containing a linear cam. A protrusion on the slider engages with the cam and pushes the slide block towards the back of the switch. The block presses the switch contacts together; neither contact is stationary and both move together after they touch (possibly to provide a wiping motion that cleans the contact surfaces). As the slider is released, the front contact returns the slide block to its home position.
The switch is SPST only; there is no space inside the switch for a second set of contacts. There is space for a diode or jumper to be fitted; the circuit diagram symbol for a diode is moulded into the bottom of the switch, but all examples to date have been found fitted with a jumper.
There are two different slide block designs:
Switches for standard 1-unit keys have a slide block with a wider, longer cam that is engaged earlier by the slider. Switches for wide keys have a shorter, narrower cam that is not engaged as early. The design of slide block found in Jacob’s wide-key switches is not quite the same as that found in my switches, but it is very similar.
The illustration above is only a rough guide based on Jacob’s photographs.
There are eight components in total: