- See also
Datanetics DC-50 is a family of tactile mechanical keyboard switches introduced in 1973. DC-50 was designed by Mike Muller of Datanetics, who filed a patent for them in 1972; this was granted as US patent 3777090 in 1973. DC-50 is a continuation of their elastic diaphragm system that began as the array keyboards, yet it was introduced in the same year as the much more conventional DC-60 pure mechanical switch series. Meryl Miller contributed the illuminated version and the original alternate action design. The goals for DC-50 were to have a discrete switch (in place of their existing batch-fabricated array system) with hysteresis and minimal contact bounce.
In his notes, Meryl placed a handwritten note to indicate that Datanetics was selling three million (presumably) DC-50 switches per year at $1 each. This is implied to be at a point in the late 1970s, possibly when they were manufacturing all of Apple’s keyboards. According to a letter from Datanetics to Joe Torzewski in February 1978, these switches were 50¢ each, or just over $2 each in December 2020 prices.
DC-50 It is a discrete switch that uses a hybrid arrangement of both membrane and mechanical switching. Flat metal strips form the switch contacts and terminals, but these are glued to Mylar membrane sheets that provide the release action for the contacts. The sandwich of metal and Mylar forms a sealed contact assembly. The connection between the plunger and contact sandwich is made by an arched leaf spring that operates a plastic prong assembly. This design may have inspired the Alps “switchplate” system, which functions in a similar manner. Although most “switchplate”-based switches (from not just Alps but also Omron and other brands) use a different type of actuator spring (specifically a vertical leaf spring attached at the top), Alps SCK switches copy the Datanetics design much more closely, and as with DC-50, they provide hysteresis.
Standard tactile switches are formed from two identical shell halves, ultrasonically welded together. Single and double pole operation is supported. All known types are normally open.
The forces given for the alternate action models are the separate latch and release forces, with the latter being higher.
|DC-51-01||Momentary single pole||0°||3±0.5 oz||DC-50 brochure (1974)||5930-01-086-2127||Listed under that NSN as being manufactured by Lombardini Srl and A.c.m.e. Anonima Costruzioni Motori|
|DC-51-03||Alternate action single pole||0°||4.5±0.5 oz, 5.5±0.5 oz||DC-50 brochure (1974)|
|DC-51-04||Momentary double pole||0°||4.5±0.5 oz||DC-50 brochure (1974)|
|DC-51-11||Alternate action single pole||Higher||Meryl Miller|
|DC-51-31||Momentary single pole||12°||Apple II and III service manuals|
|DC-51-35||Momentary single pole||12°||3.5 oz||Apple II service manual||Likely for space bar (one per keyboard)|
|DC-51-41||Momentary single pole illuminated||0°||3±0.5 oz||DC-50 brochure (1974)|
|DC-51-43||Alternate action single pole illuminated||0°||4.5±0.5 oz, 5.5±0.5 oz||DC-50 brochure (1974)|
|DC-51-44||Momentary double pole illuminated||0°||4.5±0.5 oz||DC-50 brochure (1974)|
|Momentary single pole||9 oz||Apple II service manual|
|Double action||Apple III service manual|
|DC-51-64||Momentary single pole illuminated||0°||WBParts||5930-01-090-8016||Listed as being manufactured by Lombardini Srl and A.c.m.e. Anonima Costruzioni Motori|
|DC-51-90||Immovable||?||Illuminated||Apple II service manual|
|DC-51-101||5930-01-305-7053||Not in known catalogues and not observed; manufactured by ITT Schadow|
Alternate action prototype
In April 2016, Meryl Miller sent me a package of sample parts and brochures. Included in this was a prototype DC-50 alternate action switch that he designed, that “worked well but did not do well enough in life testing to warrant putting it into production.” Nonetheless, this is the alternate action type illustrated in the DC-50 Series Key Switches brochure from November 1974, albeit with a black latch arm instead of white.
Both the standard type and Meryl’s version can be seen in the predictably deplorable photos below.
Surprisingly, the original type did indeed see commercial use, in the ARP Omni. The ARP Omni Model 2480 Service Notes from 1977 list ARP part 1903002 as “SWITCH, ALT ACTION, SP”, manufacturer part DC-51-03. Rich Diemer of BustedGear.com has serviced the Omni for decades, and thinks that the Omni used both the plastic arm and wire arm DC-50 alternate action switches, but all spare parts have sold out and this cannot presently be confirmed. The first photograph below is from the BustedGear.com site listing for UOMNI-SW (not in stock at the time of writing), while the rest were provided on request and show his one remaining switch that does not latch. These switches have a thin piece of metal on the top of the plunger that has not been explained.
Meryl does not know how these came to be sold. One possibility is that these were accidentally sold off as production switches by an employee who did not realise that these were not production switches. The answer will likely never be known.
It is interesting to note that his example was marked “PAT PEND” on both sides, while the example taken from the Omni has “PAT. 3777090” on the side that shares the shell half with momentary types. That is, the switch in photographs 2–5 above may be from a later batch, after US patent 377709—filed in 1972—was granted in 1973. The ARP Omni itself was made from 1975–1977.
All illuminated switches observed to date have the slot in the plunger to take the alternate action arm as found in the prototype switches.
Datanetics supplied keyboards to NCR. NCR required these keyboards to use their existing keycaps, so Meryl adapted the switch accordingly. This required replacing the standard cruciform mount with a flat mount. Two such Datanetics-made NCR keyboards were placed for sale on eBay; of these, he notes (in correspondence in November 2019):
I was very much involved in that particular keyboard from the beginning. Nice to see that a couple of them survived all these years. That project has to date back to the 1970s. NCR was unique as a customer. They insisted on Datanetics using their keycaps. So I designed a version of the DC-50 with a keystem having a blade type of top that would engage the slot in their keycaps. In addition, some of the larger keycaps, especially the “L” shaped ones required a dummy support under the keycap in addition the the keyswitch. That allowed the operator to depress the key from anywhere on the top surface.
Having spent the first 15 yrs of my product design career at NCR, I was worked closely with their design team on that particular project. As I recall, the end product was an accounting machine as opposed to a computer terminal.
Meryl previously sent me his single sample switch of this type:
A similar keyboard using the same keycaps, and with complicated-looking reed switches is also advertised on eBay. John Culver of The CPU Shack reports that the 73* codes are the production dates, so this keyboard would have been made somewhere around the time that DC-50 was introduced.
Similar keycaps can be found on the NCR Class 18-22 desktop calculator, although that is made in Japan and the keycap step profile is less prominent.
Switchcraft 315900 is an adaptation of DC-50 with a non-keyboard button mounting. These are marked “ITT SCHADOW” and were thus made in 1979 or later following ITT’s acquisition of Datanetics (although they went under the name ITT Datanetics).
Meryl Miller provided two separate DC-50 brochures. These have been donated to the National Museum of American History Library who provided higher quality scans.
- Datanetics DC-50 series on the Deskthority wiki for prior notes and illustrations