Mistake below, Vo should be a fraction of 5 volts, not "a volt" when operating
On Fri, Jul 10, 2020 at 2:28 PM Ian Finder <[email protected]> wrote: > A few responses- > > On Fri, Jul 10, 2020 at 12:29 PM Brent Hilpert <[email protected]> wrote: > >> Reading that datasheet, it appears that to reduce power consumption those >> sensors employ pulsed sleep/wake operation, presenting up to a 1/8 second >> delay in response time. Probably not very good for use in a keyboard, and >> something to watch out for if looking for a modern replacement sensor. >> > > Great catch, Brent. I'll keep looking at other options. I must have > skimmed the data sheet a bit too hard. > > On Fri, Jul 10, 2020 at 01:19 PM Santo Nucifora wrote: > > I can't help as I have a full keyboard that I'd like to keep intact but > this is a home made... > > I have many of the keyboards with these 3-pin switches. As mentioned > previously, what the sensors need to be electrically is the ***A sensor, > which is harder to find by a considerable margin. Sink level, vs sink > pulse, vs scan. In Al's words- non-multiplexed contact closure type. > > http://telcontar.net/KBK/Micro_Switch/SD > > The machine is a Xerox system. > > On Fri, Jul 10, 2020 at 02:01 PM Al Kossow wrote: > > It would be interesting to understand the failure mode. > > I wonder if they passivated the die correctly > > I agree. What is interesting to me is this- If I look at other hall effect > keyboards, and even other examples of the same sensor: > http://telcontar.net/KBK/Micro_Switch/images/SD/Micro_Switch_SD_Hall_sensors.jpg > > They appear to be encased in a hard, black epoxy of some kind. Some of the > other keyboards I have with the same SD switches- but alas the wrong sensor > output type- appear to exhibit this as well, and have not failed. > > My failed sensors have something with the gelatinous consistency and > transparency of RTV silicone. It did not appear to have become conductive, > but who knows. There were three distinct failure modes: > > * 1) Output stuck at Vcc (+5v) - By the spec sheet, this should never > happen. Vo should be a fraction of a volt when not actuated, dropping to 0v > when actuated. > > * 2) Output stuck at 0v - Self explanatory. > > * 3) Output appears to be correct-ish Vo, but does not actuate to 0v with > magnet. This was the rarest failure. > > Based on this, my best guess is that there are two failure modes: > > 1) RTV silicone decomposes over time, leaching something into the die that > destroys the IC's passivization layer per Al's comment > > 2) The RTV silicone (or similar) is not robust enough to shield the > delicate sensor die to ceramic substrate connections from vibration and > force-based trauma as it decomposes. > > My expectation is that most of these switches have the black epoxy style > of package for a reason- this is the correct design, hopefully should last > a long time, and the ones that use this transparent goop will all > inevitably die. > > I have never seen any other semiconductor die encased in what appears to > be silicone... > > This is, incidentally, more about keyboards than I have ever cared to > know. The fact that this is an entire hobby for some people is all the more > shocking to me. I'd far rather be debugging some logic... > > > Cheers- I > > > > > >
