Low Noise 1-wire Interface to a Switch Matrix - Page 1
Forum section: electronics / general

author: Lesolee
author_rank: Regular Contributor
timestamp: August 02, 2020, 12:50:31 pm
content: No doubt people having been doing this for years, but I don’t recall seeing it done elsewhere.I have used the Microchip Curiosity development board as it is really cheap and has a built in debugger.and a direct link for non-logged in peoplehttp://lesliegreen.byethost3.com/articles/interface.pdf

author: Ian.M
author_rank: Super Contributor
timestamp: August 02, 2020, 01:19:05 pm
content: Yes, people have been doing similar for years, e.g. back in 2012:https://www.microchip.com/forums/FindPost/694613and it wasn't novel then. You'd have to trawl through piles of paper from the previous century to find the first mention of the concept as, if you use the time constant of a RC network to sense the value of R, it may even predate the introduction of MCUs with ADCs.However, nice writeup.

author: wraper
author_rank: Unknown
timestamp: August 02, 2020, 01:25:12 pm
content: Just a reminder that you shouldn't use this anywhere where reliability matters, especially long term reliability in presence of moisture and corrosive gases. This approach backfired badly to reliability of many TV and monitor control buttons where this is the most commonly found IME. And frankly it should be avoided by all means if you can do it differently.EDIT: and never use this with tact switches in particular. They tend to grow metal dendrites over plastic between contacts which form a high resistance conductive path. I observed them many times when inspecting internals of faulty tact switches under microscope. Usually it does not affect operation when switches are used in a usual way with fairly low pull-up resistance. But it completely breaks operation when used within resistive divider like this.

author: mikerj
author_rank: Super Contributor
timestamp: August 02, 2020, 01:53:03 pm
content: This scheme has been used in cars for quite a while, e.g. sometimes on steering wheel mounted controls.

author: newbrain
author_rank: Super Contributor
timestamp: August 02, 2020, 02:10:10 pm
content: Quote from: wraper on August 02, 2020, 01:25:12 pmQuote from: mikerj on August 02, 2020, 01:53:03 pmYes, probably radio controls on the wheel are not that important, but the juxtaposition of the posts amused me.My garbage (literally)Pioneer XC-L7*, 1998 era, has a remote, cabled, control unit. It uses this principle.*: BTW, the little contraption in the linked thread is still going strong, after ~4 years

author: wraper
author_rank: Unknown
timestamp: August 02, 2020, 03:09:34 pm
content: Quote from: newbrain on August 02, 2020, 02:10:10 pmBut there were/are many computer monitors and TVs in the wild where buttons stopped working altogether or are incorrectly detected. It's not like every unit fails but such failure is not rare, especially in old equipment. Besides the issue with dendrites I already mentioned, particular circuit does not provide sufficient contact wetting current which is needed to ensure good contact over time.https://en.wikipedia.org/wiki/Wetting_current#

author: Lesolee
author_rank: Regular Contributor
timestamp: August 02, 2020, 10:49:20 pm
content: It is unfortunate that this field of knowledge (dry circuits / wetting currents) is so badly documented. You just get almost hearsay reports about it.The Keithley Low Level Measurements Handbook, for example, says that you must not apply more than 20 mV to a contact for fear of puncturing any oxide build up layer (and therefore getting a lower resistance reading). It makes me wonder if such proper documented evidence has ever been published.

author: mikerj
author_rank: Super Contributor
timestamp: August 03, 2020, 12:08:32 pm
content: Quote from: newbrain on August 02, 2020, 02:10:10 pmMy Smart Roadster uses this scheme for the gear change paddles and horn button. A known problem is a connector under the dashboard going high resistance meaning the horn sounds when you go to change gear

author: Lesolee
author_rank: Regular Contributor
timestamp: August 03, 2020, 01:01:14 pm
content: You would think there would be adequate current available in a car to put some current through the connector and stop the high resistance issue. The trouble is, designers and service technicians are often disconnected in the sense that there is no or little feedback as to what causes problems (after say 10 years of service).The R-2R ladder network versions are low power in the sense that they take zero current when no buttons are pressed. In my design you can actually force a high current through the switches (eg 3mA) by pressing two in a series pair. That should clean up any oxide film, although obviously proper wiping contacts on the switches would be a better solution.