SPDT switch for velocity measurement

[red]

Greetings,

I understand that it is a common method to use SPDT switches to measure key velocity by measuring the time between the initial break and the second make.

For this to work effectively I would think that we need at least two characteristics for the switch:

1) A fairly long travel (it is under a instrument key after all so some distance would be nice)

2) A period of time in the travel between the first connection being broken and the second connection being made. (Ie, it can't just go from one to the other, there has to be some "not connected".

Could someone please reccomend me a switch that would be good for this application?

Thanks,

Red

[Wilba]

I haven't found SPDT switches like you describe, and I've looked very hard.

They must exist, because some controllers like the Axis-64 use them, I think.

"Piano" type keyboards that have velocity sensitivity typically use two discrete switches... sometimes this is as simple as two rubber membrane switches that will close at different times of the keystroke, with different length actuators above it which are connected to the key.

The Axis-49 uses an interesting method of a rubber membrane with two conductive rings at different heights:

http://a2.vox.com/6a00cd972aa36b4cd5011015f5dc82860b-pi

http://musicscienceguy.vox.com/library/post/the-axis-49-reveled.html

I'm using a very time-consuming method to add extra contacts to Cherry MX keyswitches:

[BuriedCode]

As wilba said, they seem pretty rare, I certainly haven't seen any 'standard' off-the-shelve ones. Since the 'gap' between connections is considered metastability in electronics, and so, is discouraged in design/manufacturing for standard apps.

I'm no expert on keyboard switching, but I have only ever seen be-spoke custom switches used, mostly membrane (as wilba said) and once a cheaper springed contact, with two 'wire's running above and below it for the toggles.

There are microswitches which have a long lever, giving it a long travel, but they are 'click effect', as switch very quickly form one toggle to the other once clicked, almost independant of switching speed (ie: useless).

A rather expensive option, but one which could provide very reliable keyboard scanning is using optical sensing. IR LED's per 'bank', along with phototransistors. Complicated, but I guess its wherther you're making it copmletely form scratch or not.

BuriedCode.

[skunks]

I've used changeover switches similar to that

But the only difficulty is that they are too long to build other than piano-type keyboard.