5 Feb
2014
5 Feb
'14
1:45 p.m.
On 03/02/14 21:20, Tony Duell wrote:
Yes, that looks like the innards of the HP ones.
I think HP just had one magnet on the key plunger, no fixed magnet. The
magnet is normally near the ferrite, wehen you press the key it moves
down out of the way.
[...]
Are those the ones with a core that's
sturated by a permanent magnet, and
which is desaturated when the key is pressed so that there is increased
coupling between 2 'widnigns' on said core?
Yep, that seems to be the design principle. There are two U-shaped leads
(four terminals) -- drive and sense -- and a core-memory-sized ferrite
ring in the hoops, like this:
___ ___
/ \ / \
| | | |
| XXXXXX |
| | | |
The "X" is the ferrite ring.
I'm actually tempted to get a Nd magnet and a few small ferrites and try
this on a breadboard. Seems like it might be a fun experiment to do in
an evening and then write up (or demonstrate in a video).
Theorigianl magnets are nto that strong. I don't think you need an Nd magnet.
The circuitry that HP used should be in 'my' HP9845B schematics. From
what I rmmemebb the actual key drive/sense circuitry doesn't involve any
custom or hard-to-find parts. There is a single-chip keyboard encoder,
but you wouldn't need that if you were jsut making one key as a
demonstration.
-tony