A reminder that this thread was split off from samwisekoi’s idea
here. If this stuff looks interesting, please support his efforts to make a folded membrane and case for a 60% buckling spring keyboard.
The smaller the keyboard, the more important it is that it will accept multiple concurrent key-presses. NKRO is usually achieved in conductive keyboards by creating a one-way circuit using one diode per switch. But solderable flexible substrates are exotic and a diode anywhere near the switch might create enough of a bump to prevent the switch from working reliably. Can you tell that I have thought about this a lot?
I explained this idea in a few threads elsewhere; use a very thin PCB for the bottom membrane and route all of the traces to diodes beyond the border of the top membrane. As it so happens, there is an inexpensive surface mount diode available with specs similar to the common diode used with Cherry switches. But because they are
1x5 diode arrays, only 14 are needed for the M-60.
http://www.diodes.com/datasheets/ds31437.pdfI already have very thin flexible blank PCB – enough to do 6 of these boards. It is 8mil thick – only .003” thicker than the original IBM bottom membrane. I have the skill and tools to DIY etch these boards, but don’t have the skill or tools to mount the diodes. If anyone has reflow experience out there, I would be happy to supply the parts to make several and ask for a subset back. If I don't get any volunteers then i will likely build a prototype without the diodes and check into the cost of having the the boards etched and assembled by Sitopway Technology or other vendor.
http://www.skycraftsurplus.com/coppercladboard23x17x00080.aspxI have a very simple design for the bottom membrane that is similar to the original IBM design. I figured that “single-sided” would create less stress on the traces when clamped to the curved back plate. The controller would be made from a Teensy++ (no pins) with two FFC connectors mounted on it. The controller would end up top-center under the back plate. I’ve included some drawings below and will work on more for anyone having trouble visualizing it.
M60_bottom.pdfAs I said in my previous post, I send out 3 RFQ for a custom top membrane to companies that advertise they will do jobs as small as one piece – and today one of the vendors contacted me asking for a drawing in order to make an estimate.
The final piece of the puzzle is programming the Teensy++. I would really like to leave that to someone better qualified. I hope to be able to offer assembled parts as a bribe.