On 25 Dec 2005 "Tarkin" wrote:
> has anyone ever made DIY card-edges? If so, how?


Sorry for the late response. Too many other things going on. Others have
already pointed out lots of options, but I can add a few things:

- Buy prototype cards that already have an edge connector.
A good choice. Cards are available for any size. Kind of expensive,
though you can often find them cheaper as surplus. I like the
4.5"x6.5" boards with dual-sided 22/22 pin edge connectors; they
are very common and about the cheapest option.

- Etch your own boards, including the edge connector in the copper.
Bare copper corrodes fast, and makes unreliable connections. It's
usually plated with something; tin, solder, gold, etc.

If you must use bare copper or solder, then use a high-pressure
edge connector. High pressure can "bite through" the corrosion
on the surface to make a good connection in spite of it. But it
makes insertion and removal of boards harder, and you have a
limited number of connect/disconnect cycles before the board is
worn through.

- Pin-and-socket connectors.
These are generally more reliable than edge connectors, but cost
more. The common 0.025" square post on 0.1" centers works well,
and fits the 0.1" hole spacing on common prototyping board.

- Motherboards made with ribbon cable and insulation-displacement
connectors. This works, but you need to physically mount the
connectors or you'll have reliability problems. Using D connectors
is an interesting way to get an IDC connector with mounting ears.
The main shortcoming of D connectors is their odd lead spacing;
they are hard to mount on prototyping boards.

And now for something completely different (as Monty Python would say):

- Now that we have CMOS, there are some other interesting options.
CMOS takes negligible current on its inputs, so you don't need
a low-resistance connection between chips. I have actually DRAWN
circuits on a piece of paper with a #2 lead pencil, poked the
chips into the paper, connected power with paper clips -- and it
works!

Many consumer products use a more civilized version of this
technique. Circuits are silkscreened or drawn with a conductive
ink on a plastic substrate. No soldering needed!

Inter-board connections are made with "zebrastrip", which is one
brand of conductive rubber strip. You just place a strip of it
between two PC boards that you want connected, with a clamp to
squeeze them together. The zebrastrip conducts vertically (thru
its thickness), but not horizontally (between pads). Thus you have
a moderately reliable connection between the boards with 100 ohms
or so of series resistance. The rubber has the additional advantage
of sealing the connection, so bare copper can be used without
corroding.
--
Ring the bells that still can ring
Forget the perfect offering
There is a crack in everything
That's how the light gets in -- Leonard Cohen
--
Lee A. Hart, 814 8th Ave N, Sartell MN 56377, leeahart_at_earthlink.net