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So, I've been asked to make a GAL-like PCB that can be 'manually programmed' without need of soldering for an educational setting.

Meaning, I have signals coming in ('horizontally') that can be linked up to a wired AND ('vertically') by means of some connection, those ANDs then being linked up by an OR to go into the input of a Flipflop.

Thing is, the requirements are that I'd need to have at LEAST 5 inputs (inverted and non-inverted), 5 Flipflops, and all the flipflop outputs returned normal and inverted, with 5-10 AND-paths. Even if we assume the minimum of 5 on each, that would require

5(ff) * 5(and-paths per ff) * ( 10(input) + 10(ff-output) ) = 500 connection points per board

If I use any connector whatsoever - even just pinheaders, that makes it hard to produce and expensive, which both is not ideal.

Ideally, the connectors would be 'implemented' only on the PCB, with the connection bit (the 'jumper' if you so will) being the only thing that actually costs money per unit.

I've been thinking about solutions like having small screws (like M2 or something?) with nuts to connect the 'grid', on a PCB that is not throughplated, but that would be a pain for the users to assemble and disassemble, and would be fairly expensive at around 10-20 cents a connection, checking supplier prices for M2 screws.

Another thing I've been thinking about were those little earring needles with the retention clip - they can be bought cheaply from that one marketplace with an "express" attached to it, and would be easily affixable to the board. Problem would be the connection, as it is not spring loaded, however you could affix some conductive rubber, as is used in for example, controller buttons to it, to provide a 'natural spring' to it, holding it together.

I've also thought about just putting the pads next to each other, like in a controller button, then electrically connecting them with one of those conductive pen inks or electrically conductive stickers - that might work, but the cleanup would be a problem.

This would be a series of maybe 100-200pcs, maybe repeated every 2 years, so not extremely high volume.

Any other bright ideas how to implement an ultra-low cost connector that is implemented fully by just the connector that closes the connection, that's not one-way?

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  • \$\begingroup\$ Something like a high quality breadboard, maybe? There are also PCB terminal blocks with springs, for example. It might help us to visualise the layout and size of your device. Are there any cost restrictions? \$\endgroup\$
    – StarCat
    Oct 12 '20 at 6:39
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    \$\begingroup\$ @awjlogan reversible as in un- and repluggable, yes. \$\endgroup\$
    – Overrice
    Oct 12 '20 at 10:22
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    \$\begingroup\$ My experience of breadboards from when I was a student is that you spent 90% of your time trouble-shooting breadboard connection issues and 10% trouble-shooting the actual circuit. Why every school considers it unthinkable to teach soldering to EE students is beyond me. Far more useful knowledge to an EE than the usual Moore/Mealy theoretic crap that no sane person implements with 74HC logic anyway. Doing so went obsolete somewhere around year 1990. \$\endgroup\$
    – Lundin
    Oct 12 '20 at 10:50
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    \$\begingroup\$ Small frame challenge, might it be possible to move the routing into the connection cables? And then just use banana connectors and the input and output? Then its only 20-30 connectors. \$\endgroup\$ Oct 12 '20 at 12:20
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    \$\begingroup\$ 40-pin 0.1" headers are $5 for a pack of 100 on aliexpress, that's less than $1 for 500 pins... \$\endgroup\$
    – bobflux
    Oct 12 '20 at 12:33
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Search for "patchboard" as used in 1960s/70s analog synths. Ironically their original purpose was for programming computers, before stored-program computers, so this usage will be taking them pretty much back to their roots.

Or look for different models of PCB mounting IDC connector. available up to something like 50-way (25 x 2) on 0.1 inch pitch, or maybe larger arrays, from Samtec or Harwin or other companies.

These usually take 0.64mm sq pins, and are available in different lengths; look for relatively short ones with open backs, which will allow long pins to pass through into a second connector. If you can mount 2 of these, back to back, and insert a relatively long pin, you have your connection. The long pins are available separately, as wire wrap pins! You may want to dip them or use heatshrink tubing or something to give them insulated handles.

That could give you a 50x2 array on a single PCB (or even 50x4 if you can find SMD parts and use both sides) - with very few separate components, making assembly of these easy.

Then mount a bunch of these PCBs at right angles to a backplane using ... perhaps PCI connectors?

Looking at the Samtec link, they appear to have "bottom entry" parts which would allow an entire plane on a single PCB with the other plane on a second PCB (connected together by further connectors of course!) for a simpler arrangement. Select the "DE" variant (Double Entry) or PE for the top layer; bottom layer can be closed ended. Connections can be up to 50 x 2 rows in a single connector, giving you 500 connections with 5 parts (but you may prefer 25 x 20 in 10 parts). Further details left to your imagination. (Side note : I have found the supplier very obliging with fast prototype parts)

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How about placing two plated though holes next to each other, and when you want to join them you put a single grabber clip on so it grabs both holes? The holes would need to be quite large and less than one diameter apart for most grabbers.

You'd probably need to experiment a bit with different grabber clips, hole diameters and hole spacings. It might be hard to make it fully reliable, but it'd probably be OK for DC.

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It sounds like any solution on the board side that's more than just "plated holes" will be too expensive, but is it possible to put wire loops on each point? (Manually or by automated assembly). You can get "test hooks", "test clips" or "hook clip cables" that attach with a clip at one or both ends.

Otherwise, you could try banana plugs into holes in the PCB. You'd want to apply a lot of solder (HASL finish?) to protect the board, because the plugs will wear the plating out of the holes, and you'll want to do a test board to get the right tolerance for insertion force. The insertion lifetime of this will be difficult to predict.

In general I would look at the "test points" section of your electronics catalog; the cheapest little board clip points I can find are six cents https://www.digikey.com/en/products/detail/koa-speer-electronics-inc/RCSCTE/12749750

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  • \$\begingroup\$ Yes, ideally something like plated holes, or unplated holes even - see the earring clip or screw idea. You'd basically do the "through plating" yourself. The banana plug idea isn't bad, but banana plugs are expensive and too big physically. I've also been looking at DIP-switches, with 10 position ones being available starting at '10 cents per switch' - still, with 500 connections that would be 50$ in connectors alone per board. I have no 'upper cost constraint' for the board, but ideally it's cheap, so it can be distributed to keep for students. \$\endgroup\$
    – Overrice
    Oct 12 '20 at 10:31
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You could try to place three holes at a slightly larger spacing than what would be required for a standard pin header (maybe the 2 mm or even 1.27 mm kind to save some space), with the wired-AND line in the middle, surrounded by an inverted and noninverted input. That way, you “only” need 750 holes.

To connect them, either use a 2-pin header and a jumper, or take a long (e. g. 40×1) header, solder a thin wire across all 40 pins, then use wire cutters to cut it into 20 2×1 pieces.

Of course, you'll have to experiment to get the correct spacing for a good friction fit.

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