Connectors for access to four conductors from a 40-wire ribbon cable

Question

I have a Raspberry Pi project I'm working on that will involve connecting three printed circuit boards. I'm trying to figure out a rugged and convenient way to make the connections. The drawing below is a side view of my current rough concept for how to do this. I'm hoping for feedback or suggestions on how to flesh out this plan or what would be a better plan. I'm inexperienced with DIY electronics, and what experience I had was 30 years ago.

The bottom board has a 2x20 pin header with a pitch of 0.1", as does the other large board above it. They're designed to work together, so I can just connect them with a ribbon cable, and the pinouts will match up correctly. There are also holes pre-drilled in the boards so that I can provide appropriate mechanical support, uniting these two boards into one rigid unit.

The hard part is getting the connections to the third, small board, which is a GPS receiver. It needs four connections: ground, 3.3 V power, and two UART lines. My current thought is to take a razor blade and carefully separate out the four wires from the ribbon cable, send them to some kind of connector (the circle marked "?"), and then go from that connector to soldered connections on the small pcb.

Issues I'm worried about:

• The 3.3 V power line is also needed by the top large pcb, so I'd need to splice in an extra piece of wire to get that connected to both it and the small pcb.

• I don't know what electrical properties are required for the UART line in order to avoid problems with things like reflected signals. The data sheet (link above) describes various speeds in various setups, but maybe 9600 baud. This seems relatively low, so maybe this kind of RF engineering stuff is not relevant?

• I'm not sure what kind of connector to use. This is supposed to be a lightweight, compact unit. Molex seems like overkill. D-SUB seems kind of big and heavy.

• I'm imagining a tendency for the ribbon cable to fray or split from the places where I separated the four strands from the main cable.

• Not sure how to do appropriate strain relief for the soldered connections on the small pcb.

• If I want to get access to other lines later, I have to do more razor-blade surgery. This suggests maybe using a crimp-on connector in the middle of the ribbon cable, but I'm not clear on how to make permanent connections onto such a thing.

Answer 1

First, you generally should not power peripherals (at least beyond a single low current chip) from a pi's 3v3 bus; it is too sensitive to additional and varying loads. For low current things, use the 5v bus and regulate it yourself; for higher current ones, use an independent connection back to the power supply, or a separate power supply with a common ground, or even isolate both signals and power.

UART signals at reasonable baud rates are in general not very sensitive to impedance match over short distances.

In terms of your connector options, you have many:

• You can crimp an additional full-width IDC connector onto an existing ribbon cable. Use a vise.

• You can split out a few conductors and crimp them into a smaller IDC connector.

• You can split out a few conductors and individually crimp male or female pins on them. Note that you can also use these female pins on a male crimp IDC connector, but you likely won't have any good secondary retention so that is best for temporary setups.

• If the overall number of connections to be made is small, you can use individual crimp pins inserted into hearders (either Molex or Dupont style; they mate, but the housings and pins are not cross compatible). You can crimp these yourself, buy them pre-crimped without housings or buy them with single housings and pop those off.

• You can solder individual wires to the back of the pi or the other board, 30 gauge silicone insulated wire is great for this as you can do a neat job with the insulation right to the solder joint without it melting. You can then bundle these, maybe in some heatshrink and strap them at a mounting hole or standoff or similar.

• You can use a breakout board having multiple points of access to the signals including an additional connector footprint. Beware many of these are not reversible as they have the power pins of the pi footprint internally connected, however compact pi-zero sized breakout boards will in general work with the other models, too.

• You can make a custom board