DotStar Separate Power and Signal Cables

Skill: Custom Breadboard/Molex Connectors: Revisited.

This is a re-visit of our original Molex connector post. Besides that a blog issue wiped out most of our photos, we had a recent project that ended up having a lot of custom jumpers, and a lot of problems with those jumpers.

Most of the problems were caused by tight-deadline/lack-of-sleep/inattention-to-detail. At least one of the problems was caused by using an improperly-sized crimping tool because of an assumption. Because we want to always be improving, we took these problems and turned them in to lessons so hopefully they’ll never happen again. (At least until the next deadline…)

Quick Re-Cap

Standard jumper wires are great for standard jumper wire needs: quick-and-dirty point-to-point wiring. Where they’re not so great is when you have a specific need not met by standard wires, or you’re making custom runs but still want pluggability, or you’re making some drag-and-drop hardware with specific cabling needs, and so on.

The (cheap) pins come on strips and look like this (both male and female shown).

Both male and female pins are inserted into plastic retainers that look like this.

They can then be plugged into each other, into breadboards, and other headers.

There are two main crimp “sections”: the larger crimp that grabs the insulation for strain relief, and the smaller crimp that grabs the wire for the electrical connection.

Demo Project

We’re going to make some drag-and-drop DotStar LEDs. These require two chunks of wires, two pins for power, and two pins for signal. The non-connector ends will be soldered directly on to the strip and protected with both hot glue and some heat-shrink.

To make things a little more interesting, we’ll use a two-pin connector for the power (assuming some form of a power rail) and two single-pin connectors for the signal (although in general these pins are generally adjacent).

Use the Right Tool

Turns out our pile of crimping tools are not all the same size. Yes, we knew this. Under duress, we did not. The dies are labeled, but not overly-well. We resolved this in two ways: we used a silver Sharpie to fill in the marking on the die itself, and used a label machine for something a little more obvious.

Use the right tool for the job–for our setup, the .25 die was the correct size. Our mistake caused some wires to be fairly loose, which we didn’t even realize until it was too late. (Trust us–the Sharpie-ed version is easier to read IRL!)

Crimping Pre-Process

It’s worth mentioning again that it’s important to make the insulation/strain-relief “tines” parallel to each other before (and/or during, and/or after) putting the wire into the pin.

If they’re not parallel it’s essentially impossible to get the pin into the crimper the right way and you’ll end up with a mashed pin and terrible crimp.

Stripping

Captain obvious is obvious: if you’re using stranded wire, twist it up, yo.

Here’s the thing: the amount of insulation to take off is less than you think. If the wire is inserted much past the wire crimp it may cause a problem when you insert it into the pin retainer (“connector”). You want the bare wire to be long enough to pass completely through the wire crimping section, but really, not any further.

When you’re making a single-pin connector this is pretty easy–you can eyeball it on the wire and trim to length on a test fit. If you’re making a multi-pin connector, you want the wires to be as close to the same length as possible: eyeballing works, but is frustrating. It’s much easier to use a stripper with a depth stop and trim the wire down after the initial strip.

Assembly Line

If we’re making a lot of connectors it’s much easier to do all of each step at once rather than making one complete connector at a time. It reduces the number of tool changes and allows for a fairly streamlined process.

Crimping

The actual crimp depends on the type of wire and its insulation. For example, with the super-flexible “crazy wire”, we almost always needed to “pre-load” the wire into the pin before putting it in the crimper. Stiffer wires, even a 24 AWG stranded, allow insertion into the pin-already-in-the-crimper which is incrementally faster.

Crimp Zones
Crimp Zones: “Tall” zone is insulation, “short” zone is wire.

Pin placement in the crimper is very important. There are two crimping sections in the crimper at slightly different depths–don’t put the pin in backwards; you’ll get a too-tight crimp on the insulation/strain-relief, and nowhere near enough crimp on the wire section.

Make sure the “dividing line” between the insulation/strain-relief section and the wire connection section aligns with their respective sections in the crimper. The divide is usually in the middle of the die. It may look like the pin is in too far–go by the specific pin and crimper combination. This is important.

If the pin isn’t in far enough you’ll put a crimp on the piece that keeps the pin from being pushed out; this is almost always bad: if you destroy the piece the connector body uses to grab on to the pin when you try to plug the finished connector in the pin might just push out the back.

If the pin is in too far you’ll miss the wire crimp and get a poor electrical connection, which can complete failure, or even worse, sporadic failure. Finding these issues is a pain.

With stiff-enough wire (i.e., anything but Crazy Wire) you can pre-load the pin, and gently insert the wire into the crimper/pin combo. A gentle push will automagically get it in the right place. You can double-check by checking the exit and making sure you don’t see any wire.

A good crimp will end up looking (roughly) like this. (In fairness, we gave this one a little extra squeeze with needle-nose pliers to get a better connection.)

Pretty Good Crimp
Pretty Good Crimp

Post-Processing

Before inserting the pin into the body consider whether or not you’ll be adding any heat-shrink tubing to either the connector or to help neaten up multi-wire bundles. (Indeed, you might not even need a pin body if you’re just making simple jumpers–a length of heat-shrink tubing may be all you need.)

After crimping we sometimes have a dickens of a time getting the pin into the connector housing. Pulling the pin through the front is a multi-plier operation (one to hold the housing, one to pull the pin), and we’d occasionally destroy a pin or have to fix it up after.

Extra Squeeze for Final Pin Shaping
Extra Squeeze for Final Pin Shaping

Instead, before inserting the pin into the housing, give the end near the wire a couple of squeezes with a pair of needle nose pliers, particularly around the strain relief insulation crimp area. You might still need to pull the pin through, but it’ll be much easier (read: single-plier operation). Ideally the pin should slide right in.

When it’s fully seated you should be able to hear a tiny (and we mean tiny) little “click” or “ping” when the tab that keeps the pin from pushing out locks in to place. The pin should be sticking out 1/4″ although it’ll depend on the specific pin/body combination. You should be able to push it firmly on a table without the pin backing out.

Post-Post-Processing

While not at all necessary, adding some heat-shrink tubing around the wire and body may help with often-used cables, not so much because of any additional holding power, but because you’re more likely to pull the cable out by grabbing on to the heat-shrink tubing instead of pulling directly on the wire. the tubing also provides additional strain relief on the wire.

If it’s a multi-wire bundle (think a two-wire power or SPI cable, or a four-wire bundle with power and SPI, or a three-wire NeoPixel bundle with power and signal, etc.) and you opted not to use heat-shrink tubing (or forgot) you can use cable ties or electrical tape. Cable ties are easy, but relatively bulky. Electrical tape saves space, but will start to peel off eventually (or immediately).

If you’re really feeling sassy you can tie them up the old-fashioned way.

Why Do All This?

For a lot of projects the cabling doesn’t matter all the much, as long as everything is electrically sound. For more “serious” projects, or gifts, or installations, or if you’re making a lot of the same thing, having a consistent wiring system and process can significantly enhance the usability, durability, and extensibility of a project. Rat’s nests of wires are difficult to debug, and harder to route and hide.

Some things just beg for custom cables, like this demo strip of DotStar LEDs. They require four wires, VCC/GND and two signal wires. When prototyping they never go to the same place, and in practice we often need to chain the signals to other strips while routing power to a distribution bus. Creating custom cables provides flexibility and ease of use.

Resources

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