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The Vice-Prez Sez

Karl Lunt

I was working on my Fire-fighting robot (as are we all) and reached for a 4-pin connector to solder onto the 68332 controller board. I had already added connectors to support two stepper motors, power, and 40 kHz IR object detection. The board's prototype area was filling up with connectors, to the point that the connectors had become one of the most common elements in the design.

If you're new to hobby robotics, connector densification might strike you as an odd problem, given how difficult everything else is in this hobby. But believe me, you can't build a robust design without them, so you might as well learn to live with them. Choosing the right connector, and following a consistent wiring practice when you use them, can save you a load of time and trouble later on.

For cables of eight or fewer conductors, I usually rely on the Molex/Waldom locking K.K. series connectors. The male version of this connector contains a single row of square pins on 0.1-inch centers, formed into an open plastic base. The back portion of the male connector contains a vertical plastic ridge that acts as both a locking device and a polarization key. The matching female connector plugs into the male in only one orientation, and the vertical ridge acts as a clamp to hold the female connector firmly in place.

This style of connector is excellent if you need to build a wire harness that plugs into a PCB. The male connector simply solders in place. The female connector requires a little special effort on your part, as you must crimp or solder a single pin to each wire, then push that pin into the proper hole in the connector body.

When I assign signals to any of my K.K. series connectors, I always make the leftmost pin be pin 1, as you look into the row of pins from the board edge. This pin always contains the ground signal. If the connector also carries power, such as +5 VDC, the power signal is on pin 2.

You can buy all of the elements of this connector system, including loose pins and a good quality crimping tool, through DigiKey or locally at the Active Electronics store. Both the male and female connectors come in various pin counts, from two up to eight or more. Take a look The Basics - Connectors for more information about how to to order and use these connectors.

If you need to run more than eight wires at a time into your PCB, consider using an insulation displacement connector (IDC). These are commonly called ribbon cables and contain a flat strip of either gray- or rainbow-colored ribbon cable; you can find the IDC connectors in a variety of pin counts from ten to 40 or more. They get the name "insulation-displacement" because each pin of the female connector contains tiny teeth that actually pierce the insulation, displacing it enough to make electrical contact with the wire inside. The male connectors have a double row of pins on 0.1-inch centers, with the rows spaced 0.1-inches apart. Note that unlike ICs, pins on an IDC connector are numbered in staggered order, going from row to row, rather than down one row and up the next.

As a rule, you solder the male connector to pads on your PCB, then clamp a matching female connector to one end of the appropriately-sized ribbon cable. It can take considerable force to clamp some of the larger female shells onto a piece of ribbon cable, and you will need to resort to tools such as a bench-top press or bench vise. Make sure that whatever tool you use for clamping has a jaw wide enough to cover the entire width of the connector shell! If you try to use a narrow clamping device, such as pliers, you risk cracking the connector shell or getting a poor connection on one or more pins.

When you get ready to clamp the female shell in place, double- and triple-check the position and alignment of the connector before you begin clamping. Make sure you have pin one of the female shell (marked with a small arrow or triangle) placed exactly over the ribbon conductor you have chosen as wire one. Also make sure that you are about to clamp the connector onto the appropriate surface of the ribbon cable; if you put the connector on the opposite side of the ribbon, you will have the odd- and even-numbered wires reversed! Note that if you get a connector on wrong, you will probably have to redo the entire cable!

When you buy your female IDC connector shells, spend the extra dime or so and get the matching strain relief adapter. This is a thin plastic strip that clamps onto the back end of the connector and minimizes stress on the wires where the connector's teeth pierce the insulation.

As with the K.K. series connectors, you can get both genders of IDC connectors at all the larger mail-order houses, such as DigiKey, and also at stores such as Active Electronics.

When you lay out your ribbon cable wire assignments, remember that you don't need to use up every wire in the ribbon. In fact, you're better off leaving two or more wires spare, for possible expansion later. Believe me, using a cable that has a couple of extra pairs of wires in it is only a minor inconvenience, but redoing a whole cable harness because you have to add a few unplanned-for wires is a nightmare.

I hope this little primer on connectors will help you the next time you need to run a few wires from point A to point B. You can't avoid running lots of wires in most robot designs, but with a little planning, you can make the cabling neater, more reliable, and easier to build.

Keep on keeping on...

karllunt@seanet.com

http://www.seanet.com/~karllunt

"Technology marches on. Over you or through you, take your pick." Attributed to Stewart Brand