Percepticle 1: Wiring

Percepticle 1: Wiring

(For all images, click on the thumbnail for a larger picture.)

The first problem when attaching sensors to the Tmote Sky is making the physical connection to the board. Apparently the designers made a peculiar decision which makes this task a bit more complicated than one might expect.

As the mechanical drawing shows, there are two IDC headers that give access to the sensor channels: one 10 pin (tag 4) and one 6 pin (tag 5). The physical layout does not maintain 0.1 inch spacing between the two headers, though, so it's not possible to bridge both of them with one 16 pin connector.

However, they're not far enough apart to allow two typical connectors, either ...

Here, I've connected a 10 pin ribbon cable, with an ordinary crimp-on connector, to the Tmote's 10 pin header ... but it pushes right up against the 6 pin header and doesn't allow access to it. I haven't been able to find any other crimp-on ribbon cable connector that's narrower than this.

The solution is to use small profile headers that can fit right up next to each other. I chose to mount two such headers on a single 16 strand ribbon cable, but you could put the headers on separate cables if you prefer. Here are the part numbers for the headers I chose:

Part Molex part number Digikey part number Quantity
10 position housing 22-55-2101 WM2522-ND 1
6 position housing 22-55-2061 WM2520-ND 1
crimp terminals 16-02-0103 WM2512-ND 16

It is tedious but straightforward to crimp the 16 individual terminals onto the ribbon cable's 16 conductors.

Once the terminals have been threaded into the housings, the result is a nice neat connection to the breadboard. Of course, the pinout at the other end depends on where you put which terminal. Here's my pinout, with the connectors physically arranged to reflect the layout of the DIP plug at the other end of the ribbon cable.

Why this is of only transient importance

This cabling issue only matters for breadboarding ... once our design is solid enough for PCB fabrication, we can put the header pins on our PCB anywhere we want. (In fact, someone at Moteiv said that this is exactly why they can't change this odd layout -- other customers have already built their boards to interface with this existing layout, so Moteiv has to stay compatible with its customer base.) But breadboarding is a useful phase of design, and it's nice to be able to prototype and test the circuit before committing to PCB fabrication.

This page maintained by Wil Howitt
Last updated 30 December 2005