FITM (Flex-In-The-Middle) for power analysis

Flex-In-The-Middle device for analyzing the power consumption patterns of battery powered products

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This is my entry for the Hackaday flexible circuit contest, it is a simple board that goes in between a battery and the battery holder contact, or two batteries in series in order to interrupt the circuit. That makes operations like current measurement and power profiling possible without modifying anything in the target hardware while providing a fairly secure connection.

It works by virtue of being very thin and flexible and having isolation between the two "sides" up until the other end, where either a shunt resistor or direct connection to a current meter leads can be soldered on.

I designed this FPC using best practices regarding use of copper and smooth transitions at the bending points. Also taking into account the very economical prices OSHPark offers for flexible pcbs (as designed a set of 3 boards is only $9) allows them to be semi-disposable.

To design this project I used the great KiCad StepUp workbench, specifically the design flow from FreeCAD to KiCad using the outline exporter.

First I began on the part design workbench, by drawing a sketch on the XY plane, like so

Drawing complicated outlines in KiCad is a chore, specially if it involves arcs that need to be tangent to each other, making it in a parametric sketch on FreeCAD is much easier and powerful. Notice the generous filleted arcs in between all transitions, when designing a FPC you really want to avoid any kind of stress concentration, as that could start a tear when bending or pulling on the board. The dimensions are thought out for fitting comfortably in the negative terminal of a AAA or AA battery, and the total bounding box area is just under an inch square, while being long enough to get out of recesses. The parameters defining those dimensions can be changed at will for different size battery holders or other requirements.

Once i was happy with my outline in FreeCAD I just switched to the KiCad StepUp workbench and pressed this button: "Push sketch to PCB edge"That opens a file dialog where i have to select an existing kicad_pcb file that has nothing in the Edge.Cuts layer (in this case my empty PCB file for my project). Once I reloaded the board file I got this on KiCad

With that outline in place I just had to place some dummy "test point" footprints and a shunt resistor. Then I used planes and free vias to fully route the board.

Using planes eases the creation of a proper "Teardrop" into the round pad, without the smooth copper transition the copper foil would crack at that point on bending.

That is basically it... Will update when prototypes arrive.

  • Paper prototypes

    Jose Ignacio Romero04/30/2019 at 04:57 0 comments

    Before ordering the boards from OSHPark I wanted to do some "fit testing" with a paper prototype (basically just a printout of the board render in 1:1 scale and cut out with an x-acto knife). Here are some pictures of it in different kinds of battery holders:

    AA holder of a wiimote:

    AAA Holder in a Pokemon Mini console:

    This one really shows the usefulness of the flexible pcb, as the battery terminal is recessed into the plastic, so the board needs to bend to reach, here is a picture of the console without the battery:

    It will also even work for many button cell holders!:

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