Pedometer Teardown

Michael Stanley & EE HomePage.com
This report is licensed under a Creative Commons Attribution 3.0 Unported License.

Introduction

Figure 1: Pedometer

Pedometers like the one shown are often given out free during health-related events. They are used to estimate the distance a person travels by foot, and are often used as part of a person's daily exercise regimen. I bought this particular model for $1 at my local Dollar Store.

Theory

The design is straightforward. Components include:

a simple accellerometer to convert steps to pulses
a microcontroller to count the pulses and convert that count to distance
an LCD panel to display results
a button to cycle through the various modes of operation

Teardown

Figure 2: Rear view of the pedometer

As is the case in many EEHP reports, detailed versions of most of the illustrations are available simply by clicking on the figure of interest.

The back of the pedometer is attached via 4 screws, three of which are easily removed. The fourth could only be loosened, allowing the back of the case to be swivelled out of the way.

Figure 3: Open Case

Figure 3 shows the back of the PCB after opening the case, and removing six screws attaching the PCB to the front of the case. The spring mounted lever across the top of the PCB moves up and down in the direction of the arrow as you walk, striking a small post projecting from the back of the PCB. That completes an electrical connection, switching a logic value connected to the MCU.

In effect, the lever acts as a "poor man's accelerometer", converting steps to electrical pulses which can be counted by the MCU.

Figure 4: LCD in place within the case

After removing the PCB, we have the view shown Figure 4. This is the point at which I learned something new. The white rectangle is the rear of the LCD. The LCD has no hard connections to the PCB! It is held in place between the case and PCB by what appear to be three foam pads. Where is the electrical connection?

Figure 5: Micro-photograph of the bottom pad

After pondering awhile, inspiration struck and I pulled out my trusty microscope. Figure 5 shows the resulting closeup view of the bottom foam "pad". In this view, we see alternating bands of conducting and non-conducting material. A little research on the web shows that the "pad" is an elastomeric electronic connector; also known as a ZEBRA connector.

Figure 6: Zebra Connector

The concept is simple. Figure 6 shows a closeup of the ZEBRA connection in place on top of the PCB. You can clearly see that the spacing of the ZEBRA stripes is much less than the pitch of the PCB pads which need to be routed to the LCD. The ZEBRA bridges the gap between PCB and conductive layers on the LCD. The difference in pitch between the ZEBRA and PCB/LCD stripes ensures that each PCB route connects only to the intended conductive pattern on the LCD. Figures 11 through 17 at the bottom of this article provide a graphic illustration of the case/LCD/Zebra/PCB sandwich construction.

Figure 7: LCD

Figure 7 shows the LCD after removing it from the unit. You can just see a hint of the conductive layers along the bottom of the glass.

Figure 8: Front of the PCB

Figure 8 shows the front of the LCD after removing it and flipping it over. The black blob is epoxy used for chip-on-board mounting of the MCU die directly to the PCB. You can see the LCD signals fanning out from the MCU to the pattern that will be in contact with the ZEBRA connector. In the upper right, you can see an interleaved pattern of traces which make up part of the single button control.

Figure 9 shows closeups of various aspects of the button construction. The idea is simple, a conductive layer on the bottom of the rubber button bridges the PCB pattern, completing the circuit.

Figure 9: Button Construction

Figure 10: Back of the PCB

Finally, Figure 10 is another view of the back of the PCB after removing it from the case. I've adjusted the lighting so that the PCB appears semi-translucent, allowing you to see the metal routes on the other side of the board.

Concluding Remarks

I bought this unit with the explicit intent of ripping it apart. But I didn't really expect to learn anything new. Instead I learned of the existence of elastomeric electronic connectors, also known as ZEBRA connectors. Having spent my career in the semiconductor field, I hadn't been exposed to this technology before. So I was pleasantly surprised to discover that there is something to learn from even the most modest of products.

To Learn More

You can improve your learning experience by investigating the following resources:

Re-Use

Michael Stanley & EE HomePage.com, April 2008

The original publication of this report resides at http://www.eehomepage.com/report.php?report=20080420.

This report is licensed under a Creative Commons Attribution 3.0 Unported License.

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Auxiliary Materials

Figure 11: The LCD bracket

Figures 11 through 17 illustrate the case/LCD/ZEBRA/PCB construction. Figure 11 shows a cutout of just the case, which a clear portion through which the LCD can be viewed.