1
\$\begingroup\$

A friend and I are working on designing a power meter to affix to the crank arms on our bikes (similar to Stages, 4iiii, etc). The enclosure will house a small PCB connected to strain gauges, and we will need to either be able to replace coin cell batteries (CR2032) or charge a permanent LiPo battery. I'm leaning towards the permanent battery, but wondering: what kinds of connectors might work for this? Most waterproof, panel-mount bulkhead feedthrough connectors I've found are far too big for this application. We'd really only need to connect two wires (DC +/-), and could use a through-hole part or panel mount style connector. Has anyone found/used something like this? An example of what would work is the connector Garmin uses in their watches. However, I can't seem to find this kind of connector available for purchase, and am guessing it's proprietary.

Relatedly: would a coil for wireless charging interfere with BLE transmission? Wireless charging seems like it would be a perfect solution here, but BLE is critical in this application.

\$\endgroup\$
1

1 Answer 1

1
\$\begingroup\$

There are waterproof (IPX-8) USB-C connectors offered, but you'll have to closely follow specifications for the enclosure and PCB layout to preserve that water resistance. You'll also need a plug to prevent dirt and moisture from getting into the receptacle.

Regarding wireless charging: yes, a wireless charging coil can be compatible with BLE. You just have to make sure the BLE antenna isn't interfered with by the flat sheet of magnetic material that backs up the coil. If the BLE chip or trace antenna is located past the edge of the coil it can work, but some experimentation will be needed.

BLE is very energy efficient, and if you are careful you might be able to get quite long battery life from a coin cell (months, or even a year). Using a retainer on the PCB to hold a coin cell, you could design the device to fit in an enclosure that is sealed with gaskets. A few times a year, it could be opened to replace the coin cell.

\$\endgroup\$
4
  • \$\begingroup\$ I think the USB-C connectors definitely fit the bill size-wise, and the challenge there would be attaching the PCB to the enclosure tightly enough to maintain that seal. I had been envisioning a flange on the inside of the enclosure with a nut on the outside, but that may not exist in such a small profile. That feedback about BLE/charging coil is very helpful. I'm planning to use a chip antenna, so separating that from the coil should be very doable. \$\endgroup\$ Commented Jul 22, 2022 at 23:29
  • \$\begingroup\$ As far as BLE power, the chip series I'm using is excellent at sipping power (Nordic NRF52 series). The challenge is that the system uses a wheatstone bridge with strain gauges to measure crank arm strain, and the resistances for strain gauges are typically 350-1kohms. I plan to disconnect the wheatstone bridge when not making a measurement (via an inline n-channel MOSFET), but am not sure how much "on" time I'll need to get the performance we want. We may also need an accelerometer and potentially an external ADC, which will also draw not insignificant power. \$\endgroup\$ Commented Jul 22, 2022 at 23:33
  • \$\begingroup\$ I really would love to use gaskets and a coin cell, but making this design work for a variety of crank arms (all with different surfaces that are not always flat) makes that challenging. We may be able to 3D print our way around that, though, and it would certainly simplify the design on the electronics side. \$\endgroup\$ Commented Jul 22, 2022 at 23:35
  • \$\begingroup\$ Thank you so much for your feedback, Mark! \$\endgroup\$ Commented Jul 22, 2022 at 23:35

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.