Some slow progress, but it’s coming along. The schematic is about 90% done, but the PCB has barely been touched. I’ve just been doing some trials making sure it all translates well in Kicad from schematic to PCB layout. Below you can see the footprint layout compared to a CR2032. This is quite small. The trace PCB antenna will come off from the integrated balun from ST micro to the right. You can see the peculiar five dot layout next to the nRF51422 QFN footprint. This is ST micros integrated Balun. It’s 2mm^2.
To give you an idea on where this is going to reside I have added the picture below. The area just behind / below the chainrings seems sufficient for the circuit board. However two major hurdles.
- Battery Placement
- Antenna Placement
Coin Cell for scale. Note: I’m showing a CR2032, but I plan to use a CR2450 620mah cell. Slightly bigger physically.
Once I get the schematic and board close to finalization I will order the parts from Digikey. Most of the parts are very inexpensive except the ADS1248. This is considered the heart to the design in my mind.
When I order strain gauges I’ll also be ordering platinum RTDs. Each RTD will sense one of 3 strain gauge locations. (Top and Bottom for Right leg sensor, one for left leg sensing).
This is going to be close to a “NEVER ZERO” power meter. Rotor is pretty much there from what I’ve heard, but mine will be priced much more competitively once out of the beta. FTSH header include for reprogramming / hacking via the CMSIS-DAP.
Hoping to have the rough PCB by the end of the weekend, and a 3D concept of the enclosure which will be 3D printed. The enclosure will likely just resemble a circular battery holder by the crank arm and a simple covering over the board and strain gauges. Nothing fancy.
Have you got an idea of how you will do the connectors?
ReplyDeleteThese often take up a lot of the space, and also contribute a substantial amount of cost to the BOM.
I'm using small SMD pads. The Strain gauges will have 36 gauge wire from them, which can fit easily soldered to a 1.5mm x 2mm pad. This is to keep size small and to reduce overall height.
DeleteFantastic, just let me know if you need help with things.
DeleteI have stalled with software as its pretty hard to do much more without access to hardware (without being able to run on the device, I'll just create bugs). I could see you were trying to get hardware out the door, and my questions and queries were holding you up from doing this.
Glad to hear you have made progress on this front.
For the layout/enclosure. I would try to go with the parts on a single PCB side, with the coin cell on one side, and all the parts on the other. I would try to get the PCB to be as close to the size of the coin cell as possible. This would be optimum.
What size resistors are you using?