ZSRing - the Zephyr Smart Ring

[Kampi]

Jakob and I were discussing an open source smart ring last week and we came to the conclusion that we wanted to give this project a try. So I did some research to get a better understanding of what we really want.

The specs (roughly):

• nRF52832 to support Bluetooth
• nPM1100 battery charger
• BQ51013B Qi-compatible Power Supply
• MAX32664 + MAX30101 HR Sensor
• KXTJ3-1057 Accelerometer
• Battery (unknown - haven´t found a battery yet)

The ring should look similar to this one:

The idea behind the ring is to use it as Heart rate sensor together with the ZSWatch. Maybe we can add a few more sensors later. Unlike the ZSWatch the ring will be more complex to build up by yourself, because we need a flex PCB. The regular flex PCBs from PCBWay aren´t suitable because of the "big" structures, especially with the vias:

So I think we have to use the "Advanced PCB" with the following setup:

• Layers: 2
• Size: TBD
• Material: TG150 FR-4
• Thickness: 0.1 to 0.2 (Is FR-4 with 0.1 bendable?!)
• Traces / Spacing: <6 mil because of the BGA fanout

And we must check how we can design the ring properly in Fusion. If I see it correctly we must work with bending there.

[Kampi]

Might be interesting. Have to check it with PCBWay:

https://www.ipcb.com/pcb-blog/10066.html

For [FR-4 ultra-thin PCBs](https://www.ipcb.com/fr4-pcb.html), we can make them 0.15 mm, 0.20 mm, 0.25 mm and 0.3 mm, with or without solder mask, with copper thicknesses of 1/2 oz, 1 oz, 2 oz and 3 oz.

Thin PCBs with various thicknesses, such as 0.075mm, 0.1mm, 0.127mm, 0.152mm, 0.18mm, 0.2mm, 0.22mm to 0.6mm, come in many different materials. It depends on what materials you need. We can use flexible core materials and FR4 core materials. There is also BT core material. Or other PCB materials.

FR4 thin PCB and flexible PCB


Ultra-thin FR4 PCBs can be as thin as flexible PCBs and can be bent like flexible circuits, but they are stronger and cheaper than the same copper tracks on each board. Of course, flexible materials and BT materials can also be made into very thin PCBs. If the thickness of the circuit board you need is less than 0.1mm, flexible circuits should be used. For 1L FPC, its thickness can be 0.6 millimeters; For 2L FPC, the thickness can be 0.10 millimeters.

[blubbsy]

I don't think the PCB is the biggest problem or the CAD with bending (if fusion is difficult, for sure in Blender it can be done easily). At least I think it can be solved and aren't show stoppers.

When I was shortly thinking about it I was worried more how to design it for multiple ring sizes and came to the conclusion that a minimum ring size has to be defined and designed and components integrated.the bigger ring sizes can then be maybe being generated by scripts (at least the housing).

Regarding the battery I'm only aware(but haven't searched at all) of grepow providing LiPo for rings: https://www.grepow.com/shaped-battery/curved-battery.html / https://www.grepow.com/wearables/smart-ring.html) and I think (as far as I understand) that the battery defines the curvature and ring size.

What is the idea how to flash the software? Aren't some kind of pins needed for debugging or is the idea to do it before assembly and then updating through Bluetooth?

[Kampi]

I don't think the PCB is the biggest problem or the CAD with bending (if fusion is difficult, for sure in Blender it can be done easily). At least I think it can be solved and aren't show stoppers.

Can the output of Blender be used for CAD? I´m not very familiar with Blender.

When I was shortly thinking about it I was worried more how to design it for multiple ring sizes and came to the conclusion that a minimum ring size has to be defined and designed and components integrated.the bigger ring sizes can then be maybe being generated by scripts (at least the housing).

That´s a good point. I haven´t thought about it yet. Maybe it´s also an option to increase / decrease the thickness of the housing. But it´s hard to say for now because I haven´t prepared a design so I´m not sure how much space is needed. A 2-layer board with BGA and 0201 components looks huge in terms of space. Let´s wait and see :)

Regarding the battery I'm only aware(but haven't searched at all) of grepow providing LiPo for rings: https://www.grepow.com/shaped-battery/curved-battery.html / https://www.grepow.com/wearables/smart-ring.html) and I think (as far as I understand) that the battery defines the curvature and ring size.

I checked Alibaba and found a manufacturer who sells batteries in small quantities. The others want me to order 100 at a time...
But this one looks good to me. The question is about the price. Can you ask them?

What is the idea how to flash the software? Aren't some kind of pins needed for debugging or is the idea to do it before assembly and then updating through Bluetooth?

Our current solution are three solder pads as SWD interface because other solutions are too big. This can be used later for some initial programming and then work with BLE Updates. Maybe we can build something like a dock or stand for the PCB to be programmed (i. e. with spring contacts). It depends on the final layout, etc.

[blubbsy]

Can the output of Blender be used for CAD? I´m not very familiar with Blender.

Me neither, but it has a STEP export. so it should be okay. As you stated: Bending would probably also work.

I checked Alibaba and found a manufacturer who sells batteries in small quantities. The others want me to order 100 at a time...
But this one looks good to me. The question is about the price. Can you ask them?

i will write and ask if it is possible to order small quantities or if they could give some samples for such project (at least for prototyping this would postpone the sourcing problem). Directly they will be probably not sell small quantities, but i think this problem solves itself when smartrings and their repair are becoming more common too.
When searching on google for "Curved LiPo" some more producers pop-up, which i will then contact too.

[jakkra]

Fusion 360 can do parametric design, so supporting different ring sizes from cad point of view should be fine.

For development probably good if we do a simple normal pcb for development as the flex one will be a bit hard for that. But pads for SWD should be fine for programming them.

[Kampi]

First draft of the schematics for the Dev-Board. Each function group can be separated by solder jumpers so we can easily test and debug the functional groups.

Schematics.pdf

[jakkra]

My initial review of dev kit:

BQ51013B:

• Route out EN1/EN1, in case we realize useful
• C113 is it needed? Didn't find any about it
• R111 no value

MAX32664/MAX30101:

• Skip one of C112/C114 to save space? Reference only have one? (page 30 in DS)
• R113 seems not needed, I think hub use internal pullups as reference doesn't use. Can save space on ring.
• C116 reference use 10µF instead of 4u7
• Add pin headers connected to SDA/SCL between MAX32664 and MAX30101 as it may be possible to put the accelerometer directly connected here and let the hub act i2c master for the accelerometer. This would give option to connect accel directly here instead to host SDA/SCL.
• Why 1k pullups, would 1k8 be fine, then same as for accel (less models)

TPS6120:

• Use VSYS from nPM1100 instead of VBAT as IN
• C121 in reference is 2u2 and not 4u7

nPM1100:

• JP111 and JP112 not necessary. Those signals only connect to header.
• Leave SHPHLD floating (as otherwise it may automatically exit ship mode or? Thinking it may be good to use from SW:
  "A circuit to pull down SHPHLD is optional (see the Button switch
  shown in the following figure). If no pull-down circuit is present, Ship mode is exited when VBUS is
  connected."

Other:

• I don't understand the Battery-enable, Battery, VBAT with the transistor and mosfet. But that's just because I don't understand circuits using mosfets and transistors well :).

PCB:

• Add silkscreen to all pin in "interface" connector.

[Kampi]

Battery-Enable etc. is a circuit to measure the battery voltage :)

[Kampi]

• Route out EN1/EN1, in case we realize useful -> Done

• C113 is it needed? Didn't find any about it -> It´s from an evaluation board

• R111 no value -> I have to get the correct value. It´s still open

• Skip one of C112/C114 to save space? Reference only have one? (page 30 in DS) -> Reference is using two caps

• R113 seems not needed, I think hub use internal pullups as reference doesn't use. Can save space on ring. -> We can leave it out later. It´s here to have it in case we need it (we can set it to 0R later)

• Add pin headers connected to SDA/SCL between MAX32664 and MAX30101 as it may be possible to put the accelerometer directly connected here and let the hub act i2c master for the accelerometer. This would give option to connect accel directly here instead to host SDA/SCL. -> Good point!

• Why 1k pullups, would 1k8 be fine, then same as for accel (less models) -> Think I got it from the eval board. Reference in the datasheet lists 4k7. So 1k8 should also be fine.

• Use VSYS from nPM1100 instead of VBAT as IN -> Done

• C121 in reference is 2u2 and not 4u7 -> Done

• JP111 and JP112 not necessary. Those signals only connect to header. -> Done

• Leave SHPHLD floating -> DS says If Ship mode is not required, then SHPACT and SHPHLD pins may be tied to AVSS.. Shall we add it? We can not connect it to a push button in the ring so how do we want to enter it?

• Add silkscreen to all pin in "interface" connector. -> Will add it

[jakkra]

Leave SHPHLD floating -> DS says If Ship mode is not required, then SHPACT and SHPHLD pins may be tied to AVSS.. Shall we add it? We can not connect it to a push button in the ring so how do we want to enter it?

I was thinking we use it. For example before shipping a ring, we can from SW put it in ship mode, and then it's exited when it's put on the charger. That was my idea.

[Kampi]

Right that's an option. I will add both signals to the interface connector

[blubbsy]

Battery

From Grepow i got a response (see below). Important question is maybe to have a certain spec or idea how to do it at all at the end. Grepow asked for this:

Capacity:
Nominal Voltage:
Continuous Discharge Current:
Peak Discharge Current:
Do you require single-cell delivery, or should we add connector wires?
Is a protection board needed?

From my point of view a starting point would be this:
Capacity:
~20mAh, tbd over project-time in detail. depending how much the energy conumption is.

Nominal Voltage:
3.6V

Continuous Discharge Current_:
1C (for the moment). I think ~10-20mA current is probably the worst case. if we draw more, than there is probably some configuration tuning necessary or we change to a higher rated cell. Or if it is wanted to charge faster, then we have to increase the C-Rate.

Peak Discharge Current:
would ignore it? it is basically HR-Sensor+MCU (which again are around 10mA).

Do you require single-cell delivery, or should we add connector wires?
I would say with connector, at least i think it is easier to solder it?

Is a protection board needed?
I would say we do the protection circuit on our own to save space within the ring. if they could give us a a reference design, it is okay. at the end it is basically undervoltage, overvoltage, overcurrent and temperature that has to be monitored. The nPM1100 has only thermal protection.

Dear Adrian,

Thanks for your inquiry! This is Joey from Grepow Battery.
Grepow welcomes inquiries about shaped battery projects at any stage, and we are glad to support customers' project needs even in the early phases!

Regarding your inquiry about curved LiPo cells, we have existing solutions available and can also provide custom evaluations based on your requirements. As for the MOQ, our current battery options are quite flexible, as long as we have stock, we can meet the quantity you need for testing.

Please directly provide your requirements for the curved battery, including:

Capacity:
Nominal Voltage:
Continuous Discharge Current:
Peak Discharge Current:
Do you require single-cell delivery, or should we add connector wires?
Is a protection board needed?
Looking forward to your reply!

Best regards,

Joey

[jakkra]

Might not be possible to have a connector as it will likely be too big, I assume soldering on battery directly will consume less space. So we would want just wires.

For protection part, it depends on how small they can make it, otherwise it has to be part of our PCB yes.

Yes 1C (or even less) is fine for both peak and Continuous discharge. Max current will be when LEDs of HR are on, and that sould not be too much.

[blubbsy]

as they are referring to "connector wires" i dont think that they will add a connector. in terms of tolerance compensation, repairability etc. i think the wires would have an advantage and can be removed in the worst case. though i will anyway tell them that we are only talking about standard products they offer and nothing tailor made.

[Kampi]

I would say no wires. Instead we use contact pads and the battery is soldered directly to the PCB. We will not have space in the housing for the wires. Something like this:

[jakkra]

I would say no wires. Instead we use contact pads and the battery is soldered directly to the PCB. We will not have space in the housing for the wires. Something like this:

Yeah if it's like that, it's enough!

[Okayler]

Hey !
I had read in the past your development path for the smartwatch. It was really interesting to read.
I developed sporadically the last 2 years 2 pcb prototypes (v1 and v2) for a smartring.
One also with wireless charging feature (v1).
The second version is without wireless charging circuit. (For the moment)
The second version of the pcb is currently at home at my desk and is waiting for the assembly.
Currently I'm in holiday, so at the evening I can give you more information about my project. ( which parts I used, why I made a second version etc) Maybe we can work together ? Are you interested ? Also I made a little prototype app design for android with flutter but it is really in early stage.
My motivation is to gain some speed in my project and to share knowledge. In the past in noticed for me that I bumped on so many time boundaries. When you try to develop something alone it is really lonely and limiting.

So what do you think? Are you interested ?

[Kampi]

Hi @Okayler,

sure. Please share some details about the project and then we can take a look into it. It can be useful for both parties to share existing know-how.

[jakkra]

@Okayler very interested in any input to save us time and potential issues :)