Wearable Electronics Tips and Tricks

[mudandsnow]

Hello,
I am making some wearable electronics and wanted to share some of the things I’ve learned while also checking with the pros here before I get too far into it.

What tips or tricks do you have for making wearable electronics as small, low power and low cost as possible?

Do you see any potential issues with my plans?

1 - Always on:
Sleeping current draw is so low that I plan to have the MCU always on and put it to sleep when not in use.

2 - Shipping:
Lithium battery shipping restrictions say device should be built or packaged such that it can't be activated accidentally during shipping. Does a powered on and sleeping MCU count as not activated?

3 - Low Battery Handling Options:
3.1 - rely on battery protection PCB to shut down when low.
3.2 – use brown out detect in MCU.
3.3 – measure battery level without using any external circuitry by using Vcc as ADC reference and MCU’s internally regulated voltage as ADC input. When MCU wakes up, check the battery level and if it is too low, go back to sleep. Better explanation here:

https://wp.josh.com/2014/11/06/battery-fuel-guage-with-zero-parts-and-zero-pins-on-avr/

I’ll probably go with option 3 since that allows me to give a low battery warning.

4 - Reset:
For devices with more complicated code I want to include a reset button so it can recover from code failures and reset settings but for the simple ones, I want to skip that. Should I include a reset button for everything?

5 - Regulator:
As long as all my components can operate within the battery’s voltage range I plan to not include a regulator. I’ll make the programmer supply power during programming/debugging so the board is using the same voltage as the programmer.

6 - Oscillator:
Unless I need accurate timing, I’m going to skip the external crystal and use internal oscillator.

7 - Charging Port:
Pads on PCB and pogo pins in the charging dock. I want gold plating for wear resistance but gold plating PCB pads looks expensive so I plan to solder on gold plated coins like this S70-220102045R.

Is there a minimum spacing I should use for the charging port pads to avoid issues with sweat, water, etc getting in there? Are there any other precautions to consider for exposed contacts?

8 - Fuse:
If I use a battery that comes with a protection PCB with its own fuse is there any reason to include an extra fuse?

9 - Programming and Testing:
I plan to use 2 PCBs to hold pogo needles that will be pressed against the PCB for programming and testing. I'd like to make the pogo needles press directly onto pins/pads of my SOIC-8 MCU. Will that cause too much stress on the solder or pins of the SOIC-8?

10 - Battery Type:
I will probably use a rechargeable button cell (LIR2450) or a pouch cell like this:

http://www.batteryspace.com/polymerli-ionbatterymodule37v210mah077wh021arate-prewiredwithpcb.aspx

Pouch cell pro: can come with protection PCB installed.
Pouch cell con: more complicated enclosure design since enclosure has to hold the battery and leave room for 10% expansion.
Are lithium button cells safer than lithium pouch cells?

11 - Battery shipping:
Are lithium button cells easier to ship than lithium pouch cells? 2017 IATA Lithium Battery Guidance Document seems to be a little softer on button cells.

 

[Sparweb]

I have received both types (pouch/button) in packages shipped overseas from Asia to North America, so there seems to be a way to accomplish this. IMO, a good way to render a button cell inert during shipping is to include it in a separate package and make it the purchaser's responsibility to install the battery. I can tell from your questions that this is not preferred. As an alternative, may I mention I have received some devices with their "coin" cells installed, yet rendered inoperative by a plastic membrane slipped between the cell and the contacts. Since the plastic membrane included a long tab that protruded out of the battery compartment, it is a trivial matter for the purchaser to pull the tab of plastic out, thus removing the barrier between cell and contact. The purchaser doesn't have to open the battery compartment to do this. An external inspection reveals when the tab is in place.


STF

 

[mudandsnow]

Thank you for your feedback. I too have received all those things. What I wonder about is when I get wireless headphones with no access to the battery and the power button feels like it is just a momentary button. Does that button just wake up the MCU or does it trigger some kind of hardware that turns on power to the MCU? Either way it doesn't feel like it is disconnecting the battery.

 

[MacGyverS2000]

Does a powered on and sleeping MCU count as not activated?

If it's powered on and draining energy (no matter how little), wouldn't you think that counts as "activated"? Also, don't forget about a bumpy package ride that accidentally hits an ON/OFF button.

measure battery level without using any external circuitry by using Vcc as ADC reference and MCU’s internally regulated voltage as ADC input.

You can't use a changing voltage level (the battery) as a reference... defeats the entire purpose of a reference voltage.

For devices with more complicated code... Should I include a reset button for everything?

If there is any chance whatsoever of user settings getting borked, you had better have a way to reset them to factory defaults. If the battery cannot be easily removed, you also need a way to reset, regardless of code complexity... unless it's nothing but NOPs, something will eventually go wrong with the code.

As long as all my components can operate within the battery’s voltage range I plan to not include a regulator. I’ll make the programmer supply power during programming/debugging so the board is using the same voltage as the programmer.

These are two separate issues. The first is okay, as long as you are absolutely sure the battery can never go above the max ratings of all components... make sure you take "fresh" battery voltages into account. This method can also be wasteful of power, depending upon the circuit, and it may be better to include a switch-mode power supply, it just depends.

The second is only a good idea if you can segregate the rest of the circuit from the piece you are programming... if the remaining circuit draws too much power, your programmer will fail. Otherwise, it's often easier to just let your normal supply voltage power the circuit and let the programmer reset the processor, as necessary.

Unless I need accurate timing, I’m going to skip the external crystal and use internal oscillator.

Depends upon your circuit. For a blinky light thing, this is okay... for a watch, this is a horrible idea.

Pads on PCB and pogo pins in the charging dock. I want gold plating for wear resistance but gold plating PCB pads looks expensive so I plan to solder on gold plated coins like this S70-220102045R.

Is there a minimum spacing I should use for the charging port pads to avoid issues with sweat, water, etc getting in there? Are there any other precautions to consider for exposed contacts?

Gold plating is pretty darn cheap... a lot cheaper than purchasing separate contacts and having them soldered on. You want soft gold for component pads and hard gold for "wear" pads. Spacing is project-/condition-dependent. How many cycles and what pressure they'll be subject to will affect choosing PCB pads versus solder-on pads.

If I use a battery that comes with a protection PCB with its own fuse is there any reason to include an extra fuse?

Fuse in the battery is to protect the battery. If you use a fuse elsewhere, it's to protect some part of your circuitry.

I plan to use 2 PCBs to hold pogo needles that will be pressed against the PCB for programming and testing. I'd like to make the pogo needles press directly onto pins/pads of my SOIC-8 MCU. Will that cause too much stress on the solder or pins of the SOIC-8?

For once or twice, it'll be fine. Not ideal and certainly not industry standard, but acceptable from a hobby perspective. You shouldn't be pressing so hard it distorts the pins anyway... you want to make electrical contact, not pierce the leg like a skewered chicken. The pin pitch is wide enough on an SOIC-8 to make it pretty easy to hit your mark.


Dan - Owner

http://www.Hi-TecDesigns.com

 

[VEBill]

Spring-loaded pogo pins used to be somewhat common for use with JTAG and similar (e.g. In-Circuit Test). For JTAG, the target PCB might have simple flat pads, and the gold plated spring-loaded pogo pins would be in a fixture (or hand held for home use). The target chassis might have a punched out aperture for easy access.

(A person I know might have spent years reflashing his foreign satellite receivers with a hand held JTAG fixture as described above.)

These days it's more common to have a USB socket for charging, and reflash the device via the same USB. It requires reliable boot code to manage. Reflashing the boot code may need JTAG.

You should buy some of those comedy $15 smart watches from eBay for disection. They've often got almost all of the hardware, often just rubbish software.

 

[mudandsnow]

Thank you for the thorough response MacGyver

2 - Shipping:
I'm not sure where they draw the line for calling things "activated" but it seems some basic functions don't count. For example LiPo batteries that come with a protection PCB installed inside the battery pack. That PCB has an IC on it that is always connected and drawing about 10uA.

3.3 - Measure Battery Without External Components:
The nomenclature makes this confusing, ADC reference refers to the top of the ADC scale but that doesn't actually have to be the fixed, known value in the ADC equation. Maybe ADC reference should be called ADC full scale. I'll call it ADCfull.

In this ADC equation, there are 4 variables, if any 3 are known, the 4th can be calculated.

V_ADCin / V_ADCfull = ADCin / 2^[ADC resolution]

Typically V_ADCfull is known and V_ADCin is calculated but it can be done the other way around where V_ADCin is known and V_ADCfull is calculated.

For example V_ADCin = internally regulated 1.1V
2^[ADC res] = 1024

V_ADCfull = 1.1V * 1024 / ADCin

Ex:
ADCin = 341: V_ADCfull = 3.3V
ADCin = 375: V_ADCfull = 3.0V
ADCin = 512: V_ADCfull = 2.2V

I will probably trigger low battery warning when ADCin = 341 and shutdown when ADCin > 375.


7 - Charging Port:
I've been trying seeed lately for my PCBs. Their calculator here is helpful for comparing PCB prices:

https://www.seeedstudio.com/fusion_pcb.html

For a basic PCB that is 20mm x 30mm and 2 coins per board:
Quantity 10: Hard gold = $230
Quantity 10: HASL lead free + 20x coins = 13 + 12 = $25
Quantity 8000: Hard gold = $2000
Quantity 8000: HASL lead free + 16000x coins = 530 + 3507 = $4037

Do these costs look reasonable?

Good catch MacGyver, I underestimated the costs of buying large quantities of those coins. Looks like coins are cheaper at lower quantities, gold plating the PCB is cheaper at higher quantities, specially if more than 2 coins are used per board. I'll go with coins at first and if all goes well I'll remove the coins and get gold plating on the PCB.


9 - Programming:
Once or twice should be plenty for production. You say it doesn’t require a hard press so what’s the problem with doing it a bunch of times?
I’ve considering this:

http://www.tag-connect.com/what-is-tag-connect

but it seems wasteful to include extra pads for programming when I could multi-purpose the pads my MCU is on.

What is the industry standard?

 

[mudandsnow]

Thanks for the reply VE1BLL.

9 - Programming:
I have ordered some cheap stuff to dissect. It took almost 3 months to arrive. They told me after a month it got returned because of lithium batteries and had to re-ship. I've found tear down videos on youtube to be more efficient. Not as much fun though.

USB socket for charging and programming is my goal for the larger items that I want to offer as open source Arduino compatible but for other things I want it as small and low cost as possible and waterproof.

 

[sunnysky]

Lithium cells must be insulated on shipment with a mylar pull tab.

 

[MacGyverS2000]

Looks like my last response never posted, but I had it saved (for just such occasions):

it seems some basic functions don't count. For example LiPo batteries that come with a protection PCB installed inside the battery pack. That PCB has an IC on it that is always connected and drawing about 10uA.

That control circuit is not part of your product design, it's part of the battery itself. Those batteries are also shipped at a very low charge level to prevent the contained potential energy from becoming dangerous kinetic energy (e.g., battery case fragments becoming shrapnel). If that control circuit stops working, it's pretty much because the battery no longer contains enough energy to power it... and it's not possible to put more energy back into the battery (charge it).

You cannot consider this piece of the puzzle as part of your design when asking the question "Is the circuit active?".

ADC reference refers to the top of the ADC scale but that doesn't actually have to be the fixed, known value in the ADC equation.

Unless the ADC has a built-in voltage regulator, the full-scale reference voltage will change with the battery... basic physics. Consider a door that you've agreed to let anyone through that is at least 1/3rd as high as the doorway... if the doorway shrinks in size, smaller and smaller people can be let in. Same issue with your circuit. As the battery voltage drops, so does the height of your doorway. A smaller and smaller voltage will be acceptable as "passable". Eventually you'll hit the lower limit as the entire system scales down, but it won't be where you calculated it to be.

I'll go with coins at first and if all goes well I'll remove the coins and get gold plating on the PCB.

Make sure you're only getting hard gold on the contacts and not the entire PCB... components don't solder well to hard-gold long-term. This requires masking during the hard gold plating process and will require appropriate manufacturing/assembly notes added to the Gerbers.

You say it doesn’t require a hard press so what’s the problem with doing it a bunch of times?

You're putting stress on the chip's pins, as well as mangling them and the pads they're attached to. This eventually leads to failed PCBs. This is fine for prototyping, because who cares if a proto used 100 times eventually fails... but production units should only see this kind of pressure a few times in its life, if ever.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[mudandsnow]

3.3 – measure battery level without using any external circuitry by using Vcc as ADC reference and MCU’s internally regulated voltage as ADC input [...] Better explanation here:

https://wp.josh.com/2014/11/06/battery-fuel-guage-...

For example V_ADCin = internally regulated 1.1V

Unless the ADC has a built-in voltage regulator, the full-scale reference voltage will change with the battery... basic physics

You should read my posts or the link I shared if you want to understand this.