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Questions relating to designing PCBs
By da40flyer
#201031
Hello,

I have been using the Sparkfun ProMicro and the BNO080 breakout board with success. I would now like to try my hand at creating a consolidate PCB with these chips. This is my first attempt at doing this. I am using Eagle for this, again a first time user.

My Goal: a single compact PCB with the Atmega32u4 & BNO080 connected together via I2C, an ICSP header for programming the 32u4, a buzzer as the output, and two 2032 coin cells to drive it. This will be a 3.3V board.

Would someone be willing to review my schematic for errors/omissions?
https://drive.google.com/file/d/1AgdHB9 ... sp=sharing

Thank you in advance,
David
By jremington
#201042
It is extremely inefficient to use 2xCR2032 cells in series with a linear 3.3V regulator. One of the cells is effectively wasted as heat.

Most 3.3V electronics work fine on a single CR2032 cell with no regulator, provided that the battery capacity and current limitation of a few mA is taken into account.
By jremington
#201043
Note: the BNO055 and 32U4 consume about 12 and 6 mA current respectively, so expect less than 10 hours of total continuous, normal waking mode operation (probably much less with a poor quality CR2032).
By da40flyer
#201050
@jremington, thank you for your help! I did not know this regarding the two 2032's.

Battery selection has been a challenge. I spent two days last week scouring the internets learning about batteries for projects like this. I have little space and specifically want a replaceable battery (versus lipo recharging). I considered a 9V box battery, but lots of comments regarding their inefficiency, especially when used with a linear regulator and 3.3V needs. Opinion on this? I don't need really long life. 200 mAh battery would be fine, and I believe even a poor 9v would provide twice that.

I tried to run my bread-boarded prototype with a single 2032, but it would not run... hence ending up with two in series.

From your comments, is this due to the fact that the single 2032 battery was being run through the Pro Micro regulator along with running the on-board LED's? I can try removing the regulator and led's from one of my arduino's to see if it will run with a single 2032.
By jremington
#201053
There are many possible reasons why your test did not work, but with a fresh cell, battery voltage should not be the problem. You should get the breadboard circuit working with a single cell and evaluate its performance before designing a PCB.

Coin cells are designed for very low current applications, but can work for short periods at higher draw. You might find this discussion useful, among many others.

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