Circuit design verification: Added CO2 sensor, 5V sensor rail and USB power input


I want to add the possibility for powering the Nora using an external AC to 5V power adapter with DC jack. I will still have 2xAA batteries installed, but these should not be used when the Nora gets power from the external 5C adapter.

The Nora uses Torex XC9128B DCDC step-up converter for its 3V power supply.
From the datasheet i found the circuit posted below.

If I modify my nora to do as this circuit, will my 5V supply then be regulated to supply imp with 3V?
And other than that the resisters must be able to handle bias current, I am unsure of how to size RAEN1 and RAEN2.

Here are datasheet and schematics

No, this doesn’t regulate the input. You’d need to put a 3.3v LDO between the 5v input and the “USB” point in the above schematic. RAEN1/2 need to be adjusted so that the switcher will turn off when external power is supplied (ie AEN/ will go above its threshold).

Ok so it’s been a while since Hugo guided me in regards to the external PSU.
Since then I have done some more changes, which I would like some additional eyes on.

I have attached schematic, and would greatly appreciate if someone would take a look at it. The stuff I have been fiddling with is the blocks in the lower left corner of the .pdf

  • Added option for USB power with 3.3V LDO
  • Added a 5V regulated rail for 5V sensors using another Torex XC9129.
  • Added a power gate for the 5V_sensors
  • Added mounting for an optional Senseair S8 CO2 sensor

Why is output from the first XC9129 called 3V0 on original Nora files, according to the XC9129/XC9128 datasheet, the configuration with 270k, 43k and 12pF should give 3.3V ?
Vout = 0.45*(270k + 43k) / 43k = 3.276V (from datasheet)
What am I missing here?

I feel like the way i do power management could be done smarter/simpler, but I dont know how. Right now i use two XC9129 (one for 3.3V and one for 5.0V) and one LDO from the USB to 3.3V.

Later I will add 3.5mm stereo TRS jacks for optional external analog sensor inputs.

Some comments:

  • You should not chain the boost converters. Both of them should just be connected to the battery, otherwise you’re chaining the efficiency loss and loading the 3v rail.
  • It looks like the sensair sensor is going to be outputting 5v TTL serial. You should add a zener clamp to this on the way into the imp to ensure the input line doesn’t go above 3.3v (say, 10k series resistor and a 3.3v zener on the imp side).
  • You should check the sensair will be ok with 3.3v as a logic 1 on its uart input. Standard 0.7xVCC=3.5v which would imply you’d need a buffer.

Hello again

Thank you for the reply Hugo, again very helpful!
I have been working on a few other projects meanwhile, so haven’t finished this up yet.
However I changed the PSU configuration as you suggested.

In regards to the sensor voltages I checked with our Swedish friends at Sensair and got the following reply:
“The S8 UART is running 3.3V CMOS levels. Actually, there is an onboard regulator that supplies the microcontroller with 3.3V.”

And by opening up a which also uses Sensair S8, i could see that I had made the footprint of the Sensair S8 mirrored, so I fixed that too.

Bonus info::slight_smile: - Since the S8 is big compared to Nora standards, I am thinking about using the ‘big brother’ to the current nora case.