Arduino pro mini - simplify schematics

Hi there, for a project I want to design a pcb which incldues an arduino pro mini (3.3V/8Mhz) and a LoRa Ra-02.
I’m very inexpert with schematics (but I’m trying to learn) and new to KiCad. What I’m trying to do is basically to simplify the arduino pro mini as much as possible but I’m not sure exactly of what I can/can’t remove.

The schematics I have are taken from here.

With my limited understand I’ve removed a few things (pretty much everything) except for:

  • the crystal
  • the C3 component between GND and VCC (no idea what is it for)

This is assuming that I will power directly the ATMega328 with a battery (ATMega Vcc accepts 1.8V - 5.5V from datasheet) likely 3.7V (for the LoRA module to work correctly).

The resulting schematic is below.

My questions:

  • can I remove the C3 component? What is it?
  • Is it safe to use the battery directly to the ATMega or is it better to use something to protect the chip? In which case, what would I use?
  • I see pins for Vcc (18,4,6) and GND (5,3,21) are all connected together. Is there a reason for it or can I just only connect 1 of each to Vcc and GND? for example disconnect pins 5,3 and 6,4, leaving only pin 18 to Vcc and 21 to GND.

Any comment is much appreciated!

FYI at the moment I have the LoRa working well with the arduino pro mini board so I’m basically trying to design a pcb to put everything together without cables for less power consumption (and smaller size as possible).

Thank you!

I think you have looked at this from the wrong perspective.

In general people dont use 'uneccessary' components in designs.

The only components on the schematic and PCB design you linked to that I would ommit are the LEDs and associated resistors.

The only significant change I would make, especially if you need low power operation from battery is to use a low quiescent current low drop out voltage regulator such as a MCP1700. The MIC5205, is a poor choice in this respect.

How are you going to upload programs ?

You cannot arbitrarily wire things the way you want.

You must follow the wiring specifications in the data sheet.

You are missing a pull-up resistor on RESET’

Ceramic .1uF, C3 is for power supply decoupling, you should have one capacitor for each pin that is connected to 5v.

Use a 5V USB power supply if you have a 16MHZ crystal.

srnet: The only significant change I would make, especially if you need low power operation from battery is to use a low quiescent current low drop out voltage regulator such as a MCP1700. The MIC5205, is a poor choice in this respect.

Ok thanks. assuming it is powered by Lipo 3.7V battery, would I be mad not to use a voltage regulator? I mean, the ATMega works between 1.8V and 5.5V if I'm right

srnet: How are you going to upload programs ?

FTDI. I realized now I need either DTR or a reset button, and probably reset button would be more useful..? What do you think?

larryd: You are missing a pull-up resistor on RESET’

Thanks actually I totally missed that.

larryd: Ceramic .1uF, C3 is for power supply decoupling, you should have one capacitor for each pin that is connected to 5v.

Use a 5V USB power supply if you have a 16MHZ crystal.

I'm using 8Mhz, 3.3 chip so..would I still need .1uF C3 for each pin? I mean, if I look at the PCB of that schematic I don't see any track that connects C3 to, for example, pin D5 to D9?

Thank you!!

For reference, suggest you use this cct.

Ok thanks. assuming it is powered by Lipo 3.7V battery, would I be mad not to use a voltage regulator?

Completly mad, you will fry the LoRa device. Thats why I said to follow the schematic you linked to. Your so called 3.7v battery, a LiPo, will be 4.2V when charged, which is way in excess of the 3.6V absolute max for the LoRa device.

FTDI. I realized now I need either DTR or a reset button, and probably reset button would be more useful..? What do you think? Thanks actually I totally missed that.

A reset button can be useful, but not necessary.

C3 is clearly shown as connected between VCC and GND, it is essential. Dont understand why you think this capacitor would be connected to D5 to D9 ?

Follow the schematic you linked to, the comments in post #1 still stand.