I’ve been mainly working with 5v projects, namely with the Arduino Pro Mini and Nano, etc. I have now decided for certain reasons that I need to switch from a 5v to a 3.3v board. So I bought an Arduino Pro Mini 3.3v 8Mhz and a FTDI basic 3.3v as my Arduino FTDI 5v will not be suitable.
My problem is this, I have just one sensor that requires 5v to power it, but will work fine on a 3.3v I2c bus. I have been told I can send the initial 5v from the computer through the FTDI chip to the raw pin of the Arduino by soldering the bridges on the back of the adapter, so I can power my 5v sensor, and more importantly set it up and debug via the serial monitor.
If I do this, wont I be converting my 3.3v adapter to a 5v one? I’m a little confused and would like to be told if the above is possible (which will be fantastic!) and how, because if I solder the bridges to 5v, surely I will damage my 3.3v Arduino board?
Thanks
If you are using a Pro Mini like this one https://www.sparkfun.com/products/11114, power it and your sensor with 5V via the “raw” input power pin. Just make sure that the I2C lines are pulled up to 3.3V, not 5V. For programming, you will need to make sure that the output of the FTDI adapter on TX, etc. is 3.3V.
Thanks for the reply.
So power the board with the 3.3v FTDI and at the same time send 5v to the raw pin to supply the sensor?
I was hoping to wire the 5v sensor (DS1307 RTC) like this:
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The voltage regulator would be 5v (with 0.1uf capacitors in and out) and the Arduino board is the same one as the link you provided.
So how would I make sure the I2c lines are pulled up to 3.3v and not 5v? do I remove the 4.7K resistors from the RTC and place them somewhere else?](Photo Storage)
I guess I don’t really understand what you want to do. If the RTC needs 5V, then you have to supply that. You can also use that 5V to power the 3.3V arduino through the “raw” dc input pin. For I2C communications, you need to have two pullup resistors to Vcc. For a 3.3 V arduino, the pullup resistors have to be connected to a source of 3.3V. You may have to remove them from the RTC and install them on the arduino input pins. The data sheet for the DS1307 states that the minimum voltage for a “high” on input is 2.2 V, so this will work fine. This tutorial shows how it is done for the 3.3V Raspberry Pi http://learn.adafruit.com/adding-a-real … ng-the-rtc
Now, do you want to send information back to the PC? If so, you will need a 3.3V FTDI or other serial adapter. You can do all these things (power the arduino, RTC and provide communications) with this serial adapter from Pololu, which provides both 3.3 and 5 V power: http://www.pololu.com/catalog/product/1308
You could also power the entire project on 3.3 V, using a 5V boost converter module to supply the 5V for the RTC. An example is http://www.pololu.com/catalog/product/2564 In that case you still need to make sure the pullup resistors are connected to 3.3V.
Thanks,
I understand now regarding the I2c bus now and realise I’ll have to solder some 4.7K resistors externally, so thats fine. Also to add, I have an RTC breakout board that has no resistors soldered to is as I was using it for a short while with another I2c sensor that only worked on a 3.3v bus, so I will use that module again with this setup 
I attached the diagram as I will be powering the project (mobile) using a 9v battery which will run through a 5v regulator (with capacitors) and go into the RAW input so the board has a regulated 3.3v from the 5v and more importantly my RTC gets its initial 5v power.
This is what I want to do
BUT, this is the problem - I want to be able to connect everything including the RTC to the computer to I can see it working in harmony with everything else e.g. SD card, light sensor, etc. Sadly I cant seem to find a UK supplier yet that stocks both Pololu products, so…
Is it safe for me to connect my SparkFun 3.3v FTDI to my Arduino for communication AND send that 5v regulated power from the 9v battery for the RTC into the RAW pin concurrently without blowing anything up? Basically reference the diagram as thats how the power side of things will be, and can only be wired up so far.
That will work. Your diagram doesn’t show the FTDI Basic breakout, but it does have the user-selectable option to provide either 3.3V or 5V power for the entire circuit, so you wouldn’t need the regulator and battery.
Hint: dump the Fritzing diagrams. They may look cool but are for beginners, are error-prone and can be very confusing. Schematic diagrams are much easier to read and less prone to errors. With a little practice using any of the freely downloadable programs, I think you will find schematics easier and quicker to produce. I use LTSpice.
Cool, ok will have to pick up where I left off with Eagle or the software you suggested (as long as its compatible with osx).
Yes sorry the FTDI would be there as you figured. So if I soldered the jumper on the back of the FTDI to 5v I wouldn’t even need the external battery and regulator? Thats whats been confusing me all along is that on another forum someone said if I used mu official Arduino FTDI 5v I’d harm the 3.3v board. But wouldn’t I be doing the same if I switched my SparkFun FTDI basic from 3.3v to 5v? Or is this not the case?
Btw I found one of my UK supplies stocks the Pololu step-down/up module for £5 so I’ll get one anyway to add to my arsenal
Yep, according to the schematic of the Sparkfun FTDI Basic 3.3V board, the 3.3/5V jumper option to the “power” output also supplies VCCIO, so that is a problem to solve. A simple resistive voltage divider (e.g. 22K + 33K) from TX on the FTDI to RX on the arduino, and a direct connection from TX (arduino) to RX on the FTDI board will allow you to safely use the 5V IO option. Google “3.3V level convertor circuits” for more options.
Another possibility is to disconnect VCCIO from “power” and reconnect it to the built in 3.3 V regulator while using 5V for power output. You might ask SF about how to do that, or inspect the board closely to see if cutting a trace and rewiring will work. I use Pololu’s version of the board which gives more options.
I see what you mean. I’ve tried searching all the UK suppliers and can’t seem to find the Pololu’s version of the board. I thought I would try for now while waiting for my Pololu step-up converter, separately powering one side of my breadboard rails with an external 9v regulated to 5v (YwRobot). The other rail powered using the Vcc and Gnd from the 3.3v Pro Mini. Before I could even try my RTC I ran into problems trying to read the temperature and humidity from my AM2302 (DHT22). I noticed it has a power rating between 3.5v ~ 5.5v, so immediately I powered it using the 5v rail opposite the 3.3v rail and still it failed by delivering both zeros for temp and humidity. I immediately replaced my Arduino 3.3v pro mini with a 5v version I had lying around and it came to life. I also powered and ran the sketch on my Arduino Uno and again it was working.
Is this problem possibly because the regulated 5v rail is not part of the Arduino’s main circuitry?
I would of started a new thread but it seemed more relevant I continued with this thread, as I probably would of had the same problem with the RTC.
This is the Pololu adapter to which I was referring. http://www.pololu.com/catalog/product/391
The I/O is 3.3V but can also tolerate 5V directly. The adapter supplies both 5V and 3.3 V regulated for external devices.
Different arduinos provide different regulated output voltages. The 3.3V versions do not provide regulated 5V, but of course can run from 5V on the raw input.
If you want to “roll your own” low voltage RTC, it would be very simple. Maxim now provides a new line of chips similar to the DS1307 (DS1390-1394) that run on lower voltages. They provide free samples.
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