Connect 3 volt arduino to 5 volt arduino

Projects General Guidance
1

I have an Adafruit Feather 32u4 RFM69HCW Packet Radio (3.3volt) and an Arduino Yun (5Volt) and I want to have the Feather give a simple one-way signal to the Yun (the Yun does not need to send a signal to the Feather). The signal can be as simple as a 3.3 volt feed to an input digital pin on the Yun.

I am concerned about the voltage differences and want to protect both Arduinos.

What's the best way to do this?

Thanks,
Will

2

A 3.3V output signal is almost always read as a HIGH by a 5V Arduino input, so no conversion is needed in that direction.

In the 5V (output) to 3.3V (input) direction, the simplest way is to use a resistive voltage divider, 1K and 2K resistors.

An active level shifter works in both directions.

Plenty of info on the web: Taking It To Another Level: Making 3.3V Speak With 5V | Hackaday

3

Thanks for the quick response and the link which I will read through.

So can I just connect a 3.3v output digital pin on the Feather to a 5v digital input pin on the Yun and connect ground from both boards together?

Thanks.

4

Your 3.3V signal is going to the Atmega32u4 on the Yun?
https://store.arduino.cc/usa/arduino-yun

The 32u4 will read voltage as low as 0.2 x Vcc + 0.9V as a valid high.

Minimum: 0.2VCC+0.9V (2) (LVTTL)
2. "Min" means the lowest value where the pin is guaranteed to be read as high

With Vcc = 5V, that's 0.2 x 5 + 0.9 = 1.9V.

Table 29-2 of latest Atmega32u4 datasheet.

5

Thanks for the info. I understand your point to be that the voltage from the 3.3v signal may not send the Yun digital pin HIGH.

To answer your question "Your 3.3V signal is going to the Atmega32u4 on the Yun?":

I was going to connect a jumper from the OUTPUT digital pin on the Adafruit Feather 32u4 RFM69HCW Packet Radio 3.3v to an INPUT digital pin on my Yun (it's an original version bought in 2015 if that makes a difference.) I was also going to connect ground from one board to the other.

Most important question - is this safe without risk to either board?

Can I connect the 3.3v OUTPUT digital pin on the Adafruit Feather 32u4 to an INPUT analog pin (A0) on the YUN to detect a signal from the Adafruit Feather 32u4?

Thanks

6

Somehow you got confused. You may directly connect a 3.3V output to a 5V input, and it will work fine. Be sure to connect the grounds.

INPUT analog pin (A0)

There would be no point in using that input as an analogRead() input. Use it as a digital input.

7

No doubt I got confused. Sorry.

Thanks so much for your advice.

Will

8

Actually, to protect the devices in case one is powered and the other not, you should put a 4k7 resistor in series with the receiving device to limit the "phantom powering" current to no more than 1 mA. If this is adjacent to the input, it will not be affected by the cable capacitance between.