For a project of mine, I needed to connect a Raspberry Pi’s output to an Arduino Uno’s input. The Raspberry Pi’s digital input/output uses 3.3-volt logic while the Arduino Uno digital input/output uses 5-volt logic. How does one convert from one logic level to the other? There are several ways to do this and most methods use a transistor switch controlled by 3.3-volt logic to drive 5-volt logic.

We must first define what constitutes a HIGH and a LOW on the Arduino Uno and the Raspberry Pi. For the Arduino Uno, a digital pin, whether input or output, is considered in a LOW state for voltages below 1.5 volts and in a HIGH state for voltages between 3 and 5 volts. The Raspberry Pi’s digital input and output pins are considered in a LOW state for voltages below 1 volt and in a HIGH state for voltages between 2 and 3.3 volts.

From 3.3 Volts to 5 Volts

In order to convert the Raspberry Pi’s digital output voltage levels to the Arduino’s digital input levels, we can use a transistor switch, similar to the one used to control a relay in a previous post. I will describe two transistor circuits: the logic inverter and the level converter using the transistor’s emitter as input.

Logic Inverter

In the circuit below, we want that if the input voltage is between 0 and 1 volt, the transistor be in cutoff mode, completely off, and the output voltage be between 3 to 5 volts. We also want that if the input voltage is between 2 and 3.3 volts, the transistor be in saturation mode, completely on, and the output voltage be between 0 to 1.5 volts. In other words, if the input is LOW, the output will be HIGH and if the input is HIGH, then the output will be LOW. This circuit is called a logic inverter. It converts logic levels but it inverts logical values. Because the input is considered LOW at or below 1 volt and HIGH at or above 2 volts, we want for the transistor to go from cutoff to saturation mode between input values of 1 volt and 2 volts. To ensure that LOW and HIGH values are met, let’s make the transistor be in cutoff mode for input values at of below 1.4 volts and let it be in saturation mode for values at or above 1.6 volts.

In cutoff mode, there is no transistor base current and the base-emitter voltage (V_be) is at or below 0.7 volts. Hence, the current in R_base and R_bias are the same and the voltage across R_bias is at or below 0.7 volts. When the input voltage is 1.4 volts, the following equation holds.

V_be / R_Bias = (V_in – V_be) / R_Base

0.7 V / R_Bias = (1.4 V – 0.7 V) / R_Base = 0.7 V / R_Base

R_Bias = R_Base

In saturation mode, the voltage across the collector and emitter is 0 volts and collector current is constant at V_cc / R_Load. We want the collector current to be large enough for the transistor to work properly, but small enough to consume the least power possible. Using a 10K resistor, the collector current is 5 V / 10K, or 0.5 mA which is within the BC337-40 bipolar junction transistor working values. According to its specification, the h_fe value, the collector-base current amplification, is 400. The base current is thus

I_b = I_c / h_fe

I_b = 0.5 mA / 400 = 1.25 μA

The current through R_Base is equal to the currents through both R_Bias and the transistor’s base.  When the input voltage is 1.6 volts, the following equation holds.

(V_in – V_be) / R_Base = V_be / R_Bias + I_b

(1.6 V – 0.7 V) / R_Base = 0.7 V / R_Bias + 1.25 μA

Since, as we have seen above, R_Bias has the same value as R_Base, the equation becomes

0.9 V / R_Base = 0.7 V / R_Base + 1.25 μA

R_Base = R_Bias = 160K ≅ 220K

The circuit becomes

The following graph plots the input and output computed voltage values, in blue, and the values measured on an actual circuit implemented with the resistor values in the schematic above, in orange.

As can be seen, logical values are inverted and voltage values correspond to acceptable values for both the Raspberry Pi and Arduino’s HIGH and LOW voltage levels. Notice the shift of the target voltage values slightly to the left of 1.5 volts. This is caused by the fact that we are not using the exact resistor values as computed, because the saturated V_be is not exactly 0.7 volts and the actual h_fe is not exactly 400. Still, the results are more than acceptable, if we wanted an inverted signal.

Using the Emitter as Input

Let’s use the same circuit, but connect the base resistor to the Raspberry Pi’s supply, V_ccP, and the emitter to the Raspberry Pi digital output as in the following circuit.

This is like rotating the previous circuit clockwise by 90 degrees. Now we want that if the input voltage is LOW, below 1 volt, that the transistor be in saturation and the output be at V_in, the Raspberry Pi digital output value.  We also want that if the input voltage is HIGH, at 2 volts or higher, that the transistor be in cutoff and the output be at the Arduino’s supply voltage. Because the permissible voltages for the LOW value on the Arduino, between 0 and 1.5 volts, are higher than those for the Raspberry Pi, between 0 and 1 volt, it is acceptable to use the Raspberry Pi digital output voltages as the Arduino’s input for LOW values.

In this circuit, as it was in the previous one, in cutoff mode there is no transistor base current and the base-emitter voltage is at or below 0.7 volts. Hence, the current in R_Base and R_Bias are the same and the voltage across R_Bias is at or below 0.7 volts. Hence, when the input voltage is 1.6 volts, the following equation holds.

(V_ccP – V_be – V_in) / R_Base = V_be / R_Bias

(3.3V – 0.7V – 1.6) / R_Base = 0.7V / R_Bias

R_Bias = 0.7 • R_Base

For saturation mode, the same reasoning as for the logic inverter holds about the base current to collector current relationship. Since we want saturation of the transistor to occur at 1.4 volts, the collector current is equal to the current going through R_Load.

I_c = (V_ccA – V_in) / R_Load

I_c = (5V – 1.4V) / 10K = 0.36 mA

The base current is thus

I_b = I_c / h_fe

I_b = 0.36 mA / 400 = 0.9 μA

The current through the base is the current through R_Base minus the current through R_Bias. When the input voltage is 1.4 volts, when we want transistor saturation to occur, the following equations hold.

I_b = (V_ccP – V_be – V_in) / R_Base – V_be / R_Bias

0.9 μA = (3.3V – 0.7V – 1.4V) / R_Base – 0.7V / R_Bias

Since, as we have seen above, R_Bias is 0.7 • R_Base, we have

0.9 μA = (3.3V – 0.7V – 1.4V) / R_Base – 0.7V / (0.7 • R_Base)

0.9 μA = 1.2V / R_Base – 1V / R_Base

R_Base = 0.2V / 0.9 μA = 222.222K ≅ 220K

We then compute R_Bias from the cutoff equations

R_Bias = 0.7 • R_Base = 155.556K ≅ 150K

The circuit becomes

The following graph plots the input and output computed voltage values, in blue, and the values measured on an actual circuit implemented with the resistor values in the schematic above, in orange.

In the graph, logical values are converted to the same levels this time, and voltage values correspond to acceptable values for both the Raspberry Pi and Arduino’s HIGH and LOW voltage levels. Notice the shift of the target voltage values slightly to the right of 1.5 volts. This is caused by the fact that we are not using the exact resistor values as computed, because the saturated V_be value is not exactly 0.7 volts and the actual transistor current amplification is not exactly 400. Nevertheless, results are more than acceptable.

Putting it Together

Now that we have a method to convert 3.3-volt logic to 5-volt logic, we can connect a Raspberry Pi 3 GPIO output to an Arduino digital input. As a test, we will program the Raspberry Pi to output Morse Code, read the output using an Arduino digital input and output the value read to the Arduino’s built-in LED. The code within this post can be found on GitHub by clicking here.

Breadboarding

First, let’s have a look at the complete circuit. The following picture depicts how to connect the different parts using a solderless breadboard, jump wires, a BC337-40 transistor, and a 10K, a 220K and a 150K resistor. Through one of the breadboard’s ground rail, we connect the Raspberry Pi and Arduino’s ground pins. The Raspberry Pi’s 3.3-volt supply is connected to the transistor’s base through the 220K resistor. The Raspberry Pi’s GPIO4 output is connected directly to the transistor’s emitter. The Arduino’s 5-volt supply is connected to the transistor’s collector through a 10K resistor. The Arduino’s digital input 8 is connected directly to the transistor’s collector.

Morse Code Generator in Python

We have seen the morse code generator in a previous post. I have ported the Morse Code program to Python 3 for it to be run on the Raspberry Pi. I used IDLE (Integrated Development and Learning Environment) to code and run the Python 3 code on the Raspberry Pi. If you are new to the Raspberry Pi and want to set it up, please visit “Get started with Raspberry Pi” on the www.raspberrypi.org site.

# Morse Code Generator Python Program that translates a
# text string to Morse Code and outputs it to a GPIO pin.
# This program is used as part of a demonstration to show
# connectivity between a Raspberry Pi and an Arduino UNO.
# It is associated with the Raspberry Pi Speaks Arduino
# blog post on https://lagacemichel.com
# MIT License
# Copyright (c) 2019, Michel Lagace

import RPi.GPIO as IO
import time
UNIT_TIME = 0.1
BOARD_PIN = 7
# Characters to be encoded
characters = "abcdefghijklmnopqrstuvwxyz"

# Morse code sequences for each character
codedCharacters = [
".-", "-...", "-.-.", "-..", ".", "..-.", "--.", "....",
"..", ".---", "-.-", ".-..", "--", "-.", "---", ".--.",
"--.-", ".-.", "...", "-", "..-", "...-", ".--", "-..-",
"-.--", "--.." ]

# Setup the board. Digital port LED_BUILTIN in output mode
def setup():
    IO.setwarnings(False)
    IO.setmode (IO.BOARD)
    IO.setup(BOARD_PIN,IO.OUT)

# Function to output a dot: one unit on, one unit off
def outputDot():
    IO.output(BOARD_PIN,1)
    time.sleep(UNIT_TIME)
    IO.output(BOARD_PIN,0)
    time.sleep(UNIT_TIME)

# Function to output a dash: three units on, one unit off
def outputDash():
    IO.output(BOARD_PIN,1)
    time.sleep(UNIT_TIME*3)
    IO.output(BOARD_PIN,0)
    time.sleep(UNIT_TIME)

# Function to output a single character
def outputCharacter(c):
    # Find index of character to encode
    index = characters.find(c)
    # Ignore unencodable characters
    if (index >= 0):
        # Encode morse codeand output it
        code = codedCharacters[index]
        for ch in code:
            if ch == '-':
                outputDash()
            else: # if not '-', must be '.'
                outputDot()
        # Wait 3 units at the end of the letter
        # (2 units assuming previous dot or dash)
        time.sleep(UNIT_TIME*2)

# Function to encode a whole string
def sentence (text):
    # Output each character in turn
    for ch in text:
        # Only lower-case characters are encoded
        if ch != ' ':
            outputCharacter(ch.lower())
        # Spaces are encoded as 7 units,
        # (4 units assuming a previous character)
        else:
            time.sleep(UNIT_TIME*4)

# Function looped indefinitely
def loop():
    sentence("Mikes Electro Shack")
    time.sleep(UNIT_TIME*25) # Wait 4 spaces at the end

# Main program. Setup board then loop forever
setup()
while(True):
    loop()

Raspberry Pi Reader on the Arduino

The code on the Arduino is quite simple. It loops forever, reading the digital input connected to the Raspberry Pi, writes the value to the built-in LED, then waits 10 milliseconds befor starting over.

// Main RaspberryPiReader sketch
// Program that reads a digital input from a Raspberry Pi
// GPIO pin and outputs its value to an Arduino digital output
// pin. This sketch is used as part of a demonstration to show
// connectivity between a Raspberry Pi and an Arduino UNO.
// It is associated with the Raspberry Pi Speaks Arduino
// blog post on https://lagacemichel.com
// MIT License
// Copyright (c) 2019, Michel Lagace

// Define input and output ports
#define INPORT 8    // Input port connected to Raspberry Pi
#define OUTPORT 13  // Output port to built-in 

// Time to wait in milliseconds between samples
#define SAMPLE_DELAY 10

// Setup the board.
void setup() {
    pinMode(INPORT, INPUT);
    pinMode(OUTPORT,OUTPUT);
    digitalWrite(OUTPORT,LOW);
}

// Repeat forever
void loop() {
    // Read value from Raspberry Pi
    bool value = digitalRead(INPORT);
    
    // Output value to built-in LED
    digitalWrite(OUTPORT, value);
    
    // Wait before next sample
    delay(SAMPLE_DELAY);
}

Download the sketch on the Arduino then run the Python program on the Raspberry Pi. You will notice Morse Code being output by the Arduino’s buit-in LED.