PCB Adapter: LEGO Bricks to Qwiic Sensors

This guide will walk you through the steps and materials needed to fabricate a PCB that can connect your LEGO Brick to Qwiic Sensors to expand the possibilities of your projects!

• 1 Dual Bidirectional I2C Bus and SMBus Repeater (PCA9515BDGKR)
• 2 10kohm resistors SMD (CHP0805AFX-1002ELF)
• 1 Qwiic connector SMD (PRT-14417)
• Qwiic cables (at least one, any length)
• Qwiic sensors (at least one)
• 1.6mm thick acrylic
• Super glue
• Solder paste
• Heat gun
• Tweezers (optional)

Find and use the qwiic_gerb zipped folder that contains the gerber files for this PCB in this Github repository to download and order the printed circuit boards. Ensure the thickness is 1.6mm when ordering.

Use the qwiic_laser dxf file in this Github repository to laser cut the second layer of the adapter. Ensure the acrylic is 1/16in (~1.6mm).

Use superglue to glue the final laser cut acrylic you obtained in step 2 to the PCB. Ensure they are properly lined up, otherwise it will not fit in the LEGO Brick. This should make the thickness 3.2mm, which is the needed thickness for the LEGO Brick's socket.

Due to the small size of the electrical components that need to be soldered, solder paste allows for the most ease.

Place solder paste on the pads on the PCB and place the components on the PCB (ensure the chip is oriented so the 2A text on top of it is on the side closest to the Qwiic connector) Using the heat gun, heat up the solder paste to solder all components on the PCB.

Connect the adapter PCB to Port A in the LEGO Brick, and attach the Qwiic sensor to the PCB, as depicted in the photo.

Make sure to have Thonny Software downloaded on your computer and save the following script as softwarei2c.py onto the LEGO Brick using Thonny:

from hub import pins as Pin
from hub import uart
import time


import struct


class SoftwareI2C:
    SW_I2C_WAIT_TIME = 40  # Microseconds to wait, adjust based on your needs


    def __init__(self, scl_pin, sda_pin):
        
        self.en =  Pin.init(0, Pin.EN, Pin.OUT)
        self.en.value(1)
        
        self.scl = Pin.init(0, scl_pin, Pin.OUT)
        self.sda = Pin.init(0, sda_pin, Pin.OUT)
        
        self.scl.value(1)
        self.sda.value(1)


    def scl_high(self):
        self.scl.value(1)


    def scl_low(self):
        self.scl.value(0)


    def sda_high(self):
        self.sda.value(1)


    def sda_low(self):
        self.sda.value(0)


    def sda_input(self):
        self.sda = Pin.init(0, Pin.RX, Pin.IN)

    def sda_output(self):
        self.sda = Pin.init(0, Pin.RX, Pin.OUT)


    def delay_us(self, us):
        time.sleep_us(us)


    def start(self):
        self.sda_high()
        self.scl_high()
        self.delay_us(self.SW_I2C_WAIT_TIME)
        self.sda_low()
        self.delay_us(self.SW_I2C_WAIT_TIME)
        self.scl_low()
        self.delay_us(self.SW_I2C_WAIT_TIME * 2)


    def stop(self):
        self.sda_low()
        self.scl_high()
        self.delay_us(self.SW_I2C_WAIT_TIME)
        self.sda_high()
        self.delay_us(self.SW_I2C_WAIT_TIME)


    def check_ack(self):
        self.sda_input()
        self.scl_high()
        ack = not self.sda.value()
        self.scl_low()
        self.sda_output()
        self.delay_us(self.SW_I2C_WAIT_TIME)
        return ack


    def write_byte(self, byte):
        self.scl_low()
        for i in range(8):
            self.sda.value((byte >> (7 - i)) & 1)
            self.delay_us(self.SW_I2C_WAIT_TIME)
            self.scl_high()
            self.delay_us(self.SW_I2C_WAIT_TIME)
            self.scl_low()
        return self.check_ack()


    def read_byte(self, ack=True):
        self.sda_input()
        byte = 0
        for i in range(8):
            self.scl_high()
            byte = (byte << 1) | self.sda.value()
            self.scl_low()
        self.sda_output()
        if ack:
            self.sda_low()
        else:
            self.sda_high()
        self.scl_high()
        self.delay_us(self.SW_I2C_WAIT_TIME)
        self.scl_low()
        self.sda_high()
        return byte


    def scan(self):
        found_devices = []
        for address in range(0x01, 0x78):  # Valid I2C addresses
            self.start()
            if self.write_byte(address << 1):  # Shift address for write mode
                found_devices.append(address)
            self.stop()
        return found_devices


    def writeto(i2c, address, data):
        """
        Write data to an I2C device.


        :param i2c: SoftwareI2C object instance.
        :param address: 7-bit I2C device address.
        :param data: Bytearray or list of data to write.
        """
        i2c.start()
        if i2c.write_byte(address << 1):  # Shift address for write mode and send
            for byte in data:
                if not i2c.write_byte(byte):
                    print("Error: No ACK received after data byte.")
                    break
        else:
            print("Error: No ACK received for address.")
        i2c.stop()


    def readfrom(i2c, address, num_bytes):
        """
        Read data from an I2C device.


        :param i2c: SoftwareI2C object instance.
        :param address: 7-bit I2C device address.
        :param num_bytes: Number of bytes to read.
        :return: Data read as a bytearray.
        """
        data = bytearray()
        i2c.start()
        if i2c.write_byte((address << 1) | 1):  # Shift address for read mode and send
            for i in range(num_bytes):
                ack = i < num_bytes - 1  # ACK all but the last byte
                data.append(i2c.read_byte(ack))
        else:
            print("Error: No ACK received for address.")
        i2c.stop()
        return data


    def writeto_mem(i2c, device_addr, register_addr, data):
        """
        Write data to a specific register of an I2C device.


        :param i2c: SoftwareI2C object instance, the custom I2C implementation.
        :param device_addr: The 7-bit address of the I2C device.
        :param register_addr: The register address within the I2C device where data will be written.
        :param data: The data to write (as a bytes object or list of bytes).
        """
        i2c.start()  # Start I2C communication
        # Send the device address in write mode
        if not i2c.write_byte(device_addr << 1):
            print("Error: No ACK received for device address.")
            i2c.stop()
            return


        # Send the register address
        if not i2c.write_byte(register_addr):
            print("Error: No ACK received for register address.")
            i2c.stop()
            return


        # Write the data bytes
        for byte in data:
            if not i2c.write_byte(byte):
                print("Error: No ACK received after data byte.")
                break


        i2c.stop()  # Stop I2C communication


    def readfrom_mem(i2c, address, register, num_bytes):
        """
        Read data from a specific register of an I2C device.


        :param i2c: The SoftwareI2C object instance.
        :param address: The 7-bit address of the I2C device.
        :param register: The register address within the device from which to read.
        :param num_bytes: The number of bytes to read from the register.
        :return: A bytearray containing the data read from the device.
        """
        # Start the I2C communication and send the device address in write mode
        i2c.start()
        if not i2c.write_byte(address << 1):  # Shift address for write mode
            print("Error: Device not acknowledging write mode.")
            i2c.stop()
            return


        # Write the register address to read from
        if not i2c.write_byte(register):
            print("Error: Device not acknowledging register address.")
            i2c.stop()
            return


        # Repeated start to switch to read mode
        i2c.start()
        if not i2c.write_byte((address << 1) | 1):  # Shift address for read mode
            print("Error: Device not acknowledging read mode.")
            i2c.stop()
            return


        # Read the specified number of bytes
        data = bytearray()
        for i in range(num_bytes):
            ack = i < num_bytes - 1  # ACK for all but the last byte
            byte = i2c.read_byte(ack)
            data.append(byte)


        # Stop the I2C communication
        i2c.stop()


        return data

Run the following code to scan for I2C devices:

from hub import pins as Pin
from hub import uart
import time
from softwarei2c import SoftwareI2C


i2c = SoftwareI2C(scl_pin=Pin.TX, sda_pin=Pin.RX)  # Example GPIO pins
    
print("Scanning I2C bus...", i2c.scan())