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I have three light sensors TSL2591, of signal I2C, that I need to connect (together) to an Arduino Mega. However, I'm not very sure of how share the SDA and SCL pins of the three sensors and I don't know how to change their address for the Arduino to read and show the data of each sensor.

I have already read sensor's datasheet but there is not much information about this.

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    ` there is not much information` ... what information is there? ... also, please add a link to the data sheet to your question (do not put the link into comments) – jsotola Jul 3 '18 at 1:27
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Looking at the datasheet, the TSL2591 does not appear to have a comfigurable address mechanism, therefore you are most likely obliged to use an I2C Multiplexer.

One example of such a device is the TCA9545A, Low Voltage 4-channel I2C and SMbus Switch With Interrupt Logic and Reset Functions, that can split one I2C line into 4 buses.

TCA9545A pinout

They are very easy to use and the pinout (and datasheet) are self explanatory.

Don't forget that you will need 3k3 Ω resistors on the INT, RESET, SDA and SCL lines from the Arduino and on each of the INTx, SDx, and SCx lines from the TCA9545A:

TCA9545A Typical Application Schematic

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    Adafruit has a module with the TCA9548A. The advantage is that these multiplexer chips will also do level shifting for sda and scl. That is needed because the arduino mega is a 5v board and the sensor is a 3.3v sensor. An arduino mega has 10k pullup resistors. Pullup resistors for all the sensors should still be added if they are not on the tsl2591 module. – Jot Jul 3 '18 at 6:11

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