HELLO I have already connected with I2C 2 Arduino's all is working with comunication on Bus I2C. My Setup is : MASTER with LCD I2C , RTC1037 SLAVE some sensor not I2C

I allready put 2 pullup resistor 10Kohm on SDA and SCL

I need to switch off the slave arduino but when i switch off the Slave board the Master Arduino stop working.Why? It's possible to continue working with master board and the slave board switch off? Thank's for help

  • Define "shut down". Also, define "stop working"...
    – Majenko
    Commented Feb 28, 2017 at 11:14
  • switch off :) sorry for my english Commented Feb 28, 2017 at 11:18
  • "switch off" as in remove the power completely? Then you may need to isolate the I2C bus from it as well.
    – Majenko
    Commented Feb 28, 2017 at 11:24
  • Yep, remove power completely,how to isolate the i2c bus? 2 relay 1 for sda 1 for scl when i need to switch off ? Commented Feb 28, 2017 at 11:28
  • Relay is kind of overkill and power hungry. MOSFETS (as Adafruit use in the I2C level shifters) are a better choice. See Adafruit's app note here: cdn-shop.adafruit.com/datasheets/an97055.pdf - ignore the level shifting as you would have both sides the same voltage, but section 2.3 is of interest. Your slave would be the 3.3V side of the shifter (but at 5V not 3.3V). Paragraph 2.3.4 tells you what you need to know for this application.
    – Majenko
    Commented Feb 28, 2017 at 11:36

1 Answer 1


You need to isolate the slave device from the I2C bus since it is causing interference with the signals.

Adafruit have published a good application note about level shifting in I2C busses using MOSFETs, and it also contains a bit about bus isolation:

2.3.4 Isolation of the powered-down “Lower voltage” section.

An additional feature of the level shifter circuit in figure 2 is the isolation of the “Lower voltage” section when VDD1 is switched off. In that case VDD1 is about 0 Volt and the MOS-FET’s [sic] are switched off because VGS is below the threshold voltage. The “Higher voltage” section is not hindered and stays operational. To assure a noise margin, the MOS-FET’s should have a minimum threshold voltage VGS(th) of e.g. 0.4V and VDD1 must stay below this value. The isolation feature can also be applied if no level shifting is required, VDD1 and VDD2 may have the same value, e.g. both 3.3V or both 5V.

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