Using divider circuit to make 5V device 3.3v tolerant on I2C bus

So I have an old device which has an Adafruit LCD shield and it’s a 5V device with an LCD, some buttons and a MCP23017 I2C interface to it all. The ESP32 is a 3.3v device and the Arduino Pro Mini I’d used is a 5v device so to get the I2C interface inline to connect with the ESP32, I was thinking of putting 2 10K pull downs on the SCL and SDA lines to take the 4.7K → 5V pullups down to 3.4V.

Does this seem sane?

There’s another device I will be putting on the bus and I will remove its pullups and I know it already works at 3.3V.

I’m still a bit fuzzy on what to do when I have one I2C bus and I have a 3.3V system, one 3.3V sensor and one 5V sensor. I figure I have 2 options on this one:

  1. float the pull-ups on the 5V part and make sure not to put the I2C bus in highspeed mode(driven) and let the 3.3V pullups of the other sensor do the work.
  2. add a 5V->3.3V level shifter on the 2 I2C lines of the 5V module.

But in my current situation, using the ESP32 which has 2 I2C buses so I can put the 3.3V sensor on I2C bus1 and the 5V sensor(with the 10K pull downs) on I2C bus2.

This might be useful?


I would investigate these. I have used them for other purposes but understand they can be used to interface and level shift I2C:


Looking at the spec for the MCP23017 chip, it seems input H is 0.8*VDD which would be 4V when powered by 5V… So in theory, putting pull-ups on SDA/SCL to 3.3/3.4V should not work but in practice it is working. So I should be using level shifters of some sort(chip or transistors).

Since I2C is open-drain( only actively pulls signal down ) and what I have is working(for now), ie my 2nd bus device( the 5V LCD display/button module ) options could be to either lift the 5V leg of each pull-up resistor and bring 3.3V from the ESP32 over to them or get the pull-up voltage into acceptable range for the 3.3V ESP32 and adding the 10K resistors from SDA/SCL to ground can do that. Acceptable esp32 input voltage is VDD+0.3V and max VDD is 3.6V so 3.9V is the input pin max voltage for the esp32 and the 3.4V of the 5V->4.7K->x->10K->Gnd divider circuit is within range.

If I had to put it on the same bus with another 3.3V device I would definitely go with the level shifting.

Seems that the amazon 8 channel breakout is easier to implement than a bunch of discrete pullups :slight_smile:

Been following… Don’t know if you’ve looked at this type of bi-directional interface… I assume data is going both ways… Takes about 6 components for sda/scl. I’ve used this before… it’s from

Just a thought…


Currently all the SDA/SCL pullups are already on the 2 devices so grabbing 2 10K resistors and soldering one end to a common ground and each of the other ends to the SDA and SCL side of the pull-ups was quite a simple task.

I’m also going from an Adafruit ATINY85 Trinket to an ESP32 D1 Mini plus another relay and 2 of these drivers for the relays while trying to do it in the same case.

I had made a little timer device to turn our HVAC fan-only on for varying intervals of 5 minutes per button press. Our existing thermostat only has 3 daily temp settings so in the AM going from 60F to 69F just heats the room air way too much for comfort. I want to stair-step the heating process so I’m creating a Thermostat out of an ESP32 and I’m hoping to shove it inside the existing Fan-only control box since I also want it to have that same feature too. I’m using Omron G5V-1 relay already for the fan(HVAC-G) and will add a 2nd relay for heating(HVAC-W). Doh, I just realized I now either need a switch to change from heating(W) to cooling(Y) or I need another relay and 5v driver circuit… Once again, ‘talking’ about this helps me figure out bugs in my processes. :slight_smile:

![Relay Switch Circuit]

And I just realized that my Thermostat setup is only setup for heating so a cooling mode needs to be added along with the hysteresis. And possibly it’s just a switch to change the hysteresis direction.

What I ended up doing, after learning that the I2C bus drivers are open-collector types, was to use a poker pin to get under the resistors of the Adafruit LCD button assembly and pulled off the 4.7K pull-ups. This is the 5V device. I then put the Adafruit LCD Button device on the I2C bus and then chained the HTU21(temp/humidity) sensor which has 10K pullups to 3.3V. Seems to be working.

for the relay driver I used this design using an NPN 2n2222 transistor.

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