I have the Sparkfun MPR121 I2C Capacitive touch sensor breakout board

I have read that it uses 3.3 V, I know that I can't just connect up the 3.3 V power supply output from the Arduino as the communications lines would require 3.3 V as well. How do I connect up the other lines?

I have found a tutorial at Sparkfun's website. The 3rd diagram using the diode - would that be the best way to interface the sensor to the Arduino?

Any ideas or information would be great, my knowledge is pretty low.


2 Answers 2


You can do it using exactly two transistors, which is pretty slick.

alt text

This comes straight from a Philips application note.

  • \$\begingroup\$ That Phillips appnote is practically a classic. \$\endgroup\$ Commented Jan 18, 2011 at 4:08
  • \$\begingroup\$ @reemrevnivek yes, and very clever indeed \$\endgroup\$
    – vicatcu
    Commented Jan 18, 2011 at 14:10
  • \$\begingroup\$ I count 2 more pull-ups than normal. Not quite exact :P \$\endgroup\$
    – Nick T
    Commented Jan 18, 2011 at 18:53

Depending on the current draw of the device in question the 3.3V output of the arduino should suffice. As for the communication lines, you'll need some type of level shifter. Looks like SparkFun has a solution for that too:


  • \$\begingroup\$ I have one of those at home, just not sure on how to hook it up as I2C has more than one line. and is bi-directional \$\endgroup\$ Commented Jan 19, 2011 at 0:46
  • \$\begingroup\$ According to some of the user comments on that page, the 'TX' lines are supposed to be bi-directional. So I assume you'd connect SDA to TX1 and SCL to TX2, for example. \$\endgroup\$
    – Craig
    Commented Jan 19, 2011 at 12:01
  • 1
    \$\begingroup\$ If you look at the Data sheet It is almost the same as the Schematic that vicatcu posted in his answer. And as experience goes they are bi-directional. \$\endgroup\$
    – jsolarski
    Commented Mar 5, 2011 at 23:01

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