Im using an SRf02 ultrasonic sensor, and I need to make the wires long to elevate the sensor. They turn on and blink and are able to transmit a signal, yet they are not able to receive a signal. What is the error?

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  • 1
    Which interface are you using - UART or I2C?
    – Majenko
    Dec 10, 2015 at 11:21
  • im using i2c as the interface
    – Umbus
    Dec 10, 2015 at 18:54
  • I thought you might be. Are you using the internal pullups of the Arduino IO pins or have you added external pullup resistors?
    – Majenko
    Dec 10, 2015 at 18:55
  • internal, i havent added any resistors
    – Umbus
    Dec 10, 2015 at 19:23
  • Again, I suspected as much.
    – Majenko
    Dec 10, 2015 at 19:40

1 Answer 1


Using the internal pullup resistors for I2C is not recommended. Arduino are very wrong in providing those as the default option and basically "hiding" the fact that I2C needs decent pullup resistors from you.

The internal pullup resistors, because they are so weak (i.e., a high resistance, typically > 50KΩ), do not pull the clock and data lines up adequately, and as such are only suitable for very short connections (in the order of a few inches). Anything longer and the induced noise and inductance and capacitance of the wires will corrupt the I2C waveform.

I2C requires pullup resistors in the range of 2.2KΩ to 4.7KΩ. As you can see that is more than 10x the pullup strength of the internal resistors.

For off-board connections (i.e., wires) you want the lower end of that range. So add a 2.2KΩ resistor between SCL and +5V and another between SDA and +5V and you should find it works much better.

  • so what do you mean, add the resistor between SDL and V+, i cant just place it in between SDL on arduino and the TX pin of the sensor?
    – Umbus
    Dec 10, 2015 at 20:13
  • I mean exactly what I say. Connect a 2.2k resistor with one end connected to the SCL pin on the Arduino and the other end connected to +5V on the arduino. The same with the SDA pin.
    – Majenko
    Dec 10, 2015 at 20:14
  • i uploaded a picture in the original post, is this what you mean?
    – Umbus
    Dec 10, 2015 at 20:58
  • I don't know, it's too small to make out.
    – Majenko
    Dec 10, 2015 at 20:59
  • i put a better picture
    – Umbus
    Dec 10, 2015 at 21:25

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