I have encountered problem with Arduino UNO and water flow sensor. So here's situation:

I have connected a 12 V power supply from DC battery and then used 5 V regulator which output is used as input to 3.3 V regulator.

Ground of all power supply is the same that is ground of 12 V. It is connected to 5 V and then 3 V. I have a water flow sensor which uses 5 V from output of the 5 V regulator also this output is used to power the Arduino.

When serial port is connected to a laptop, the sensor works fine and gives accurate output. But when serial port disconnected sensor gives output even if there is no flow of water.

  • \$\begingroup\$ I am confused about where you talk about a 3.3 volt regulator. Do not all Arduino Uno board operate at 5 volts? ... Oh, I see, there' \$\endgroup\$
    – st2000
    Dec 4, 2016 at 16:27
  • \$\begingroup\$ @st2000 It is not clear what is the purpose of the 3.3 V. But the OP said that the Arduino is connected to the output of the 5 V regulator. \$\endgroup\$ Dec 4, 2016 at 16:29
  • \$\begingroup\$ ...which is not good enough for the on board linear regulator. That is, if he/she is connecting their 5 volt regulator output to the barrow jack. Which is another 5 volt regulator. Which need at least 7 volts to generate 5 volts internally to the Arduino. \$\endgroup\$
    – st2000
    Dec 4, 2016 at 16:34
  • 1
    \$\begingroup\$ Add a schematic please and current ratings of the regulators. Unfortunately we cannot see how did you made your connections. (via 5 V pin, barrel jack, else?) (For next time: A proper sentence starts with an upper case and usually have 1 . at the and not 4-5. If you expect anyone to take time and help you, you should take time and ask properly, your original question was a mess to read.) \$\endgroup\$ Dec 4, 2016 at 16:34
  • \$\begingroup\$ I looked at the schematic of the Uno. Looks like there is an on board 3v3 regulator just to supply peripherals w/3v3. It does not look like 3v3 is used inside an Uno. \$\endgroup\$
    – st2000
    Dec 4, 2016 at 16:35

1 Answer 1


In the Arduino Uno design, if power is detected from the USB port, the power from the external barrel jack is disconnected.

That explains how the Arduino is working while connected to your laptop.

Now for the bit when it is not working. From the Aruduno Uno web page:

The board can operate on an external supply from 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may become unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts.

So, if you are connecting your 5 volt regulated output to the barrel jack of the Arduino, you will likely encounter unexpected results.


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