We are using an Arduino Nano for simple serial comms to fire a relay from a Windows 10 PC.

Initial issue we had was windows not recognizing the device (COM Port) when the pc is powered off (as per this link: Arduino Nano - USB not present after reboot ) so we powered the arduino nano over a 5V pin and now when Windows starts up the device is recognized.

However, as I'm learning with these chips, nothing is ever that simple. Now the relay that we are firing does not work with external power, but if you unplug the external power then the relay does fire (powered over USB)

The units are at a mine and at the moment and I believe that the problem is that the serial chip is now not responding as per this - from the official Arduino Nano page:

The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high.

Note - I included the above from another question but this no longer appears on the official arduino nano page (can't past link as it's my first post)

The question is how can we get the digital pins on the arduino nano to fire using an external power source and input from a Windows PC?


Diagram: enter image description here

  • Are you able to put up a schematic of how the relay is connected ?
    – D-on
    Aug 21, 2016 at 8:26
  • Thanks, added the diagram. Very simple, works fine with USB input. Not 100% sure how they've wired with the external power, trying to find out.
    – onemorecupofcoffee
    Aug 21, 2016 at 9:13
  • 1
    "units are at a mine" I don't know what your are doing, but if you are operating an Arduino and an off-the-shelf relay in a mining environment, you are likely violating safety rules and/or laws related to electronics in hazardous environments. Have you considered the hazardous location equipment qualifications needed for electronics? Aug 21, 2016 at 14:18
  • The relay is to fire a turnstyle and it's not related to any industrial equipment and the guys on site are working under supervision so I don't think that this is an issue Aug 22, 2016 at 6:25

2 Answers 2


I think you may need to back up and start over again.

Rather than changing the power supply, you need to fix the problem that made you think you needed to change power supplies.

Following a link in a comment on the question you linked to leads to this thread on the Arduino forums.

The suggestion there is that a particular pin on the FTDI chip on the Nano isn't connected as it should be.

To be exact, the Test pin (pin 26 on the FT232RL chip) needs to be grounded to fix the original problem you were having with communications with the Nano.

The description there fits your description. The Nano can't talk to the PC if the Nano is connected to the PC at power on. The same thing happens with the circuit referenced in that Arduino forum thread, but they are using another embedded system instead of a PC.

If you fix that, then you won't need to use an external 5V supply.

To back up the conclusion from the Arduino forum, here's an excerpt from page 8 (pin descriptions) of the FT232RL datasheet:

26 TEST Input Puts the device into IC test mode. Must be tied to GND for normal operation, otherwise the device will appear to fail

You need to connect pins 25 (ground) and pin 26 (Test.)

Excerpt from the Nano schematic: enter image description here

Pins 25 (ground) and 26 (Test) are right next to one another, so easy to fix.

You need to put a blob of solder on the red marked spot in this picture: enter image description here

  • Thanks. I do agree this is probably the right way to do it - but because the site is so remote we did not want to risk blowing the board. I've accepted as answer and then added what we did as a "workaround" Aug 22, 2016 at 6:27

I accepted the answer from JRE as I believe this is the best answer. For certain reasons we weren't able to do this so I've posted this as a workaround.

We ended up splitting the USB input to the arduino nano by using a Y cable. We used the one end for the power to the device and the other end for comms from the PC (with power disabled). This solved our issue as the chip now received power and responded to comms and the USB driver also initialised correctly on Windows when starting up.

If you are unable to solder the chip then this is a workaround that we have used.

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