Powering Arduino Using 6V?

I'm working on a project to remotely control a small robotic arm called the OWI Robotic Arm Edge, using an Arduino Nano and 3 TB6612FNG-based motor driver circuit boards, and an nRF24L01+ RF board, to allow it to be controlled by a remote controller that uses an Pro Mini and its own nRF24L01+ board.

It works great and I'm happy with the results. I'm just not sure how to power the Nano that controls the arm, once I'm done programming it and it's no longer powered via USB.

The arm uses 5 small DC motors that are powered by 4 D batteries, yielding 6V that's connected to the motor drivers' motor voltage-in pins. Ideally, I'd like to power the Nano from it as well, but I'm not sure how to power it using 6V.

If the arm used 5V I'd connect it to the Nano's 5V pin. If it was 7V or more I'd connect it to its Vin pin. But 6V is an awkward voltage to use with an Arduino.

I've tried connecting the 6V to the Vin pin, using a 7805 voltage regulator, and a buck converter, but none gave satisfactory results, with the arm either not moving or just barely moving.

D batteries put out a lot of current so power shouldn't be a problem. It's just that 6V presents a challenge in powering an Arduino, to me at least. Any ideas or suggestions?

I posted a longer version of this in the Robotics forum, but only got one response. I'm re-posting it here in a shorter version, since the problem I'm facing isn't specifically robotics-related.

Some motor driver boards can supply 5V logic power, can you post a link to your board?

habanero:
But 6V is an awkward voltage to use with an Arduino.

Agreed.

A simple solution would be to power the Arduino from a separate pack of 3 x AA cells (4.5v) connected to the 5v pin. It also avoids any risk of voltage spikes from the motors finding their way to the Arduino.

...R