I have a wheeled robot with two dual motor shields (one to power two wheels, and one to power two motors with rotating brushes attached). Also attached is 3 ultrasonic distance detectors.

The whole thing was running from a single 4 AA battery pack. I realised that there was too much voltage going to the main TinyDuino board from that. So I removed resistor R3 from both motor shields, so the A4s can power just the 4 motors, and I would run the logic from the 3v coin cell.

With this setup, 9 times out of 10 it does not work. There's no motors running at all. On the occasion that it does work, the motors run, but the distance sensors do not react. Now when I attach the USB cable to the TinyDuino, it all runs perfectly - sensors and motors.

It seems like it is a power issue. Is the 3v coin cell too small to power the logic for the sensors and the motors? The docs say it should be able to run from 2.7 v.