Keep motors off if battery power below voltage threshold

[laurb9]

Do not activate the ENABLE line if the Vin voltage drops below the minimum required for the drivers/motor combination to function, or the minimum acceptable battery voltage.

A threshold slightly above 5V would prevent the drivers being activated while powered from USB, if the step-down leaks current from Vcc to Vin.

Note: also add 10K pull-up on ENABLE to avoid activating the drivers on power-up until setup() got around to setting up the pin.

[laurb9]

Just not activating ENABLE is probably fine. The other case, where Vin was good and then drops due to battery depletion, could be a problem. Turning off motors with a heavy lens can slam the lens into the pano unit, so maybe a controlled descent is better (but we don't know when to stop).

[stale]

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