Docs missing requirement to power PCA9685 servo rail separately

[nbertagnolli]

Thanks for the great docs! I ran into some issues during my build with figuring out how to power the servos. As a new builder it might be helpful to add some information to the docs about powering the servos and how that servo power then gets used on the PA9685 board. I can update the docs if you're open to PRs : ). The problem I ran into was that the hardware build guide didn't mention that the PCA9685 servo rail
(V+) requires its own 5–6V power source.

Following the guide exactly, the PCA9685 receives logic power from the Pi
(I²C works), but the servos remain completely unpowered unless:

• the ESC provides a 5–6V BEC on its 3-wire connector, or
• a separate 5–6V battery/BEC is connected to the V+ screw terminals.

If neither is present, users will see:

• servos not moving during donkey calibrate
• no twitch/buzz on startup
• I²C detection at 0x40 working but no PWM output response

This small clarification would prevent a common silent-failure scenario for new builders like me : ).

[zlite]

Good point! Please do submit a PR

[TCIII]

@nbertagnolli
Taken from Step 4: Connect Servo Shield to Raspberry Pi of the Donkey Car Build a car documentation:

• 5v VIN should NOT be provided by the GPIO bus because motors/servos may draw too much power. Most Electronic Speed Controllers actually provide the necessary power via the 3 pin cables that get plugged into the PCA9685, so it is generally not necessary to provide power directly to VIN.
  TCIII

[zlite]

Ah, yes Tom is right. The assumption is that you're using an ESC for your motor, which has a BEC that provides sufficient power. But if you're not (why?) then yes, you will have to power the servos some other way