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JEJunior Engineer LabPico W + micro:bit course
09 35 minutes

Servo motion

Something finally moves, and the safety rules get serious

Servo motion project
Illustration of the finished idea. Build from the diagram and connection table, not the photo.

Mission goal: Drive a positional servo from GP13 using 50 Hz pulses between 1200 and 1800 microseconds, powered from a separate regulated 5 V supply with a shared ground.

Section 1

Understand

New ideas

  • Why motors need their own power supply
  • Shared ground between two power systems
  • Servo control by pulse width at 50 Hz
  • Clamping a value to a safe range
  • 1A servo moves to a position set by pulse width (1200–1800 µs here).
  • 2It needs its own regulated 5 V supply with a shared ground.
  • 3Test with the horn off. Cut motor power if it binds or heats.

Safety first

  • The servo must be powered from an external REGULATED 5 V supply able to deliver at least 1 A, with a suitable insulated connector. That supply may be an extra purchase beyond the kit, and an adult must check it before it is used.
  • Never power a servo from a GPIO pin or from the 3V3 OUT pin. A servo pulls far more current than those pins can give, and trying it can damage the board.
  • The external supply's positive wire connects to NO Pico pin at all. Not VBUS, not VSYS, not 3V3. The Pico stays powered from the USB cable.
  • The only thing shared between the two power systems is ground: the external supply's 0 V connects to Pico GND at physical pin 38. Without that shared ground the servo cannot understand the signal.
  • Secure the servo to something solid, such as taping it to a heavy book, before powering it. A loose servo whips around.
  • Test with the horn removed, or at least with nothing attached to it. The very first command can make the servo jump suddenly to a new position.
  • Actual lead markings on your servo override any colour convention. Red is usually positive and brown or black is usually ground, but check the printed markings or the supplier's documentation.
  • If the servo buzzes continuously, gets hot, or is clearly straining against something, cut the motor power immediately.
  • Switch BOTH the USB power and the external supply off before any rewiring, and before fitting or removing the horn.