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JEJunior Engineer LabPico W + micro:bit course
10 30 minutes

Robot control knob

A human hand steering a motor through a number

Robot control knob project
Illustration of the finished idea. Build from the diagram and connection table, not the photo.

Mission goal: Combine the Mission 09 servo circuit with the Mission 07 potentiometer so turning the knob commands the servo's pulse width in real time.

Section 1

Understand

New ideas

  • Mapping one range onto another
  • Integer division for clean whole numbers
  • Low-voltage sensing and high-current motion in one project
  • Why a clamp still matters after a map
  • 1The knob reading becomes a servo pulse between 1200 and 1800 µs.
  • 2Pot on 3.3 V, servo on its own 5 V, grounds shared.
  • 3The drawn angle is illustrative, not measured.

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.
  • The knob is powered from Pico 3V3 OUT at physical pin 36. Keep the 5 V supply well away from the potentiometer.
  • Give each potentiometer leg its own breadboard row so you cannot short 3.3 V to ground.