Design Notes
Conservative power-budget decision
Eight MG90S servos dominate the design. Because vendor/clone stall data vary, use 1.0 A per servo at 5 V as the conservative interface design point: 8 A simultaneous stall. Add 25% rail margin, yielding a 10 A minimum Buck #1 and connector rating. This is deliberately above typical walking current and prevents selecting a common 2–3 A module that would brown out or overheat.
Buck #1 and Buck #2 remain off-board regulated module interfaces. Their conversion circuitry is not placed on this schematic because exact module MPNs, thermal implementation, switching frequency, and verified transient ratings were not supplied.
Input reflection
At low battery (9.0 V) and 90% assumed conversion efficiency:
- Buck #1 at 5 V × 10 A: 50 W / (9.0 V × 0.90) = 6.17 A battery current.
- Buck #2 at 5 V × 5 A: 25 W / (9.0 V × 0.90) = 3.09 A battery current.
- Combined full interface rating: 9.26 A steady equivalent, before wiring/fuse margin.
Use a 15 A transient-capable battery connector/wiring path and an appropriately selected external fuse. Confirm pack C-rating and voltage sag experimentally.
Grounding and domains
All grounds are electrically common. Servo and LED return currents should use dedicated connector returns and converge at a low-impedance power entry region in layout; they must not flow through logic/audio return traces. Relay contacts switch LED +5 V only.
Safe startup
- PCA9685 OE has a 10 kΩ pull-up to 3.3 V so PWM outputs are disabled during reset.
- Relay IN1/IN2 each have 10 kΩ pull-ups to 3.3 V so active-low channels remain off during reset.
- Firmware drives relay pins HIGH before configuring them as outputs.
Open confirmations
- Actual MG90S manufacturer and measured stall current.
- LED strip voltage/current per meter and maximum connected length.
- Exact buck-module MPNs and connector family.
- Relay module input threshold/current and whether VCC/JD-VCC are separate.