Module 09 — Connect the Physical AX-12A

Wire the ESP32 to the AX-12A through the 74HCT245 electrical interface and verify safe, observable physical actuation.

Hardware module — opt-in

This module requires the PixStars reference hardware: ESP32, 74HCT245 level-shifter, Dynamixel AX-12A servo, and the physical anglepoise lamp. It is an explicit extension of the simulation-first architecture. Modules 4, 7, and 12 can be completed without hardware. Only proceed here if you have the physical system wired and powered.

Goal

Connect the ESP32 edge adapter to the physical AX-12A through the 74HCT245 electrical interface. Execute a safe, ordered calibration sequence. Only after every step succeeds may the nod gesture be enabled.

The physical chain

ESP32 edge adapter
     ↓
74HCT245 (electrical interface — voltage levels, bus, direction)
     ↓
Dynamixel AX-12A
     ↓
Mechanical lamp head pivot

The 74HCT245 is purely physical communication. It is where Open Engineering abstractions cross a hardware boundary of voltage levels, buses, and electrical communication. It never appears as a semantic Open Engineering entity. See the edge adapter reference for the ESP32’s full responsibility set.

Why this module exists

Modules 1 through 8 built the complete software architecture: definitions, digital twin, simulation, Crossplane composition, MiniKube runtime, Manifold execution, and the ESP32 edge adapter firmware. That entire chain works without hardware. This module is the first point where a physical servo moves.

Physical actuation introduces safety requirements that purely virtual systems do not have. Unrestricted servo movement can damage the lamp, the servo, or injure the learner. Every step in this module exists because of that reality.

The safe sequence

The safe sequence is defined in memo5 section 14:

Discover
   ↓
Connect
   ↓
Read current state
   ↓
Establish safe limits
   ↓
Test small movement
   ↓
Return to center
   ↓
Enable gesture

Each step must complete successfully before the next begins. There are no shortcuts.

Step 1 — Connectivity test

Verify that the ESP32 can communicate with the AX-12A over the 74HCT245. This tests the electrical boundary: UART direction management, Dynamixel Protocol 1.0 packet exchange, and servo presence detection.

pico runtime inspect --lab make-the-lamp-nod --pico pixstars-head-pitch

Expected: the actuator reports connected: true. If the actuator is unreachable, check wiring, power, UART direction control on the ESP32, and 74HCT245 bus orientation before proceeding.

Evidence

Connectivity is the first piece of evidence. A connected: false response is a clear, observable failure — not an assumption that things are probably wired correctly.

Step 2 — Telemetry read

Read the current servo state: position, temperature, torque status, and error flags. This confirms the telemetry ascent path works:

AX-12A → ESP32 → Pico twin → Open Engineering → observable state

The Pico twin’s observed state fields should populate with live values. A missing or stale telemetry value means the ascent path is broken — do not proceed to movement until telemetry flows.

Step 3 — Safe-limit configuration

Load the safe limits from rules/nod-gesture.yaml:

safety:
  minPosition: -30
  maxPosition: 30
  movementSpeed: safe
  commandTimeoutMs: 2000
  emergencyStop: true
  safeStartupState: center
  safeShutdownState: center

These boundaries govern every physical movement the system will perform. The learner MUST NOT execute unrestricted servo movement. The configured limits are the outer boundary; the nod sequence positions (center 0, down +15, up -8) sit well within them.

Safety boundary

The positions in nod-gesture.yaml are illustrative until calibrated against the physical lamp. The logical nod sequence (center down up center) is fixed; the numeric values are configurable and must be calibrated before any gesture executes. This distinction between logical gesture and physical calibration is a core teaching point — see the architecture semantic layering.

Step 4 — Small movement

Command a small, controlled movement within the safe limits. Verify the servo responds and that the telemetry reports the changing actualPosition. Verify the movement does not exceed configured limits.

This is the first physical motion in the course. The descent path is:

command → ESP32 → Dynamixel Protocol 1.0 → 74HCT245 → AX-12A → rotation

And the ascent path returns:

rotation → telemetry → ESP32 → Pico twin → observed state

Both directions must work before proceeding.

Step 5 — Return to center

Command the servo back to center (position 0). Verify the Pico twin’s actualPosition converges to the desiredPosition. This confirms the closed-loop cyber-physical architecture: commanded state and observed state closing to agreement.

A difference between desired and observed is not inherently an error. desiredPosition = 0 and actualPosition = 3 means the servo is moving toward the desired state. Evidence over assumption.

Step 6 — Stop test

Trigger an emergency stop. Verify the servo halts immediately and reports the stopped state. The emergency stop must work at any point during movement. If it does not, do not enable gesture execution.

Enabling the gesture

Only after all six steps succeed may the nod gesture be enabled. Enabling the gesture means the Manifold runtime is permitted to execute the nod rule against the physical actuator. Until this point, the system has been in a calibration-only mode.

The nod sequence itself is defined in rules/nod-gesture.yaml:

nod:
  center: 0
  down: 15
  up: -8
  repetitions: 1
  speed: safe

The logical gesture is fixed: center, slightly down, slightly up, return to center. The numeric positions are calibrated values, not hard-coded generic definitions. The ESP32 receives actuator-level commands (set-position, center, execute-motion) and never interprets the gesture’s semantic meaning.

Checklist

Step	What you verified
1	ESP32 ↔︎ AX-12A connectivity through 74HCT245
2	Telemetry flows: position, temperature, torque, errors
3	Safe limits loaded: min, max, speed, timeout, emergency stop
4	Small movement succeeds within safe limits
5	Return to center succeeds; desired/observed converge
6	Emergency stop halts servo immediately
7	Gesture enabled only after steps 1-6 pass

What you have proven

The physical boundary is working. The ESP32 edge adapter converts Open Engineering semantics into Dynamixel Protocol 1.0 packets, the 74HCT245 manages the electrical interface, and the AX-12A rotates the lamp head. Telemetry returns through the same chain to the Pico twin.

This is the point where the cyber-physical architecture becomes real. The architecture page arrow map now has physical evidence behind every arrow from Manifold downward.

Module 10 — Expose the Capability, where Home Assistant becomes the first human control interface for the physical lamp.