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commit 58be310d63
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+# Created by https://www.toptal.com/developers/gitignore/api/venv,python
+# Edit at https://www.toptal.com/developers/gitignore?templates=venv,python
+
+### Python ###
+# Byte-compiled / optimized / DLL files
+__pycache__/
+*.py[cod]
+*$py.class
+
+# C extensions
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+# Distribution / packaging
+.Python
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+.installed.cfg
+*.egg
+MANIFEST
+
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+# End of https://www.toptal.com/developers/gitignore/api/venv,python
@@ -0,0 +1,219 @@
+# Raspberry Pi Signal Light API
+
+A REST API for controlling a Raspberry Pi equipped with a 4-channel relay HAT.
+
+The API controls four relays connected to the following GPIO pins:
+
+| Function | GPIO |
+| -------- | ---: |
+| Green    |   26 |
+| Yellow   |   19 |
+| Red      |   13 |
+| Alert    |    6 |
+
+## Features
+
+* REST API built with **FastAPI**
+* Interactive Swagger UI
+* Green, Yellow and Red are mutually exclusive
+* Alert relay blinks independently of the color relays
+* Demo mode continuously cycles:
+
+```
+Green → Yellow → Red → Yellow → ...
+```
+
+* Automatic relay reset during startup and shutdown
+* Status endpoint for monitoring the current state
+
+---
+
+# Requirements
+
+* Raspberry Pi
+* Python 3.10 or newer
+* 4-channel relay HAT
+* Raspberry Pi OS
+
+---
+
+# Installation
+
+Clone the repository and create a virtual environment.
+
+```bash
+python3 -m venv .venv
+source .venv/bin/activate
+```
+
+Install the dependencies:
+
+```bash
+pip install fastapi uvicorn gpiozero
+```
+
+---
+
+# Starting the API
+
+Run the application with Uvicorn:
+
+```bash
+uvicorn relay_api:app --host 0.0.0.0 --port 8000
+```
+
+If your Python file has a different name, replace `relay_api` with the filename (without the `.py` extension).
+
+Example:
+
+```bash
+uvicorn signal_api:app --host 0.0.0.0 --port 8000
+```
+
+The API will then be available on port **8000**.
+
+---
+
+# API Documentation
+
+FastAPI automatically provides interactive documentation.
+
+## Swagger UI
+
+```
+http://<RASPBERRY_PI_IP>:8000/docs
+```
+
+Swagger allows you to:
+
+* View all available endpoints
+* Execute requests directly from your browser
+* Inspect request and response schemas
+
+## ReDoc
+
+```
+http://<RASPBERRY_PI_IP>:8000/redoc
+```
+
+---
+
+# Endpoints
+
+## Colors
+
+Only one color can be active at a time.
+
+| Method | Endpoint  | Description           |
+| ------ | --------- | --------------------- |
+| POST   | `/green`  | Activate green relay  |
+| POST   | `/yellow` | Activate yellow relay |
+| POST   | `/red`    | Activate red relay    |
+
+Selecting a color automatically turns the other two color relays off.
+
+---
+
+## Alert
+
+The alert relay operates independently from the color relays.
+
+| Method | Endpoint     | Description                |
+| ------ | ------------ | -------------------------- |
+| POST   | `/alert`     | Start blinking alert relay |
+| POST   | `/alert/off` | Stop blinking alert relay  |
+
+The alert relay blinks continuously:
+
+* 1 second ON
+* 1 second OFF
+
+The currently active color remains unchanged.
+
+---
+
+## Demo Mode
+
+Demo mode automatically cycles through the colors.
+
+```
+Green
+↓
+Yellow
+↓
+Red
+↓
+Yellow
+↓
+(repeat)
+```
+
+| Method | Endpoint    | Description     |
+| ------ | ----------- | --------------- |
+| POST   | `/demo`     | Start demo mode |
+| POST   | `/demo/off` | Stop demo mode  |
+
+Starting demo mode overrides manual color selection until demo mode is stopped.
+
+The alert relay continues to operate independently during demo mode.
+
+---
+
+## System
+
+| Method | Endpoint  | Description                                   |
+| ------ | --------- | --------------------------------------------- |
+| POST   | `/off`    | Turn off all relays and stop demo/alert modes |
+| GET    | `/status` | Retrieve the current relay status             |
+
+---
+
+# Example Usage
+
+Activate the green relay:
+
+```bash
+curl -X POST http://<RASPBERRY_PI_IP>:8000/green
+```
+
+Start the alert relay:
+
+```bash
+curl -X POST http://<RASPBERRY_PI_IP>:8000/alert
+```
+
+Start demo mode:
+
+```bash
+curl -X POST http://<RASPBERRY_PI_IP>:8000/demo
+```
+
+Check the current status:
+
+```bash
+curl http://<RASPBERRY_PI_IP>:8000/status
+```
+
+Turn everything off:
+
+```bash
+curl -X POST http://<RASPBERRY_PI_IP>:8000/off
+```
+
+---
+
+# Relay Logic
+
+| Relay  | Behavior                                           |
+| ------ | -------------------------------------------------- |
+| Green  | Mutually exclusive with Yellow and Red             |
+| Yellow | Mutually exclusive with Green and Red              |
+| Red    | Mutually exclusive with Green and Yellow           |
+| Alert  | Blinks independently of the color relays           |
+| Demo   | Cycles Green → Yellow → Red → Yellow until stopped |
+
+---
+
+# License
+
+This project is provided as-is for educational and personal use.
@@ -0,0 +1,500 @@
+from contextlib import asynccontextmanager
+from threading import Event, Lock, Thread
+from typing import Dict, Optional
+from time import sleep
+
+from fastapi import FastAPI
+from gpiozero import OutputDevice
+
+
+# BCM GPIO pin numbers
+RELAY_PINS = {
+    1: 26,  # Green
+    2: 19,  # Yellow
+    3: 13,  # Red
+    4: 6,   # Alert
+}
+
+# Most relay HATs are active-low.
+# Change this to True if your relay board is active-high.
+RELAY_ACTIVE_HIGH = False
+
+# Alert: one second on, one second off.
+ALERT_INTERVAL_SECONDS = 1.0
+
+# Demo: duration of each color in the sequence.
+DEMO_INTERVAL_SECONDS = 0.25
+
+# Green -> Yellow -> Red -> Yellow -> repeat
+DEMO_SEQUENCE = (
+    (1, "green"),
+    (2, "yellow"),
+    (3, "red"),
+    (2, "yellow"),
+)
+
+
+relays: Dict[int, OutputDevice] = {}
+
+# Protects access to the three color relays.
+color_lock = Lock()
+
+# Protects alert thread state.
+alert_lock = Lock()
+
+# Protects demo thread state.
+demo_lock = Lock()
+
+
+active_color: Optional[str] = None
+
+alert_stop_event = Event()
+alert_thread: Optional[Thread] = None
+
+demo_stop_event = Event()
+demo_thread: Optional[Thread] = None
+
+def reset_relays() -> None:
+    """
+    Reset the entire relay system.
+
+    - Stops demo mode
+    - Stops alert mode
+    - Turns every relay off
+    """
+    stop_demo(turn_colors_off=False)
+    stop_alert()
+
+    with color_lock:
+        relays[1].off()
+        relays[2].off()
+        relays[3].off()
+
+        global active_color
+        active_color = None
+
+    if 4 in relays:
+        relays[4].off()
+
+
+def turn_colors_off_locked() -> None:
+    """
+    Turn off green, yellow, and red.
+
+    color_lock must already be held.
+    Alert is not affected.
+    """
+    global active_color
+
+    relays[1].off()
+    relays[2].off()
+    relays[3].off()
+
+    active_color = None
+
+
+def set_color_locked(relay_number: int, color_name: str) -> None:
+    """
+    Activate exactly one color relay.
+
+    color_lock must already be held.
+    Alert is not affected.
+    """
+    global active_color
+
+    turn_colors_off_locked()
+    relays[relay_number].on()
+    active_color = color_name
+
+
+# ---------------------------------------------------------------------------
+# Alert mode
+# ---------------------------------------------------------------------------
+
+def alert_worker() -> None:
+    """
+    Toggle the alert relay until alert_stop_event is set.
+    """
+    alert_relay = relays[4]
+
+    try:
+        while not alert_stop_event.is_set():
+            alert_relay.on()
+
+            if alert_stop_event.wait(ALERT_INTERVAL_SECONDS):
+                break
+
+            alert_relay.off()
+
+            if alert_stop_event.wait(ALERT_INTERVAL_SECONDS):
+                break
+    finally:
+        alert_relay.off()
+
+
+def start_alert() -> None:
+    """
+    Start alert blinking.
+
+    Color and demo modes are not affected.
+    Repeated calls do not create additional threads.
+    """
+    global alert_thread
+
+    with alert_lock:
+        if alert_thread is not None and alert_thread.is_alive():
+            return
+
+        alert_stop_event.clear()
+
+        alert_thread = Thread(
+            target=alert_worker,
+            name="relay-alert-thread",
+            daemon=True,
+        )
+        alert_thread.start()
+
+
+def stop_alert() -> None:
+    """
+    Stop alert blinking.
+
+    Color and demo modes are not affected.
+    """
+    global alert_thread
+
+    with alert_lock:
+        alert_stop_event.set()
+
+        thread = alert_thread
+        alert_thread = None
+
+    # Do not hold alert_lock while waiting for the worker.
+    if thread is not None and thread.is_alive():
+        thread.join()
+
+    if 4 in relays:
+        relays[4].off()
+
+
+def is_alert_running() -> bool:
+    with alert_lock:
+        return alert_thread is not None and alert_thread.is_alive()
+
+
+# ---------------------------------------------------------------------------
+# Demo mode
+# ---------------------------------------------------------------------------
+
+def demo_worker() -> None:
+    """
+    Continuously cycle through:
+
+    green -> yellow -> red -> yellow
+    """
+    try:
+        while not demo_stop_event.is_set():
+            for relay_number, color_name in DEMO_SEQUENCE:
+                if demo_stop_event.is_set():
+                    break
+
+                with color_lock:
+                    set_color_locked(relay_number, color_name)
+
+                if demo_stop_event.wait(DEMO_INTERVAL_SECONDS):
+                    break
+    finally:
+        # Leave all color relays off when demo mode ends.
+        with color_lock:
+            turn_colors_off_locked()
+
+
+def start_demo() -> None:
+    """
+    Start demo mode.
+
+    Alert mode is not affected.
+    Repeated calls do not create additional demo threads.
+    """
+    global demo_thread
+
+    with demo_lock:
+        if demo_thread is not None and demo_thread.is_alive():
+            return
+
+        demo_stop_event.clear()
+
+        demo_thread = Thread(
+            target=demo_worker,
+            name="relay-demo-thread",
+            daemon=True,
+        )
+        demo_thread.start()
+
+
+def stop_demo(turn_colors_off: bool = True) -> None:
+    """
+    Stop demo mode.
+
+    Alert mode is not affected.
+
+    When turn_colors_off is True, all three color relays are turned off.
+    """
+    global demo_thread
+
+    with demo_lock:
+        demo_stop_event.set()
+
+        thread = demo_thread
+        demo_thread = None
+
+    # Do not hold demo_lock while waiting for the worker.
+    if thread is not None and thread.is_alive():
+        thread.join()
+
+    if turn_colors_off and relays:
+        with color_lock:
+            turn_colors_off_locked()
+
+
+def is_demo_running() -> bool:
+    with demo_lock:
+        return demo_thread is not None and demo_thread.is_alive()
+
+
+# ---------------------------------------------------------------------------
+# Manual color mode
+# ---------------------------------------------------------------------------
+
+def activate_manual_color(relay_number: int, color_name: str) -> None:
+    """
+    Stop demo mode and activate a manually selected color.
+
+    Alert mode is not affected.
+    """
+    stop_demo(turn_colors_off=True)
+
+    with color_lock:
+        set_color_locked(relay_number, color_name)
+
+
+def turn_everything_off() -> None:
+    """
+    Stop demo and alert modes and turn off every relay.
+    """
+    stop_demo(turn_colors_off=True)
+    stop_alert()
+
+    if relays:
+        with color_lock:
+            turn_colors_off_locked()
+
+
+# ---------------------------------------------------------------------------
+# FastAPI lifecycle
+# ---------------------------------------------------------------------------
+
+@asynccontextmanager
+async def lifespan(app: FastAPI):
+    try:
+        # Initialize GPIOs
+        for relay_number, gpio_pin in RELAY_PINS.items():
+            relays[relay_number] = OutputDevice(
+                pin=gpio_pin,
+                active_high=RELAY_ACTIVE_HIGH,
+                initial_value=False,
+            )
+
+        # Give the GPIO driver a moment to initialize
+        sleep(0.5)
+
+        # Force every relay into the OFF state
+        reset_relays()
+
+        yield
+
+    finally:
+        reset_relays()
+
+        for relay in relays.values():
+            relay.close()
+
+        relays.clear()
+
+app = FastAPI(
+    title="Raspberry Pi Signal Light API",
+    description=(
+        "Controls mutually exclusive green, yellow, and red relays. "
+        "Alert blinking operates independently. "
+        "Demo mode cycles through green, yellow, red, and yellow."
+    ),
+    version="3.0.0",
+    lifespan=lifespan,
+)
+
+
+# ---------------------------------------------------------------------------
+# API endpoints
+# ---------------------------------------------------------------------------
+
+@app.get("/")
+def root():
+    return {
+        "service": "Raspberry Pi Signal Light API",
+        "active_color": active_color,
+        "alert_active": is_alert_running(),
+        "demo_active": is_demo_running(),
+        "endpoints": {
+            "green": "POST /green",
+            "yellow": "POST /yellow",
+            "red": "POST /red",
+            "start_alert": "POST /alert",
+            "stop_alert": "POST /alert/off",
+            "start_demo": "POST /demo",
+            "stop_demo": "POST /demo/off",
+            "all_off": "POST /off",
+            "status": "GET /status",
+        },
+    }
+
+
+@app.post("/green")
+def green():
+    activate_manual_color(
+        relay_number=1,
+        color_name="green",
+    )
+
+    return {
+        "active_color": "green",
+        "demo_active": False,
+        "alert_active": is_alert_running(),
+    }
+
+
+@app.post("/yellow")
+def yellow():
+    activate_manual_color(
+        relay_number=2,
+        color_name="yellow",
+    )
+
+    return {
+        "active_color": "yellow",
+        "demo_active": False,
+        "alert_active": is_alert_running(),
+    }
+
+
+@app.post("/red")
+def red():
+    activate_manual_color(
+        relay_number=3,
+        color_name="red",
+    )
+
+    return {
+        "active_color": "red",
+        "demo_active": False,
+        "alert_active": is_alert_running(),
+    }
+
+
+@app.post("/alert")
+def alert():
+    start_alert()
+
+    return {
+        "active_color": active_color,
+        "alert_active": True,
+        "demo_active": is_demo_running(),
+        "mode": "blinking",
+        "on_seconds": ALERT_INTERVAL_SECONDS,
+        "off_seconds": ALERT_INTERVAL_SECONDS,
+    }
+
+
+@app.post("/alert/off")
+def alert_off():
+    stop_alert()
+
+    return {
+        "active_color": active_color,
+        "alert_active": False,
+        "demo_active": is_demo_running(),
+    }
+
+
+@app.post("/demo")
+def demo():
+    start_demo()
+
+    return {
+        "demo_active": True,
+        "alert_active": is_alert_running(),
+        "sequence": [
+            color_name
+            for _, color_name in DEMO_SEQUENCE
+        ],
+        "interval_seconds": DEMO_INTERVAL_SECONDS,
+    }
+
+
+@app.post("/demo/off")
+def demo_off():
+    stop_demo(turn_colors_off=True)
+
+    return {
+        "active_color": None,
+        "demo_active": False,
+        "alert_active": is_alert_running(),
+    }
+
+
+@app.post("/off")
+def off():
+    turn_everything_off()
+
+    return {
+        "active_color": None,
+        "alert_active": False,
+        "demo_active": False,
+    }
+
+
+@app.get("/status")
+def status():
+    with color_lock:
+        color_status = {
+            "green": {
+                "gpio": RELAY_PINS[1],
+                "on": relays[1].is_active,
+            },
+            "yellow": {
+                "gpio": RELAY_PINS[2],
+                "on": relays[2].is_active,
+            },
+            "red": {
+                "gpio": RELAY_PINS[3],
+                "on": relays[3].is_active,
+            },
+        }
+
+        current_color = active_color
+
+    alert_running = is_alert_running()
+    demo_running = is_demo_running()
+
+    return {
+        "active_color": current_color,
+        "alert_active": alert_running,
+        "demo_active": demo_running,
+        "relays": {
+            **color_status,
+            "alert": {
+                "gpio": RELAY_PINS[4],
+                "on": relays[4].is_active,
+                "blinking": alert_running,
+            },
+        },
+    }
@@ -0,0 +1,3 @@
+fastapi==0.140.0
+gpiozero==2.0.1.post2
+uvicorn==0.51.0