sudo apt update
sudo apt full-upgrade -y
sudo reboot

# Session 4 - set up the project. Run this on the Pi, over SSH.
sudo apt update
sudo apt install -y python3-venv

mkdir ~/piweb && cd ~/piweb
python3 -m venv .venv
source .venv/bin/activate

pip install django RPi.GPIO

django-admin startproject controller .
python manage.py startapp hardware

# hardware/gpioHw.py - everything that touches a pin, in one place.
#
# A web app is many short-lived requests, and GPIO.setup() is not something to
# run on each one. So the pins are configured once at import, and a background
# thread owns the automatic mode. Views only read and write these variables.
import threading
import time

import RPi.GPIO as GPIO

LED = 17
LDR = 27

GPIO.setmode(GPIO.BCM)
GPIO.setwarnings(False)     # the module is imported once per worker
GPIO.setup(LED, GPIO.OUT)
GPIO.setup(LDR, GPIO.IN)

_state = {"led": False, "auto": False, "dark": False}
_lock = threading.Lock()    # views and the worker thread both touch _state


def _worker():
    while True:
        dark = GPIO.input(LDR) == GPIO.HIGH
        with _lock:
            _state["dark"] = dark
            if _state["auto"]:
                _state["led"] = dark
                GPIO.output(LED, GPIO.HIGH if dark else GPIO.LOW)
        time.sleep(0.2)


# daemon=True so Ctrl+C on the dev server actually exits.
threading.Thread(target=_worker, daemon=True).start()


def setLed(on: bool):
    with _lock:
        _state["auto"] = False      # a manual command turns automatic off
        _state["led"] = on
        GPIO.output(LED, GPIO.HIGH if on else GPIO.LOW)


def setAuto(on: bool):
    with _lock:
        _state["auto"] = on


def readState():
    with _lock:
        return dict(_state)

# blink.py - session 2.
# Wiring: pin 11 (GPIO17) -> 330 ohm resistor -> LED anode;
#         LED cathode -> pin 9 (GND).
import RPi.GPIO as GPIO
import time

LED = 17

GPIO.setmode(GPIO.BCM)      # number pins by GPIO, not by physical position
GPIO.setup(LED, GPIO.OUT)

try:
    while True:
        GPIO.output(LED, GPIO.HIGH)
        time.sleep(1)
        GPIO.output(LED, GPIO.LOW)
        time.sleep(1)
except KeyboardInterrupt:
    pass
finally:
    # Without this, the pin keeps its last state after Ctrl+C and the next
    # run warns "This channel is already in use".
    GPIO.cleanup()

# ldr.py - session 3. LED on in the dark, off in the light.
#
# Uses an LDR *module* (the small board with a potentiometer on it), which
# outputs a clean digital HIGH/LOW. A bare LDR is analogue, and the Pi has no
# ADC - that is why the module is worth the few rupees.
#
# Wiring: module VCC -> 3.3V, GND -> GND, DO -> pin 13 (GPIO27).
#         LED still on GPIO17.
import RPi.GPIO as GPIO
import time

LED = 17
LDR = 27

GPIO.setmode(GPIO.BCM)
GPIO.setup(LED, GPIO.OUT)
GPIO.setup(LDR, GPIO.IN)

try:
    while True:
        # Most of these modules pull DO LOW when light crosses the threshold
        # set by the on-board pot. If yours is inverted, swap the branches.
        dark = GPIO.input(LDR) == GPIO.HIGH
        GPIO.output(LED, GPIO.HIGH if dark else GPIO.LOW)
        time.sleep(0.2)
except KeyboardInterrupt:
    pass
finally:
    GPIO.cleanup()

# hardware/views.py
from django.http import JsonResponse
from django.shortcuts import render

from . import gpioHw


def index(request):
    return render(request, "hardware/index.html")


def state(request):
    return JsonResponse(gpioHw.readState())


def led(request, action):
    gpioHw.setLed(action == "on")
    return JsonResponse(gpioHw.readState())


def auto(request, action):
    gpioHw.setAuto(action == "on")
    return JsonResponse(gpioHw.readState())

# controller/urls.py
from django.urls import path

from hardware import views

urlpatterns = [
    path("", views.index),
    path("api/state/", views.state),
    path("api/led/<str:action>/", views.led),
    path("api/auto/<str:action>/", views.auto),
]

# controller/settings.py - the dev server refuses unknown Host headers.
# For a workshop on a trusted network this is enough:
ALLOWED_HOSTS = ["*"]

INSTALLED_APPS = [
    # ...
    "hardware",
]

<!-- hardware/templates/hardware/index.html -->
<!DOCTYPE html>
<html>
<head>
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Pi Control</title>
</head>
<body style="font-family: sans-serif; text-align: center; padding-top: 2rem">
  <h1>Raspberry Pi Control</h1>

  <p>LED: <b id="led">?</b> &nbsp; Light: <b id="dark">?</b> &nbsp; Auto: <b id="auto">?</b></p>

  <p>
    <button onclick="send('/api/led/on/')">LED on</button>
    <button onclick="send('/api/led/off/')">LED off</button>
  </p>
  <p>
    <button onclick="send('/api/auto/on/')">Auto on</button>
    <button onclick="send('/api/auto/off/')">Auto off</button>
  </p>

<script>
function paint(s) {
  document.getElementById('led').textContent  = s.led  ? 'ON' : 'OFF';
  document.getElementById('auto').textContent = s.auto ? 'ON' : 'OFF';
  document.getElementById('dark').textContent = s.dark ? 'dark' : 'bright';
}
function send(url) { fetch(url).then(r => r.json()).then(paint); }

// Poll, so the page follows the LED when automatic mode changes it.
setInterval(() => fetch('/api/state/').then(r => r.json()).then(paint), 500);
</script>
</body>
</html>

# Let other machines on the network reach it - the default only binds
# localhost, which on a headless Pi means nothing can connect.
python manage.py runserver 0.0.0.0:8000

# Then open http://<your-pi-ip>:8000/ from any device on the same network.