WiFi-Controlled Lamp — Arduino L3

Learning Goals 5 min

Cluster F's capstone: a real "smart lamp" you can toggle from any phone on your home WiFi. The control surface is a phone-friendly webpage. The lamp itself is a low-voltage LED today (we'll talk about real mains separately — never wire 230 V in class). By the end of this lesson you will:

Combine yesterday's sensor dashboard with L03-32's control endpoints into a single mobile-friendly UI: button to turn on/off, slider for brightness, live state read-back.
Add "timer" commands — turn the lamp on for N minutes, then auto-off — using non-blocking timing.
Discuss (not wire) how to scale this from a 20 mA LED to a 230 V household lamp using a relay module and proper electrical safety.

Warm-Up 10 min

Hardware for today is minimal: one LED + 220 Ω resistor on a PWM pin. (D5 on NodeMCU = GPIO14 = PWM-capable.) Wire as L03-32.

What we mean by "lamp"

For the school build, the "lamp" is a single LED. The architecture — phone UI sends commands; ESP toggles a pin — is identical to a real smart bulb. Only the load changes:

Load	Driver	Safety
LED + 220 Ω	Direct from GPIO	None needed (3.3 V, < 20 mA)
Small DC motor / 12 V LED strip	MOSFET or driver IC	Power supply isolation
230 V mains bulb	Optoisolated relay module	Wired by adult; behind a fuse; inside a sealed enclosure

Never wire 230 V mains in a classroom project. The discussion is about how the architecture scales, not about doing it yourself. For a real smart-home build, buy a commercial smart plug or use a low-voltage path.

New Concept · Combined dashboard + control 25 min

What we're building

One HTML page with:

Big "ON / OFF" toggle.
Brightness slider (0–255).
"Timer" quick-buttons: 1 min / 5 min / 15 min.
Live state read-back so multiple phones stay in sync.

The endpoints

Method	Path	Args	Effect
GET	/	—	HTML control page
GET	/state.json	—	Returns {on, brightness, secondsLeft}
POST	/on	—	Turn on at last brightness; clear timer
POST	/off	—	Turn off; clear timer
POST	/bright	?value=N	Set brightness; if N==0 also turn off
POST	/timer	?mins=N	Turn on, schedule auto-off after N min

Why POST for the actions and GET for reads

HTTP convention: GET = safe / idempotent / read-only. POST = causes a state change. Following this means a browser's "refresh" on a state page won't accidentally trigger another command. Tools like cURL, scripts, and home-automation hubs also expect this convention.

The basic ESP8266WebServer handles GET and POST identically by default. To distinguish, use the three-arg form: server.on("/on", HTTP_POST, handler).

Non-blocking auto-off timer

unsigned long autoOffAt = 0;       // 0 = no timer

void scheduleAutoOff(unsigned long mins) {
  autoOffAt = millis() + mins * 60UL * 1000UL;
}

void clearAutoOff() {
  autoOffAt = 0;
}

void loop() {
  server.handleClient();

  if (autoOffAt && (long)(millis() - autoOffAt) >= 0) {
    digitalWrite(LED_PIN, LOW);
    autoOffAt = 0;
    Serial.println("# Auto-off fired");
  }
}

The (long)(millis() - autoOffAt) >= 0 cast handles the (unlikely-in-this-app) millis() overflow correctly. millis() overflows after ~49 days.

State management — one struct of truth

struct LampState {
  bool on;
  int  brightness;       // 0..255
  unsigned long autoOffAt;
};

LampState lamp = {false, 128, 0};

void applyLamp() {
  analogWrite(LED_PIN, lamp.on ? lamp.brightness : 0);
}

All handlers mutate lamp then call applyLamp(). /state.json serialises lamp. Single source of truth — no risk of the actual LED disagreeing with what the API reports.

Worked Example · The smart lamp 25 min

Step 1 — wire one LED to D5 (PWM)

Same as L03-32 / L03-33.

Step 2 — the sketch

// L03-34 · WiFi-Controlled Lamp
#if defined(ESP8266)
  #include <ESP8266WiFi.h>
  #include <ESP8266WebServer.h>
  ESP8266WebServer server(80);
#elif defined(ESP32)
  #include <WiFi.h>
  #include <WebServer.h>
  WebServer server(80);
#endif

const char* SSID     = "YourNetwork";
const char* PASSWORD = "YourPassword";
const int   LED_PIN  = 14;

struct LampState {
  bool on;
  int  brightness;
  unsigned long autoOffAt;
};
LampState lamp = {false, 128, 0};

void applyLamp() {
  analogWrite(LED_PIN, lamp.on ? lamp.brightness : 0);
}

const char index_html[] PROGMEM = R"HTML(
<!doctype html><html><head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Smart Lamp</title>
<style>
  :root { color-scheme: light dark; }
  body { font-family: -apple-system, sans-serif; max-width: 380px; margin: 0 auto; padding: 1rem; }
  h1 { font-size: 1.4rem; }
  .row { display: flex; gap: .5rem; margin: .8rem 0; }
  button { flex: 1; padding: 1rem; font-size: 1rem; border: 0; border-radius: 8px; background: #4f46e5; color: #fff; }
  button.off { background: #6b7280; }
  input[type=range] { width: 100%; }
  .state { font-size: 1.1rem; margin: .8rem 0; padding: .8rem; background: #f1f5f9; border-radius: 8px; }
</style></head><body>
  <h1>Smart Lamp</h1>
  <div class="state" id="state">--</div>

  <div class="row">
    <button onclick="post('/on')">ON</button>
    <button class="off" onclick="post('/off')">OFF</button>
  </div>

  <label>Brightness <span id="brval">128</span></label>
  <input type="range" min="0" max="255" value="128" id="br"
    oninput="document.getElementById('brval').textContent = this.value"
    onchange="post('/bright?value=' + this.value)">

  <div class="row">
    <button onclick="post('/timer?mins=1')">1 min</button>
    <button onclick="post('/timer?mins=5')">5 min</button>
    <button onclick="post('/timer?mins=15')">15 min</button>
  </div>

  <script>
    async function post(p) { await fetch(p, { method: 'POST' }); refresh(); }
    async function refresh() {
      try {
        const r = await fetch('/state.json');
        const j = await r.json();
        const t = j.secondsLeft > 0
                 ? ' (auto-off in ' + j.secondsLeft + 's)'
                 : '';
        document.getElementById('state').textContent =
          (j.on ? 'ON' : 'OFF') + ' @ ' + j.brightness + t;
        document.getElementById('br').value = j.brightness;
        document.getElementById('brval').textContent = j.brightness;
      } catch (e) { /* offline */ }
    }
    setInterval(refresh, 1000);
    refresh();
  </script>
</body></html>
)HTML";

void handleRoot() {
  server.send_P(200, "text/html", index_html);
}

void handleState() {
  String json = "{\"on\":";
  json += (lamp.on ? "true" : "false");
  json += ",\"brightness\":";
  json += lamp.brightness;
  json += ",\"secondsLeft\":";
  if (lamp.autoOffAt) {
    long remaining = (long)(lamp.autoOffAt - millis()) / 1000;
    if (remaining < 0) remaining = 0;
    json += remaining;
  } else json += 0;
  json += "}";
  server.send(200, "application/json", json);
}

void handleOn()   { lamp.on = true;  lamp.autoOffAt = 0; applyLamp(); server.send(204); }
void handleOff()  { lamp.on = false; lamp.autoOffAt = 0; applyLamp(); server.send(204); }

void handleBright() {
  int v = constrain(server.arg("value").toInt(), 0, 255);
  lamp.brightness = v;
  if (v == 0) lamp.on = false;
  else if (!lamp.on) lamp.on = true;
  applyLamp();
  server.send(204);
}

void handleTimer() {
  int m = constrain(server.arg("mins").toInt(), 1, 60);
  lamp.on = true;
  lamp.autoOffAt = millis() + (unsigned long)m * 60UL * 1000UL;
  applyLamp();
  server.send(204);
}

void setup() {
  Serial.begin(115200);
  pinMode(LED_PIN, OUTPUT);
  applyLamp();

  WiFi.mode(WIFI_STA);
  WiFi.begin(SSID, PASSWORD);
  while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.print("."); }
  Serial.println();
  Serial.print("# Open http://"); Serial.println(WiFi.localIP());

  server.on("/", handleRoot);
  server.on("/state.json", handleState);
  server.on("/on",     HTTP_POST, handleOn);
  server.on("/off",    HTTP_POST, handleOff);
  server.on("/bright", HTTP_POST, handleBright);
  server.on("/timer",  HTTP_POST, handleTimer);
  server.begin();
}

void loop() {
  server.handleClient();

  // Auto-off check
  if (lamp.autoOffAt && (long)(millis() - lamp.autoOffAt) >= 0) {
    lamp.on = false;
    lamp.autoOffAt = 0;
    applyLamp();
    Serial.println("# Auto-off fired");
  }
}

Step 3 — upload and open from your phone

Phone on the same WiFi → open http://<esp-IP>/.
Big "Smart Lamp" page appears.
Tap ON. LED lights up. State updates to "ON @ 128".
Drag slider. Brightness changes live (releases of the slider trigger the POST; the on-input event updates the on-screen number).
Tap "1 min". LED lights up; state shows "ON @ 128 (auto-off in 60s)"; countdown updates every second.
Wait 60 s — LED switches off; state shows "OFF @ 128".

Step 4 — open on two phones at once

Two family members can both have the page open. When one toggles, the other sees the state update within 1 s (refresh interval). Multi-user smart-home control.

Step 5 — scaling to a real lamp (talk only, no wiring)

To control a real 230 V household bulb you'd add an opto-isolated relay module between the ESP's GPIO and the mains-side switch. The module electrically isolates the mains from the microcontroller; the GPIO never sees mains voltage. Hardware:

A 5 V opto-isolated relay module (cheap, sold as "ESP relay" or "Arduino relay").
Wire the ESP GPIO + 5 V + GND to the relay's low side.
The relay's high side (COM + NO contacts) goes in series with the lamp's live wire — done by a qualified electrician inside a sealed plastic enclosure.

Do not do this without adult supervision and proper enclosure. A loose live wire is a fatal hazard. The architecture is the same as this lesson's LED; only the load and the safety bar change.

For a no-electrician build: buy a Sonoff smart plug, flash Tasmota or ESPHome, expose HTTP endpoints — the work is identical to this lesson but the manufacturer already did the mains-safety engineering.

Step 6 — bookmark on your phone's home screen

Both iOS and Android let you "Add to Home Screen" from the browser. Your lamp page becomes a one-tap launchable icon — feels like a native app, runs entirely off your ESP's tiny web server.

Try It Yourself 15 min

🟢 Easy

Goal: Add a "Cancel timer" button that clears the auto-off without turning off the lamp.

Hint

server.on("/cancel-timer", HTTP_POST, []() {
  lamp.autoOffAt = 0;
  server.send(204);
});

Add a button on the page. The state.json already reports secondsLeft; the UI just shows it.

🟡 Medium

Goal: Save the lamp's last state (on/off and brightness) to flash so it survives a reboot. On boot, restore the saved state.

Hint

Use the EEPROM library — read/write to addresses 0..3. Persist on every state change. On boot, read addresses 0..3 and apply.

#include <EEPROM.h>

void saveState() {
  EEPROM.write(0, lamp.on ? 1 : 0);
  EEPROM.write(1, lamp.brightness);
  EEPROM.commit();
}

// in setup, before WiFi:
EEPROM.begin(8);
lamp.on = EEPROM.read(0) == 1;
lamp.brightness = EEPROM.read(1);
if (lamp.brightness == 0) lamp.brightness = 128;
applyLamp();

Beware: flash has limited write cycles (~100,000). For something that changes often, don't save every change — debounce / save only after settling.

🔴 Stretch

Goal: Add a daily schedule. The lamp turns on at 7 PM and off at 11 PM. Needs the current time — preview of L03-35 (NTP).

Hint

Use configTime(0, 0, "pool.ntp.org") (built into both ESP cores) and time(nullptr) + localtime_r to get the current hour. In loop(), compare to the schedule and toggle. Tomorrow's lesson dives into this properly.

Mini-Challenge · Ship the smart lamp 10 min

Put the ESP + LED + wiring inside a small clear-plastic enclosure (a takeaway box works). Cable management.
Print a sticker: device name + URL (http://192.168.1.42). Stick it on the box.
Verify the home-screen icon works on your phone.
Hand the phone to a parent / sibling. Can they toggle the lamp without instruction?
Take a 30-second video: phone, page, lamp on, brightness slider, 1-min timer countdown.

Ship-ready test: if a non-coder can turn it on from across the room, set a 5-minute timer, and walk away knowing it'll go off — you've shipped a real smart-home device.

Cluster F is done. Cluster G (Time and Internet Glue) starts tomorrow — short cluster that gives your device a real-world clock and the ability to push notifications elsewhere.

Recap 5 min

Smart lamp = ESP web server + HTML control page + a few POST endpoints + one struct of state + a non-blocking auto-off timer. The architecture is exactly the same as commercial smart bulbs (the cloud part is added on top by manufacturers but the device itself works fine as a LAN-only device). For real mains loads, opt for a commercial smart plug or work with a qualified electrician — never wire 230 V yourself. Cluster G next: real time-of-day clocks via NTP and the "send a Discord message when something happens" webhooks pattern.

POST vs GET: Conventions for HTTP methods. GET = read, no side effects, safe to refresh. POST = state change. Following the convention makes scripts and integrations work as expected.
HTTP 204 No Content: Success response with no body. Used when the POST succeeded but there's nothing meaningful to return.
State of truth: The single in-memory struct that represents the current state. All handlers mutate it; all reads serialise it. Avoids divergence between hardware and reported state.
Auto-off timer: A future timestamp; loop() checks each iteration whether millis() has passed it. Non-blocking; cooperates with everything else in the loop.
Opto-isolator: A device that uses an LED and a phototransistor to pass signals between electrically isolated circuits. Used in relay modules so mains can never reach the microcontroller.
Relay module: An opto-isolated relay on a breakout PCB with a logic-level input. The standard way to switch mains from a microcontroller — but must be installed by a qualified electrician inside a sealed enclosure.
Add to Home Screen: Browser feature on iOS / Android that creates a launchable icon for a webpage. Makes your ESP's page feel like a native app.
Smart bulb / smart plug architecture: Microcontroller + WiFi + a switching circuit + a phone app. Commercial products add a cloud layer for remote access; LAN-only devices skip the cloud.

Homework 5 min

Build the smart lamp. Video a 30-second demo (page + lamp).
Save the sketch as esp-lamp.ino.
Read ahead to ARD-L03-35 (NTP Time Sync). Tomorrow we give the device a real clock.

Bring back next class:

Working lamp + video.
ESP still wired (will use for L03-35).

Learning Goals 5 min

Combine yesterday's sensor dashboard with L03-32's control endpoints into a single mobile-friendly UI: button to turn on/off, slider for brightness, live state read-back.
Add "timer" commands — turn the lamp on for N minutes, then auto-off — using non-blocking timing.
Discuss (not wire) how to scale this from a 20 mA LED to a 230 V household lamp using a relay module and proper electrical safety.

Warm-Up 10 min

Hardware for today is minimal: one LED + 220 Ω resistor on a PWM pin. (D5 on NodeMCU = GPIO14 = PWM-capable.) Wire as L03-32.

What we mean by "lamp"

For the school build, the "lamp" is a single LED. The architecture — phone UI sends commands; ESP toggles a pin — is identical to a real smart bulb. Only the load changes:

Load	Driver	Safety
LED + 220 Ω	Direct from GPIO	None needed (3.3 V, < 20 mA)
Small DC motor / 12 V LED strip	MOSFET or driver IC	Power supply isolation
230 V mains bulb	Optoisolated relay module	Wired by adult; behind a fuse; inside a sealed enclosure

New Concept · Combined dashboard + control 25 min

What we're building

One HTML page with:

Big "ON / OFF" toggle.
Brightness slider (0–255).
"Timer" quick-buttons: 1 min / 5 min / 15 min.
Live state read-back so multiple phones stay in sync.

The endpoints

Method	Path	Args	Effect
GET	/	—	HTML control page
GET	/state.json	—	Returns {on, brightness, secondsLeft}
POST	/on	—	Turn on at last brightness; clear timer
POST	/off	—	Turn off; clear timer
POST	/bright	?value=N	Set brightness; if N==0 also turn off
POST	/timer	?mins=N	Turn on, schedule auto-off after N min

Why POST for the actions and GET for reads

The basic ESP8266WebServer handles GET and POST identically by default. To distinguish, use the three-arg form: server.on("/on", HTTP_POST, handler).

Non-blocking auto-off timer

unsigned long autoOffAt = 0;       // 0 = no timer

void scheduleAutoOff(unsigned long mins) {
  autoOffAt = millis() + mins * 60UL * 1000UL;
}

void clearAutoOff() {
  autoOffAt = 0;
}

void loop() {
  server.handleClient();

  if (autoOffAt && (long)(millis() - autoOffAt) >= 0) {
    digitalWrite(LED_PIN, LOW);
    autoOffAt = 0;
    Serial.println("# Auto-off fired");
  }
}

The (long)(millis() - autoOffAt) >= 0 cast handles the (unlikely-in-this-app) millis() overflow correctly. millis() overflows after ~49 days.

State management — one struct of truth

struct LampState {
  bool on;
  int  brightness;       // 0..255
  unsigned long autoOffAt;
};

LampState lamp = {false, 128, 0};

void applyLamp() {
  analogWrite(LED_PIN, lamp.on ? lamp.brightness : 0);
}

All handlers mutate lamp then call applyLamp(). /state.json serialises lamp. Single source of truth — no risk of the actual LED disagreeing with what the API reports.

Worked Example · The smart lamp 25 min

Step 1 — wire one LED to D5 (PWM)

Same as L03-32 / L03-33.

Step 2 — the sketch

// L03-34 · WiFi-Controlled Lamp
#if defined(ESP8266)
  #include <ESP8266WiFi.h>
  #include <ESP8266WebServer.h>
  ESP8266WebServer server(80);
#elif defined(ESP32)
  #include <WiFi.h>
  #include <WebServer.h>
  WebServer server(80);
#endif

const char* SSID     = "YourNetwork";
const char* PASSWORD = "YourPassword";
const int   LED_PIN  = 14;

struct LampState {
  bool on;
  int  brightness;
  unsigned long autoOffAt;
};
LampState lamp = {false, 128, 0};

void applyLamp() {
  analogWrite(LED_PIN, lamp.on ? lamp.brightness : 0);
}

const char index_html[] PROGMEM = R"HTML(
<!doctype html><html><head>
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>Smart Lamp</title>
<style>
  :root { color-scheme: light dark; }
  body { font-family: -apple-system, sans-serif; max-width: 380px; margin: 0 auto; padding: 1rem; }
  h1 { font-size: 1.4rem; }
  .row { display: flex; gap: .5rem; margin: .8rem 0; }
  button { flex: 1; padding: 1rem; font-size: 1rem; border: 0; border-radius: 8px; background: #4f46e5; color: #fff; }
  button.off { background: #6b7280; }
  input[type=range] { width: 100%; }
  .state { font-size: 1.1rem; margin: .8rem 0; padding: .8rem; background: #f1f5f9; border-radius: 8px; }
</style></head><body>
  <h1>Smart Lamp</h1>
  <div class="state" id="state">--</div>

  <div class="row">
    <button onclick="post('/on')">ON</button>
    <button class="off" onclick="post('/off')">OFF</button>
  </div>

  <label>Brightness <span id="brval">128</span></label>
  <input type="range" min="0" max="255" value="128" id="br"
    oninput="document.getElementById('brval').textContent = this.value"
    onchange="post('/bright?value=' + this.value)">

  <div class="row">
    <button onclick="post('/timer?mins=1')">1 min</button>
    <button onclick="post('/timer?mins=5')">5 min</button>
    <button onclick="post('/timer?mins=15')">15 min</button>
  </div>

  <script>
    async function post(p) { await fetch(p, { method: 'POST' }); refresh(); }
    async function refresh() {
      try {
        const r = await fetch('/state.json');
        const j = await r.json();
        const t = j.secondsLeft > 0
                 ? ' (auto-off in ' + j.secondsLeft + 's)'
                 : '';
        document.getElementById('state').textContent =
          (j.on ? 'ON' : 'OFF') + ' @ ' + j.brightness + t;
        document.getElementById('br').value = j.brightness;
        document.getElementById('brval').textContent = j.brightness;
      } catch (e) { /* offline */ }
    }
    setInterval(refresh, 1000);
    refresh();
  </script>
</body></html>
)HTML";

void handleRoot() {
  server.send_P(200, "text/html", index_html);
}

void handleState() {
  String json = "{\"on\":";
  json += (lamp.on ? "true" : "false");
  json += ",\"brightness\":";
  json += lamp.brightness;
  json += ",\"secondsLeft\":";
  if (lamp.autoOffAt) {
    long remaining = (long)(lamp.autoOffAt - millis()) / 1000;
    if (remaining < 0) remaining = 0;
    json += remaining;
  } else json += 0;
  json += "}";
  server.send(200, "application/json", json);
}

void handleOn()   { lamp.on = true;  lamp.autoOffAt = 0; applyLamp(); server.send(204); }
void handleOff()  { lamp.on = false; lamp.autoOffAt = 0; applyLamp(); server.send(204); }

void handleBright() {
  int v = constrain(server.arg("value").toInt(), 0, 255);
  lamp.brightness = v;
  if (v == 0) lamp.on = false;
  else if (!lamp.on) lamp.on = true;
  applyLamp();
  server.send(204);
}

void handleTimer() {
  int m = constrain(server.arg("mins").toInt(), 1, 60);
  lamp.on = true;
  lamp.autoOffAt = millis() + (unsigned long)m * 60UL * 1000UL;
  applyLamp();
  server.send(204);
}

void setup() {
  Serial.begin(115200);
  pinMode(LED_PIN, OUTPUT);
  applyLamp();

  WiFi.mode(WIFI_STA);
  WiFi.begin(SSID, PASSWORD);
  while (WiFi.status() != WL_CONNECTED) { delay(500); Serial.print("."); }
  Serial.println();
  Serial.print("# Open http://"); Serial.println(WiFi.localIP());

  server.on("/", handleRoot);
  server.on("/state.json", handleState);
  server.on("/on",     HTTP_POST, handleOn);
  server.on("/off",    HTTP_POST, handleOff);
  server.on("/bright", HTTP_POST, handleBright);
  server.on("/timer",  HTTP_POST, handleTimer);
  server.begin();
}

void loop() {
  server.handleClient();

  // Auto-off check
  if (lamp.autoOffAt && (long)(millis() - lamp.autoOffAt) >= 0) {
    lamp.on = false;
    lamp.autoOffAt = 0;
    applyLamp();
    Serial.println("# Auto-off fired");
  }
}

Step 3 — upload and open from your phone

Phone on the same WiFi → open http://<esp-IP>/.
Big "Smart Lamp" page appears.
Tap ON. LED lights up. State updates to "ON @ 128".
Drag slider. Brightness changes live (releases of the slider trigger the POST; the on-input event updates the on-screen number).
Tap "1 min". LED lights up; state shows "ON @ 128 (auto-off in 60s)"; countdown updates every second.
Wait 60 s — LED switches off; state shows "OFF @ 128".

Step 4 — open on two phones at once

Two family members can both have the page open. When one toggles, the other sees the state update within 1 s (refresh interval). Multi-user smart-home control.

Step 5 — scaling to a real lamp (talk only, no wiring)

A 5 V opto-isolated relay module (cheap, sold as "ESP relay" or "Arduino relay").
Wire the ESP GPIO + 5 V + GND to the relay's low side.
The relay's high side (COM + NO contacts) goes in series with the lamp's live wire — done by a qualified electrician inside a sealed plastic enclosure.

Do not do this without adult supervision and proper enclosure. A loose live wire is a fatal hazard. The architecture is the same as this lesson's LED; only the load and the safety bar change.

Step 6 — bookmark on your phone's home screen

Both iOS and Android let you "Add to Home Screen" from the browser. Your lamp page becomes a one-tap launchable icon — feels like a native app, runs entirely off your ESP's tiny web server.

Try It Yourself 15 min

🟢 Easy

Goal: Add a "Cancel timer" button that clears the auto-off without turning off the lamp.

Hint

server.on("/cancel-timer", HTTP_POST, []() {
  lamp.autoOffAt = 0;
  server.send(204);
});

Add a button on the page. The state.json already reports secondsLeft; the UI just shows it.

🟡 Medium

Goal: Save the lamp's last state (on/off and brightness) to flash so it survives a reboot. On boot, restore the saved state.

Hint

Use the EEPROM library — read/write to addresses 0..3. Persist on every state change. On boot, read addresses 0..3 and apply.

#include <EEPROM.h>

void saveState() {
  EEPROM.write(0, lamp.on ? 1 : 0);
  EEPROM.write(1, lamp.brightness);
  EEPROM.commit();
}

// in setup, before WiFi:
EEPROM.begin(8);
lamp.on = EEPROM.read(0) == 1;
lamp.brightness = EEPROM.read(1);
if (lamp.brightness == 0) lamp.brightness = 128;
applyLamp();

Beware: flash has limited write cycles (~100,000). For something that changes often, don't save every change — debounce / save only after settling.

🔴 Stretch

Goal: Add a daily schedule. The lamp turns on at 7 PM and off at 11 PM. Needs the current time — preview of L03-35 (NTP).

Hint

Mini-Challenge · Ship the smart lamp 10 min

Put the ESP + LED + wiring inside a small clear-plastic enclosure (a takeaway box works). Cable management.
Print a sticker: device name + URL (http://192.168.1.42). Stick it on the box.
Verify the home-screen icon works on your phone.
Hand the phone to a parent / sibling. Can they toggle the lamp without instruction?
Take a 30-second video: phone, page, lamp on, brightness slider, 1-min timer countdown.

Ship-ready test: if a non-coder can turn it on from across the room, set a 5-minute timer, and walk away knowing it'll go off — you've shipped a real smart-home device.

Cluster F is done. Cluster G (Time and Internet Glue) starts tomorrow — short cluster that gives your device a real-world clock and the ability to push notifications elsewhere.

Recap 5 min

POST vs GET: Conventions for HTTP methods. GET = read, no side effects, safe to refresh. POST = state change. Following the convention makes scripts and integrations work as expected.
HTTP 204 No Content: Success response with no body. Used when the POST succeeded but there's nothing meaningful to return.
State of truth: The single in-memory struct that represents the current state. All handlers mutate it; all reads serialise it. Avoids divergence between hardware and reported state.
Auto-off timer: A future timestamp; loop() checks each iteration whether millis() has passed it. Non-blocking; cooperates with everything else in the loop.
Opto-isolator: A device that uses an LED and a phototransistor to pass signals between electrically isolated circuits. Used in relay modules so mains can never reach the microcontroller.
Relay module: An opto-isolated relay on a breakout PCB with a logic-level input. The standard way to switch mains from a microcontroller — but must be installed by a qualified electrician inside a sealed enclosure.
Add to Home Screen: Browser feature on iOS / Android that creates a launchable icon for a webpage. Makes your ESP's page feel like a native app.
Smart bulb / smart plug architecture: Microcontroller + WiFi + a switching circuit + a phone app. Commercial products add a cloud layer for remote access; LAN-only devices skip the cloud.

Homework 5 min

Build the smart lamp. Video a 30-second demo (page + lamp).
Save the sketch as esp-lamp.ino.
Read ahead to ARD-L03-35 (NTP Time Sync). Tomorrow we give the device a real clock.

Bring back next class:

Working lamp + video.
ESP still wired (will use for L03-35).