Sunset Sunrise Switch, a ESP32 project

The Sunset Sunrise Switch is an ESP32-based automation controller that turns devices on and off based on sunrise and sunset times at your location. It calculates solar events using your geographic coordinates and automatically switches a relay-controlled load at dawn and dusk, making it ideal for outdoor lighting, irrigation systems, or other time-of-day automation.

This guide provides a complete wiring diagram, parts list, and step-by-step assembly instructions for connecting the relay module to the ESP32. You'll also get the full firmware with a web-based setup page where you enter your Wi-Fi credentials and location coordinates. The device stores these settings in non-volatile memory and begins controlling your load immediately after setup.

Wiring diagram

Gather all the parts

Qty	Component
1	1-Channel 5V Relay Module, 3.3V Logic Compatible 5 V, active-low, 3.3 V logic compatible A small relay board that lets the ESP32 safely turn one low-voltage or suitably enclosed mains load on and off.

Assemble it in 5 steps

1. Keep the load side safe

Mount the ESP32 and relay module where you can reach their low-voltage pins. If you are switching a mains-powered lamp or appliance, use a fully enclosed, correctly rated relay enclosure and have a qualified electrician make the mains connections; do not put mains wiring on a breadboard.

Test the project first with a battery-powered low-voltage lamp or other low-voltage load.
The relay screw terminals are the load side; keep them physically separated from the ESP32 pins.
Touching exposed mains wiring can cause serious injury or fire.

2. Power the relay board

With the ESP32 unplugged, connect relay_1 VCC to the ESP32 VIN/5V pin (power) and relay_1 GND to an ESP32 GND pin (ground).

Use short wires for these two connections so the relay has steady power.
Make sure VCC and GND are not swapped — swapped power can damage the relay board.

3. Connect the relay control wire

Connect relay_1 IN to ESP32 GPIO27 (signal). This relay turns on when GPIO27 is pulled low by the ESP32.

GPIO27 is the only control wire needed between the ESP32 and relay board.
Do not connect relay_1 IN directly to the 5V pin — that can leave the load permanently switched on.

4. Connect the load through the relay

Route the supply wire that you want to switch into relay_1 COM, then route relay_1 NO to the load's supply connection (switched power). Leave NC unused so the load starts off whenever the relay is off.

COM and NO act like a switch that closes only after sunset.
For a low-voltage test load, switch its positive wire and connect its negative wire directly back to the supply negative terminal.
Never exceed the relay's printed contact rating, and do not use exposed mains wiring.

5. Enter Wi-Fi and location on the setup page

Plug the ESP32 into USB. On a phone or computer, join the Wi-Fi network named Astronomical-Switch-Setup and open http://192.168.4.1. Select your normal Wi-Fi network, enter its password, then enter the installation latitude and longitude before pressing Save and connect.

Latitude is positive north of the equator and negative south; longitude is positive east of Greenwich and negative west.
The ESP32 saves the Wi-Fi details and coordinates even after it is unplugged, then restarts and obtains UTC time from the internet.
The switch stays off until it has both a saved location and valid UTC network time, so an incomplete first setup will not accidentally turn the load on.

Review all connections

1. Connections between "relay_1" and "ESP32"

Function	relay_1	ESP32
`power`	VCC	VIN
`ground`	GND	GND
`digital`	IN	GPIO 27
`power`	COM → incoming supply wire for the load being switched	EXT
`power`	NO → outgoing supply wire to the load being switched	EXT

Deploy the firmware

main.cppOpen in Schematik

#include <Arduino.h>
#include <WiFi.h>
#include <WebServer.h>
#include <Preferences.h>
#include <time.h>
#include <math.h>

// Set the installation position. North/east are positive; south/west are negative.

// Forward declarations
double toRadians(double degrees);
double toDegrees(double radians);
bool clockIsValid();
String pageHeader(const String &title);
void handleRoot();
void handleSave();
void startSetupServer();
int solarEventMinuteUtc(const tm &utcTime, bool sunrise);
void setRelay(bool on);
void updateSwitch();
void connectWiFi();

// These values are entered on the first-run setup page and retained after power is removed.
double latitude = 0.0;
double longitude = 0.0;
bool locationSaved = false;

constexpr int RELAY_PIN = 27;
constexpr bool RELAY_ON_LEVEL = LOW;   // This relay module is active-low.
constexpr bool RELAY_OFF_LEVEL = HIGH;
constexpr unsigned long WIFI_RETRY_MS = 30000;
constexpr unsigned long CLOCK_RETRY_MS = 10000;
constexpr double PI_VALUE = 3.14159265358979323846;

WebServer server(80);
Preferences preferences;
String savedSsid;
String savedPassword;
bool setupServerRunning = false;
bool relayState = false;
int cachedYear = -1;
int cachedDayOfYear = -1;
int sunriseMinute = -1;
int sunsetMinute = -1;
unsigned long lastWifiAttempt = 0;
unsigned long lastClockAttempt = 0;

double toRadians(double degrees) { return degrees * PI_VALUE / 180.0; }
double toDegrees(double radians) { return radians * 180.0 / PI_VALUE; }
bool clockIsValid() { return time(nullptr) > 1700000000; }

String pageHeader(const String &title) {
  return "<!doctype html><html><head><meta name='viewport' content='width=device-width,initial-scale=1'>"
         "<title>" + title + "</title><style>body{font-family:sans-serif;max-width:36rem;margin:2rem auto;padding:0 1rem}"
         "input,select,button{box-sizing:border-box;width:100%;padding:.7rem;margin:.35rem 0;font-size:1rem}button{background:#1769aa;color:white;border:0;border-radius:.25rem}small{color:#444}</style></head><body>";
}

void handleRoot() {
  String html = pageHeader("Astronomical Switch Setup");
  html += "<h1>Connect the switch to Wi-Fi</h1><p>Select your network, then enter its password. The switch uses network time in UTC.</p>";
  int networkCount = WiFi.scanNetworks();
  html += "<form action='/save' method='post'><label>Wi-Fi network</label><select name='ssid' required>";
  if (networkCount <= 0) html += "<option value=''>No networks found — refresh this page and try again.</option>";
  for (int i = 0; i < networkCount; ++i) {
    String network = WiFi.SSID(i);
    if (network.length() == 0) continue;
    html += "<option value='" + network + "'" + (network == savedSsid ? " selected" : "") + ">" + network + " (" + String(WiFi.RSSI(i)) + " dBm)</option>";
  }
  html += "</select><label>Password</label><input name='password' type='password' required autocomplete='current-password'>"
          "<label>Latitude</label><input name='latitude' type='number' step='any' min='-90' max='90' required value='" + String(latitude, 6) + "' placeholder='Example: 40.7128'>"
          "<small>Use positive numbers north of the equator and negative numbers south.</small>"
          "<label>Longitude</label><input name='longitude' type='number' step='any' min='-180' max='180' required value='" + String(longitude, 6) + "' placeholder='Example: -74.0060'>"
          "<small>Use positive numbers east of Greenwich and negative numbers west.</small>"
          "<button type='submit'>Save and connect</button></form><p><small>The Wi-Fi details and location are kept even if power is removed. The switch uses network time in UTC.</small></p></body></html>";
  server.send(200, "text/html", html);
}

void handleSave() {
  String ssid = server.arg("ssid");
  String password = server.arg("password");
  double enteredLatitude = server.arg("latitude").toDouble();
  double enteredLongitude = server.arg("longitude").toDouble();
  if (ssid.length() == 0 || server.arg("latitude").length() == 0 || server.arg("longitude").length() == 0 ||
      enteredLatitude < -90.0 || enteredLatitude > 90.0 || enteredLongitude < -180.0 || enteredLongitude > 180.0) {
    server.send(400, "text/html", pageHeader("Check your details") + "<h1>Enter a network and valid location</h1><p>Latitude must be from -90 to 90 and longitude from -180 to 180.</p><p><a href='/'>Return to setup</a></p></body></html>");
    return;
  }
  preferences.begin("astro-switch", false);
  preferences.putString("ssid", ssid);
  preferences.putString("password", password);
  preferences.putDouble("latitude", enteredLatitude);
  preferences.putDouble("longitude", enteredLongitude);
  preferences.putBool("location-set", true);
  preferences.end();
  server.send(200, "text/html", pageHeader("Saved") + "<h1>Wi-Fi details saved</h1><p>The switch is now trying to join <b>" + ssid + "</b>. You can close this page.</p></body></html>");
  delay(750);
  ESP.restart();
}

void startSetupServer() {
  if (setupServerRunning) return;
  WiFi.mode(WIFI_AP_STA);
  WiFi.softAP("Astronomical-Switch-Setup");
  server.on("/", HTTP_GET, handleRoot);
  server.on("/save", HTTP_POST, handleSave);
  server.begin();
  setupServerRunning = true;
  Serial.println("Setup Wi-Fi: Astronomical-Switch-Setup, browse to http://192.168.4.1");
}

int solarEventMinuteUtc(const tm &utcTime, bool sunrise) {
  const double zenith = 90.833;
  const int dayOfYear = utcTime.tm_yday + 1;
  const double longitudeHour = longitude / 15.0;
  const double approximateTime = dayOfYear + ((sunrise ? 6.0 : 18.0) - longitudeHour) / 24.0;
  const double meanAnomaly = (0.9856 * approximateTime) - 3.289;
  double trueLongitude = meanAnomaly + 1.916 * sin(toRadians(meanAnomaly)) + 0.020 * sin(2.0 * toRadians(meanAnomaly)) + 282.634;
  trueLongitude = fmod(trueLongitude + 360.0, 360.0);
  double rightAscension = toDegrees(atan(0.91764 * tan(toRadians(trueLongitude))));
  rightAscension = fmod(rightAscension + 360.0, 360.0);
  rightAscension = (rightAscension + floor(trueLongitude / 90.0) * 90.0 - floor(rightAscension / 90.0) * 90.0) / 15.0;
  const double sinDeclination = 0.39782 * sin(toRadians(trueLongitude));
  const double cosDeclination = cos(asin(sinDeclination));
  const double cosHourAngle = (cos(toRadians(zenith)) - sinDeclination * sin(toRadians(latitude))) / (cosDeclination * cos(toRadians(latitude)));
  if (cosHourAngle > 1.0 || cosHourAngle < -1.0) return -1;
  double hourAngle = toDegrees(acos(cosHourAngle));
  if (sunrise) hourAngle = 360.0 - hourAngle;
  const double utcHours = fmod(hourAngle / 15.0 + rightAscension - 0.06571 * approximateTime - 6.622 - longitudeHour + 24.0, 24.0);
  return static_cast<int>(round(utcHours * 60.0)) % 1440;
}

void setRelay(bool on) {
  if (relayState == on) return;
  relayState = on;
  digitalWrite(RELAY_PIN, on ? RELAY_ON_LEVEL : RELAY_OFF_LEVEL);
  Serial.printf("Relay is now %s\n", on ? "ON" : "OFF");
}

void updateSwitch() {
  if (!clockIsValid()) { setRelay(false); return; }
  time_t now = time(nullptr);
  tm utcTime;
  gmtime_r(&now, &utcTime);
  if (cachedYear != utcTime.tm_year || cachedDayOfYear != utcTime.tm_yday) {
    cachedYear = utcTime.tm_year;
    cachedDayOfYear = utcTime.tm_yday;
    sunriseMinute = solarEventMinuteUtc(utcTime, true);
    sunsetMinute = solarEventMinuteUtc(utcTime, false);
    Serial.printf("UTC sunrise=%d min, sunset=%d min\n", sunriseMinute, sunsetMinute);
  }
  if (sunriseMinute < 0 || sunsetMinute < 0) { setRelay(false); return; }
  const int nowMinute = utcTime.tm_hour * 60 + utcTime.tm_min;
  const bool shouldBeOn = (sunsetMinute < sunriseMinute) ? (nowMinute >= sunsetMinute || nowMinute < sunriseMinute) : (nowMinute >= sunsetMinute && nowMinute < sunriseMinute);
  setRelay(shouldBeOn);
}

void connectWiFi() {
  if (savedSsid.length() == 0 || WiFi.status() == WL_CONNECTED || millis() - lastWifiAttempt < WIFI_RETRY_MS) return;
  lastWifiAttempt = millis();
  WiFi.mode(setupServerRunning ? WIFI_AP_STA : WIFI_STA);
  WiFi.begin(savedSsid.c_str(), savedPassword.c_str());
  Serial.println("Connecting to saved Wi-Fi...");
}

void setup() {
  Serial.begin(115200);
  pinMode(RELAY_PIN, OUTPUT);
  digitalWrite(RELAY_PIN, RELAY_OFF_LEVEL);
  preferences.begin("astro-switch", true);
  savedSsid = preferences.getString("ssid", "");
  savedPassword = preferences.getString("password", "");
  latitude = preferences.getDouble("latitude", 0.0);
  longitude = preferences.getDouble("longitude", 0.0);
  locationSaved = preferences.getBool("location-set", false);
  preferences.end();
  setenv("TZ", "UTC0", 1);
  tzset();
  if (savedSsid.length() == 0 || !locationSaved) startSetupServer();
  lastWifiAttempt = millis() - WIFI_RETRY_MS;
}

void loop() {
  if (setupServerRunning) server.handleClient();
  connectWiFi();
  if (WiFi.status() == WL_CONNECTED && !clockIsValid() && millis() - lastClockAttempt >= CLOCK_RETRY_MS) {
    lastClockAttempt = millis();
    configTime(0, 0, "pool.ntp.org", "time.nist.gov");
  }
  updateSwitch();
  delay(250);
}

Wiring diagram

Gather all the parts

Assemble it in 5 steps

1. Keep the load side safe

2. Power the relay board

3. Connect the relay control wire

4. Connect the load through the relay

5. Enter Wi-Fi and location on the setup page

Review all connections

1. Connections between "relay_1" and "ESP32"

Deploy the firmware

Make it yours in one click

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