Blink both the relay and LED

src/main.cpp

@@ -1,8 +1,19 @@
 #include <Arduino.h>
 
-void setup() { Serial.begin(9600); }
+#define RELAY 2 // GPIO4
+#define LED 5   // GPIO5
+
+void setup() {
+  Serial.begin(9600);
+  pinMode(LED, OUTPUT);
+  pinMode(RELAY, OUTPUT);
+}
 
 void loop() {
-  Serial.println("Hello world!");
+  digitalWrite(LED, HIGH);
+  digitalWrite(RELAY, HIGH);
+  delay(1000);
+  digitalWrite(LED, LOW);
+  digitalWrite(RELAY, LOW);
   delay(1000);
 }