Basic Reed switch ..

src/main.cpp

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+#include <Arduino.h>
+
+const int buttonPin = 4;  // Reed switch - don't forget the pull down resistor.
+const int ledPin =  2;    // Internal LED
+
+void setup() {
+  Serial.begin(9600);  
+  // initialize the pushbutton pin as an input
+  pinMode(buttonPin, INPUT);
+  // initialize the LED pin as an output
+  pinMode(ledPin, OUTPUT);
+}
+
+void loop() {
+  int buttonState = digitalRead(buttonPin);
+  Serial.println(buttonState);
+
+  if (buttonState == HIGH) {
+    digitalWrite(ledPin, HIGH);
+  } else {
+    digitalWrite(ledPin, LOW);
+  }
+  delay(500);
+}