diff --git a/.gitignore b/.gitignore
index 954bdcc..8bebd2a 100644
--- a/.gitignore
+++ b/.gitignore
@@ -6,3 +6,4 @@
 **/target/
 /hello_world/main
 /aoc*/**/input
+/arduino/build-*/
diff --git a/arduino/.envrc b/arduino/.envrc
new file mode 100644
index 0000000..5301374
--- /dev/null
+++ b/arduino/.envrc
@@ -0,0 +1,2 @@
+export ARDMK_DIR=/usr/share/arduino
+export ARDMK_VENDOR=archlinux-arduino
diff --git a/arduino/Makefile b/arduino/Makefile
new file mode 100644
index 0000000..97e64ce
--- /dev/null
+++ b/arduino/Makefile
@@ -0,0 +1,3 @@
+BOARD_TAG    = uno
+
+include $(ARDMK_DIR)/Arduino.mk
diff --git a/arduino/sos.ino b/arduino/sos.ino
new file mode 100644
index 0000000..6cb7e20
--- /dev/null
+++ b/arduino/sos.ino
@@ -0,0 +1,31 @@
+#define DIT_DURATION_MS 88
+#define LETTER_PAUSE_MS 1000
+
+void setup() {
+	pinMode(13, OUTPUT);
+}
+
+void dit() {
+	digitalWrite(13, HIGH);
+	delay(DIT_DURATION_MS);
+	digitalWrite(13, LOW);
+	delay(DIT_DURATION_MS);
+}
+
+void dah() {
+	digitalWrite(13, HIGH);
+	delay(DIT_DURATION_MS * 3);
+	digitalWrite(13, LOW);
+	delay(DIT_DURATION_MS);
+}
+
+void loop() {
+	dit(); dit(); dit();
+	delay(LETTER_PAUSE_MS);
+
+	dah();
+	delay(LETTER_PAUSE_MS);
+
+	dit(); dit(); dit();
+	delay(LETTER_PAUSE_MS);
+}