diff --git a/mise.toml b/mise.toml
index 95c0ea9..241d4a8 100644
--- a/mise.toml
+++ b/mise.toml
@@ -49,6 +49,18 @@ arduino-cli upload --fqbn "$FQBN" --port "$PORT" ./boards/$BOARD
 description = "Monitor (uses BOARD env var)"
 run = """
 source ./boards/$BOARD/board-config.sh
+
+LOCKFILE="/tmp/doorbell-$BOARD.lock"
+if [ -f "$LOCKFILE" ]; then
+    echo "Error: Another instance is running (lockfile exists: $LOCKFILE)"
+    echo "Remove with: rm $LOCKFILE"
+    exit 1
+fi
+trap 'rm -f "$LOCKFILE"' EXIT
+touch "$LOCKFILE"
+
+echo "Acquired lock: $LOCKFILE (Ctrl+C to release)"
+
 PORT="${PORT:-$PORT}"
 TARGET="$(readlink -f "$PORT" 2>/dev/null || echo "$PORT")"
 arduino-cli monitor -p "$TARGET" --config baudrate=115200