Auto-detect relay port via ID probe in relay script

relay

@@ -1,9 +1,42 @@
 #!/usr/bin/env python3
-"""Toggle the USB relay via serial: ./relay 1|0|id [serial_port]."""
+"""Toggle the USB relay via serial: ./relay 1|0|id [serial_port].
+
+Without an explicit port the relay board is found automatically: every
+USB serial port is probed with the ID command until one answers.
+"""
 import sys
 import serial
+import serial.tools.list_ports
 
-DEFAULT_PORT = "/dev/cu.usbserial-2110"
+
+def find_relay_port() -> str | None:
+    """Find the relay board by probing CH340 ports with the ID command."""
+    candidates = [
+        p.device
+        for p in serial.tools.list_ports.comports()
+        if "1A86:7523" in (p.hwid or "")
+    ]
+    for dev in candidates:
+        try:
+            s = serial.Serial()
+            s.port = dev
+            s.baudrate = 115200
+            s.timeout = 1
+            s.dtr = False
+            s.rts = False
+            s.open()
+            try:
+                s.readline()  # gooi boot-banner weg
+                s.reset_input_buffer()
+                s.write(b"?")
+                s.flush()
+                if s.readline().decode(errors="replace").startswith("ID "):
+                    return dev
+            finally:
+                s.close()
+        except serial.SerialException:
+            continue
+    return None
 
 def main() -> int:
     if len(sys.argv) not in (2, 3) or sys.argv[1] not in ("0", "1", "id"):
@@ -11,7 +44,15 @@ def main() -> int:
         return 1
     command = sys.argv[1]
     serial_cmd = "?" if command == "id" else command
-    port = sys.argv[2] if len(sys.argv) == 3 else DEFAULT_PORT
+    if len(sys.argv) == 3:
+        port = sys.argv[2]
+    else:
+        port = find_relay_port()
+        if port is None:
+            print("ERROR: no relay module found on any USB serial port",
+                  file=sys.stderr)
+            return 1
+        print(f"Using {port}", file=sys.stderr)
     try:
         # dtr/rts expliciet uit vóór open: voorkomt (of beperkt) de CH340
         # auto-reset (DTR->RST) bij het openen van de poort. pyserial 3.5