Arduino caps lock LED indicator

README.md

This script reads the state of the Caps Lock key and sends it to an Arduino connected to a serial port. The Arduino then controls an LED to indicate the state of the Caps Lock key.

The script can be run multiple times, but only one instance will be running at a time. If another instance is started, it will send a message to the running instance to update the state of the Caps Lock key.

If you use Hyprland, paste this into your config file:

bindrlti = , Caps_Lock, exec, $HOME/.bin/update-caps
exec-once = $HOME/.bin/update-caps

capsLED.ino

int ledG = 3, ledH = 4;

void setup() {
  pinMode(ledG, OUTPUT);
  pinMode(ledH, OUTPUT);
  digitalWrite(ledG, LOW);
  digitalWrite(ledH, LOW);
  Serial.begin(9600);
}

void loop() {
  if (!Serial.available()) return;
  switch (Serial.read()) {
    case '0':
      digitalWrite(ledH, LOW);
      break;
    case '1':
      digitalWrite(ledH, HIGH);
      break;
  }
}

update-caps.py

#!/bin/env python
import os
import sys
import socket
import atexit
import evdev
from time import sleep

arduino_port = '/dev/serial/by-id/usb-1a86_USB_Serial-if00-port0'
keyboard_event = '/dev/input/by-id/usb-Logitech_USB_Receiver-if01-event-kbd'

pidfile = '/tmp/update-caps.pid'
socketfile = '/tmp/update-caps.sock'

if os.path.exists(pidfile):
    with open(pidfile, 'r') as f:
        pid = f.read()
        if os.path.exists('/proc/' + pid):
            # already running, send message to it instead
            try:
                with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as sock:
                    sock.connect(socketfile)
                    sock.sendall('update'.encode('utf-8'))
            except ConnectionRefusedError:
                print("No running instance to send the message to.")
            sys.exit(0)
        os.unlink(pidfile)

tty = open(arduino_port, 'w')
sock = None

def cleanup():
    if sock is not None:
        sock.close()
    tty.close()
    if os.path.exists(pidfile):
        os.unlink(pidfile)
    if os.path.exists(socketfile):
        os.unlink(socketfile)

atexit.register(cleanup)

with open(pidfile, 'w') as f:
    f.write(str(os.getpid()))

if os.path.exists(socketfile):
    os.unlink(socketfile)

def update_caps():
    # Keep serial port open to avoid Arduino restarting
    device = evdev.InputDevice(keyboard_event)

    led_state = device.leds()

    if evdev.ecodes.LED_CAPSL in led_state:
        print("Caps Lock is on")
        tty.write('1')
    else:
        print("Caps Lock is off")
        tty.write('0')
    tty.flush()

update_caps()

# wait 2 seconds for the Arduino to boot
sleep(2)

update_caps()

sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
sock.bind(socketfile)
sock.listen(1)
print("Listening for messages...")
while True:
    conn, _ = sock.accept()
    with conn:
        data = conn.recv(1024)
        if data:
            if data.decode('utf-8') == 'update':
                update_caps()
            else:
                print(f"Received unknown message: {data.decode('utf-8')}")