Blink a plugged in USB keyboard's LEDs on a USB host-capable CircuitPython board

usb_host_blink_keyboard_leds.py

# Blink a plugged in USB keyboard's LEDs 
# Designed for FruitJam but works on any other CircuitPython board with USB host
# 3 Aug 2026 - @todbot / Tod Kurt

import time
import array
import usb.core
import usb_host
import adafruit_usb_host_descriptors

REQTYPE_OUT_CLASS_IFACE = 0x21
HID_SET_REPORT = 0x09
HID_REPORT_TYPE_OUTPUT = 0x02

LED_NUMLOCK    = 0x01
LED_CAPSLOCK   = 0x02
LED_SCROLLLOCK = 0x04
LED_COMPOSE    = 0x08
LED_KANA       = 0x10

DESC_INTERFACE, DESC_ENDPOINT = 0x04, 0x05

def find_kbd_boot_interface(device):
    cfg = adafruit_usb_host_descriptors.get_configuration_descriptor(device, 0)
    i = 0
    iface_num = ep_in = None
    while i < len(cfg):
        blen, btype = cfg[i], cfg[i + 1]
        if btype == DESC_INTERFACE:
            # bInterfaceClass 0x03 = HID, bInterfaceSubClass 0x01 = boot,
            # bInterfaceProtocol 0x01 = keyboard
            if cfg[i + 5] == 0x03 and cfg[i + 6] == 0x01 and cfg[i + 7] == 0x01:
                iface_num = cfg[i + 2]
            elif iface_num is not None and ep_in is not None:
                break          # moved past the keyboard interface
        elif btype == DESC_ENDPOINT and iface_num is not None and ep_in is None:
            if cfg[i + 2] & 0x80:
                ep_in = cfg[i + 2]
        i += blen
    return iface_num, ep_in

def set_kbd_leds(device, interface, leds, report_id=0):
    return device.ctrl_transfer(
        REQTYPE_OUT_CLASS_IFACE,
        HID_SET_REPORT,
        (HID_REPORT_TYPE_OUTPUT << 8) | report_id,
        interface,
        array.array("B", [leds]),
    )


device = usb.core.find(find_all=False)      # or filter by VID/PID
iface, ep_in = find_kbd_boot_interface(device)

if device.is_kernel_driver_active(iface):
    device.detach_kernel_driver(iface)
device.set_configuration()


while True:
    print("hi", time.monotonic())
    set_kbd_leds(device, iface, LED_CAPSLOCK )
    time.sleep(0.2)
    set_kbd_leds(device, iface, LED_NUMLOCK)
    time.sleep(0.2)
    set_kbd_leds(device, iface, LED_CAPSLOCK | LED_NUMLOCK)
    time.sleep(0.2)