Building a Mouse Jiggler

A microcontroller that pretends to be a keyboard and mouse is one of the most useful weekend projects I’ve put together. It can keep a machine awake during a long compile, replay a tedious sequence of shortcuts at the press of a button, or type out whatever you want—all from a board that costs a couple of euros.

I built mine on an RP2040¹ ¹ The board. Roughly the size of a thumbnail—the USB-A plug is etched into the PCB instead of soldered on. —Raspberry Pi’s first in-house silicon, whose native USB controller makes HID emulation possible without any extra hardware. Source and prebuilt firmware are on GitHub.

A single press of the BOOTSEL button starts the automation; another press stops it.

Hardware §

Any RP2040 board works. I picked up a cheap MINI USB RP2040 Development Board Module from AliExpress—dual core, 4 MB flash, around three euros shipped.

Invoice — Figure 1: **The order.** €3.86 a board with free shipping—I grabbed seven so a brick or two along the way wouldn’t end the project.

The official Raspberry Pi Pico is the better documented option, but these no-name clones are pin-compatible and the same firmware runs on either.

Flashing the firmware §

While holding the BOOTSEL button, plug the board into your computer. It enumerates as a USB mass storage device. Identify it with lsblk, create a mount point, and mount it.

$ lsblk
$ sudo mkdir /mnt/micro
$ sudo mount /dev/sda1 /mnt/micro

Grab the latest .uf2 from the releases page. Three binaries are published: fake_keyboard for keyboard only, fake_mouse for mouse only, and fake_keyboard_mouse for both. Copy whichever one you want.

$ cp fake_keyboard.uf2 /mnt/micro

The board reboots automatically and re-enumerates as a keyboard, a mouse, or both. Press BOOTSEL to start the automation, press it again to stop.

Building from source §

If you’d rather build it yourself, install the ARM toolchain.² ² arm-none-eabi-gcc is the cross-compiler targeting ARM microcontrollers; arm-none-eabi-newlib provides a slim C standard library suited for embedded targets.

$ sudo pacman -S arm-none-eabi-gcc arm-none-eabi-newlib  # Arch
$ sudo apt-get install gcc-arm-none-eabi libnewlib-arm-none-eabi  # Debian

Fetch the Pico SDK and picotool submodules.³ ³ The Pico SDK ships the C/C++ libraries for RP2040 development; picotool is a command-line utility for inspecting boards and uploading firmware.

$ git submodule update --init --recursive

Build with CMake, pointing PICO_SDK_PATH at the SDK submodule.

$ PICO_SDK_PATH=./lib/pico-sdk cmake -B build -G Ninja
$ cmake --build build

The build emits three .uf2 files under build, matching the binaries on the releases page. Flash one as described above.

$ cp ./build/fake_keyboard.uf2 /mnt/micro

Hello, world §

The fastest way to confirm the board, toolchain, and SDK are all wired up correctly is to flash a minimal program and watch it print over USB serial.

#include <stdio.h>

#include "pico/stdlib.h"

auto main() -> int {
  stdio_init_all();
  while (true) {
    printf("Hello, world!\n");
    sleep_ms(1000);
  }
}

A matching CMakeLists.txt

cmake_minimum_required(VERSION 3.13)

include(./lib/pico-sdk/pico_sdk_init.cmake)
project(HelloWorld C CXX ASM)

set(CMAKE_EXPORT_COMPILE_COMMANDS ON)

pico_sdk_init()

add_executable(hello_world main.cpp)
pico_enable_stdio_usb(hello_world 1)
pico_enable_stdio_uart(hello_world 0)
target_link_libraries(hello_world pico_stdlib)
pico_add_extra_outputs(hello_world)

Build and flash the same way as before.

$ PICO_SDK_PATH=./lib/pico-sdk cmake -B build -G Ninja
$ cmake --build build
$ cp ./build/hello_world.uf2 /mnt/micro

Once it reboots, attach to the serial TTY⁴ ⁴ The board exposes USB CDC as /dev/ttyACM0 on Linux. Exit screen with Ctrl-a k. and watch the greeting roll in.

$ screen /dev/ttyACM0
Hello, world!
Hello, world!
Hello, world!

If that works, you have everything you need to iterate on your own HID firmware.

Specifications §

The only specs the manufacturer shipped with the board are these photos—no datasheet, no pinout diagram, nothing.⁵ ⁵ Welcome to no-name AliExpress electronics. The RP2040 itself is well documented, so in practice the official datasheet is what you’ll lean on.

That’s all the moving parts. The repository ships a few example scripts to get you started—swap them out, recompile, and the board will type⁶ ⁶ The keyboard variant in action. or wiggle in whatever pattern you like.

My mailing list is free, occasional, and covers a variety of topics. I will never sell or share your email address.

Have feedback? Email me at david@alvarezrosa.com.