Simple program written in C# to control Neopixel (WS2812B & SK6812) leds directly from a PC without a microcontroller. The data stream is formed in the program memory and send to a common USB to Serial bridge.
Suported usb to serial bridges
Any usb to serial converter should work as long as it can send data at 2Mbps minimum. 3Mbps produces a signal within the limits specified in the datasheet and would allow higher refresh rates. The following modules had been tested and work as expected:
Code tested only in Windows, but it should work on Linux too, with Mono. You can drive up to 512 leds with the Virtual Com Port drivers. This limit is within the drivers themselves: they send data in chunks of 4096 bytes. Because a delay in the bit stream sent to the NeoPixels could be interpreted as a reset signal, any delay inserted between these chunks introduces glitches in the intended sequence.
Because 8 bytes are used to encode the color data for a single Neopixel, at most 512 leds can be controlled glitch free (4096B / 8).
The CP2102N's driver doesn't have this 4KB limit and can drive many more leds. It has been tested with up to 2048 leds without any specific change. The FT232R can also go higher than 512 leds but only when the direct API is used. The code does not use any manufacturer specific feature.
You'll need an inverter at the TX pin. A simple NPN inverter was used for this task. The FT232R could work without this inverter as it can be configured to invert the TX signal updating the firmware.