This is a basic firmware for the cheap USB relay boards that are sold on eBay and report themselves as being from www.dcttech.com. They're based on an Atmel ATtiny chip and run V-USB as the core of their firmware. I wanted to play around with some ATtiny programming so figured a good way to start was to build a new firmware for the device I already had that mimicked the existing firmware. Currently it only supports the 1 port board, but I'm sure it could be easily extended by anyone with access to the boards with more relays on them. Patches welcome.

apt install avr-libc avrdude should install the appropriate build requirements on Debian (gcc-avr will be automatically pulled in) assuming you already have build-essential installed for make.

make will then build you a main.hex which you can program to your device using avrdude. With my Bus Pirate I use:

avrdude -p attiny45 -c buspirate -P /dev/ttyUSB0 -U flash:w:main.hex:i

Pin outs on the 1 port relay board look like:

  |RELAY|
  +-----+               [Bus Pirate connections]
           o RESET      CS (white)
           o SCK        CLK (purple)
   +--+    o MISO       MISO (black)
   |AT|    □ MOSI       MOSI (grey)
   +--+
           □ 5V         +5V (orange)
           o GND        GND (brown)
  | USB |
  +-----+

USB D- is connected to Port B pin 1, USB D+ to Port B pin 2 (INT0) and the relay is hanging off Port B pin 3.

Given that V-USB is GPLv2+ or commercial all of my code is released as GPLv3+, available at https://the.earth.li/gitweb/?p=usb-relay-firmware.git;a=summary or on GitHub for easy whatever at https://github.com/u1f35c/usb-relay-firmware