ATtiny85 CDC ACM Relay Controller

USB CDC ACM device (ttyACM) on DigiSpark ATtiny85 for controlling an induction relay via serial break signal.

Relay Logic

PB0 State	Relay	Trigger
LOW	ON (conducting)	Normal state / break cleared
HIGH	OFF (open, NO)	Serial Break asserted

Custom USB Device Name

The USB product name is stored in EEPROM and can be changed at runtime. Send the command 0xAA <length> <name bytes> on the ttyACM data channel. The device writes the name to EEPROM, reloads the USB descriptor, and re-enumerates — no reboot required. The new name is visible in lsusb. Maximum length: 64 characters.

Default name: ATtiny Relay

Firmware

Requires: avr-gcc, micronucleus

make            # build firmware
make flash      # flash via micronucleus (re-plug DigiSpark when prompted)

V-USB library is cloned automatically on first build.

Python Installation

pip install .          # API library only
pip install .[mcp]     # API + MCP server

Python API

from relay_control import RelayControl

with RelayControl() as r:
    r.relay_off()               # PB0 HIGH, relay OFF (serial break)
    r.relay_on()                # PB0 LOW, relay ON (break cleared)
    r.set_name("My Relay")      # change USB device name (auto-reconnects)
    print(r.name)               # read current USB product name

MCP Server

The package provides an MCP server for use with Claude Code via mcp-compressor.

Setup

Add to ~/.claude.json in the mcpServers section:

{
  "mcpServers": {
    "usb-relay": {
      "command": "mcp-compressor",
      "args": ["-n", "usb-relay", "-c", "max", "usb-relay-mcp"]
    }
  }
}

Restart Claude Code after editing the config.

Tools

Tool	Description
relay_list	List connected relay devices (port, USB address, name)
relay_on(device)	Turn relay ON — accepts /dev/ttyACMx or USB address like 1-2
relay_off(device)	Turn relay OFF
relay_set_name(device, name)	Change USB device name (1–64 ASCII chars)

Tests

make test       # runs pytest against real hardware