Grid power

Home Assistant custom integration for detecting mains presence with an isolated AC detector and an FT232 USB-UART adapter.

The integration reads the FT232 modem-control input directly with Linux TIOCMGET and monitors TIOCM_CTS.

How it works

The optocoupler's low-voltage OUT signal is connected to the FT232's CTS input. Home Assistant asks the Linux kernel for the current CTS modem-control bit using TIOCMGET. The integration interprets that bit as mains present or mains absent and polls it at the configured interval.

This is a hardware-status reading, not serial communication: no bytes are received through the FT232 RX path, and no serial data format or baud rate is involved.

Safety

This project interfaces with mains voltage. Use an appropriately rated, enclosed, certified isolation module and follow local electrical regulations. Disconnect power before wiring. Keep the mains and low-voltage sides physically separated. The author is not responsible for damage, injury, or incorrect outage detection.

Hardware

The following detector board is required:

• 1-Bit AC 220V Optocoupler Isolation Module Voltage Detect Board

You also need an FT232 USB-UART adapter with an exposed CTS input. The detector board provides the isolation between the mains side and the FT232 low-voltage side.

Hardware used

1-Bit AC 220V Optocoupler Isolation Module Voltage Detect Board

FT232 USB-UART adapter with CTS exposed

The low-voltage connections are:

AC detector	FT232 adapter
VCC	3.3 V or 5 V supported by the detector
GND	GND
OUT	CTS

The tested setup uses an FT232BL adapter and /dev/ttyUSB1 on Home Assistant OS. Other FT232 boards and device paths may work, but must be tested with the specific hardware.

3D-printed enclosure

The repository includes STL files for a simple 3D-printed enclosure:

• Enclosure box
• Enclosure cap

The assembled device:

The enclosure is a project example. It is not a certified mains-voltage enclosure. Use a suitable certified enclosure and follow local electrical regulations for any permanent installation.

Installation with HACS as a custom repository

This integration is intentionally distributed as a custom HACS repository and is not part of the official HACS store.

1. Make sure this repository is available on GitHub.
2. Open HACS → Integrations in Home Assistant.
3. Open the menu and select Custom repositories.
4. Enter the GitHub repository URL and choose Integration.
5. Add the repository and download Grid power.
6. Restart Home Assistant.
7. Open Settings → Devices & services → Add integration.
8. Search for Grid power and open it.

Manual installation

Copy the custom_components/grid_power directory into the Home Assistant configuration directory:

/config/custom_components/grid_power/

Restart Home Assistant, then add Grid power from Settings → Devices & services.

Configuration

The setup form contains:

• Serial device — for example /dev/ttyUSB1.
• Poll interval — defaults to 1 second.
• Invert CTS state — enable this if the detector's logic level is opposite to the desired mains state.

The integration creates a binary_sensor with device class power. The entity is normally named Grid power and reports on when mains power is present.

The USB device must be accessible to the Home Assistant Core container. A successful test from the Terminal & SSH add-on alone does not prove that Core can access the device, because the add-on and Core run in separate containers.

Troubleshooting

• The device cannot be opened: confirm the adapter is connected, verify the path in the integration options, and check that Home Assistant Core can access the device.
• The entity is unavailable: check Home Assistant logs for the exact device path and permission error.
• The state is reversed: enable Invert CTS state in the integration options.
• The device path changes after reboot: use a stable serial-by-ID path if it is visible to Home Assistant Core, or update the integration's device option.
• The integration is missing from Add integration: reload the Home Assistant page or clear the browser cache after installing a custom integration.

Development and publishing

This repository intentionally contains one integration only:

custom_components/grid_power/
├── __init__.py
├── binary_sensor.py
├── config_flow.py
├── const.py
├── coordinator.py
├── device.py
├── manifest.json
├── strings.json
└── translations/en.json