The RelAC Is an Espressif ESP32-Powered Two-Channel Relay for Apple HomeKit Automation Setups

Pseudonymous maker "mr4lexndr" has released an open-hardware smart home board designed to control mains voltage appliances from within an Apple HomeKit ecosystem — with each board providing two remotely-controllable channels.

"It is mainly intended to be used with mains voltage appliances controlled in parallel with the [Apple] Home app and [a] physical switch," mr4lexndr explains of his compact board design, dubbed the RelAC. "It could be hidden e.g. behind a wall light switch. I have created this board as I couldn't find a reliable module on the market that could fulfill all the requirements."

The board in question, which is only barely bigger than the footprint of its components combined, uses a pair of Panasonic ADW1203HLW latching relays to perform the switching — drawing minimal power themselves by staying in their last state without requiring additional energy. Screw terminals and provided for wiring, while an Espressif ESP32 module controls everything and provides Wi-Fi connectivity.

The support for Apple's HomeKit ecosystem comes courtesy of the HomeSpan library, which implements Apple's HomeKit Accessory Protocol Specification Release R2 (HAP-R2) on compatible Espressif ESP32 microcontrollers. Once running, the board provides control over two devices with a supply voltage between 100V and 240V AC — though its creator notes it can be built as a single-channel device if preferred, to reduce the bill of materials.

The design files and firmware source code for the RelAC have been published on GitHub under the reciprocal Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license, with mr4lexndr currently working on reducing the board footprint still further and designing a 3D-printed case.