Tiny battery powered ultra low power arduino (atmega328p) compatible sensor module with a RFM69 radio module.

The purpose is to mount these devices inside wall light switches (instead of a normal mains switch) and send a wireless signal to a gateway if someone presses a button. The gateway forwards the received message to a home automation module to initiate the appropriate action (e.g. turn on/off the corresponding light or wall plug or…). Up to 4 different switches can be connected to a single device. The device also has a small SMD piezo buzzer on-board, so audible feedback is possible (e.g. gateway did not acknowledge the transmission). Normally, the device also sends a “i’m still alive” message at regular intervals containing current battery voltage. This allows for quick and automated detection of faulty devices and monitoring of link quality.

Battery life is estimated >40 years and is based on an average of 5 message/hour non-stop (=120 presses/day avg) and a 1200mAh Li-SOCl2 battery (self discharge is accounted for). Deep sleep current is measured to be <300nA, waking up and sending a message is measured to consume about 1400uJoule of energy. This ultra long lifetime is only possible with Lithium Thionyl Chloride batteries.

Since this device is battery powered and generates no heat, it is completely safe to mount in any type of wall (or outside or…).

The zipfile contains the gerber files and a pdf with the current schematic, BOM, board assembly views, dimensions drawing, drill info, 3D model… Full documentation, firmware and 3D printable enclosure is still in development and not (yet) included.

Changelog:

v 1.0 Original version (works and tested OK)

v 1.1 Small hardware modification to reduce power consumption when a button is pressed or locked