smarter letterbox – exercise saver

The letterbox is five doors and fifty metres from my desk. The uncertainty of the post lady arriving in the afternoon means that I make wasted journeys to the gate. OMG something had to be done.

I fitted a 433MHz door/windows sensor to a letterbox lid. The sensor is inexpensive (£4) and works for months on one AAA cell. A wifi based sensor would need constant power or lithium cells.

  • 433MHz door/windows sensors are found in wireless home security systems. Some door sensors emit two RF codes: when post lady opens the letterbox lid, the sensor is separated from a magnet so it sends an open signal. When the lid drops it sends a closed signal, but as the box is metal I never see that signal.
  • Just 20 metres away, in line of sight behind a window, is a Sonoff RF Bridge. It’s powered by a USB charger and with no modification it will ‘hear’ the closed signal and send a message to the Sonoff app (Ewelink) on a phone.
  • That’s pretty much a full solution but I wanted the letterbox sensor to alert other devices round the house. Furthermore I’d later be fitting sensors to monitor someone at the door; someone at the back; several room temperatures. So I flashed the Sonoff RF Bridge with more versatile software called Tasmota. This software runs as a web server and it sends MQTT messages to Home Assistant, my home automation hub.
  • A Raspberry Pi runs Home Assistant (as I show here). Home Assistant has an addon that receives MQTT messages – it is called the Mosquitto MQTT broker. When the letterbox is opened and sends an MQTT message, a wide range of actions are available – from sending alerts in all directions, blinking a light or sounding a chime.
  • Details to follow

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