DIY Wireless Door Sensor

Transmitter parts list:
- Arduino Nano
- 9V battery connector
- Generic 433mhz transmitter (you can get them dirt cheap on eBay)
- Normally closed magnetic door switch

Receiver parts list:
- Wemos D1 Mini
- Generic 433mhz receiver
- Apple wall plug (the square one)


The transmitter only turns on when the door is open. To save power, it will go in to a deep sleep mode after 60 seconds.
The receiver constantly listens for the signal from the transmitter. Once received it sends a MQTT message to my Home Assistant server. The transmitter knows the door is closed after it stops receiving the the transmitter signal for a couple of seconds.