With open platforms like Home Assistant, smart homes have become quite popular over the past few years. I have accumulated some smart plugs, bulbs, and sensors myself, and I like Zigbee, because it supports low-power devices and decent ranges. One thing that always annoyed me was that most wireless sensors run on disposable coin cells. Even if they last for over a year, you always find yourself replacing batteries. Ikeas Parasoll contact sensor is one of the few devices powered by a (rechargeable) AAA battery but unfortunately it’s not free of design flaws. In general, there is a lack of open hardware or software among smart home sensors. I set out to change this by designing my own smart home device. 

 I wanted to build a Zigbee sensor primarily for temperature and humidity measurements. And since most of the hardware is identical, I would also throw in a contact sensor. After all, this just meant adding a cheap hall effect switch to the device, so it can detect when a magnet is in proximity. My initial design goals were as follows:  

• The device should be powered by a single AAA cell like the Ikea Parasoll, so it needs to support voltages down to 0.9V.
• Despite the small battery, it should have at least a full year of battery life.
• It has to support smart home systems like Home Assistant and Zigbee2MQTT
• The parts should be easy to obtain and the overall cost per sensor should be below 10€.

 Usually, I am cautious about investing too much time into a project before making certain that I can complete it. With a simple sensor like this, I was sure that the hardware side was manageable and the software could not be too complicated. After all, it just has to take measurements and send them to a Zigbee hub. That can’t be rocket science, can it? That was before I knew about things like Zigbee Clusters.