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Leak sensor prevented me from some pain!
(feddit.org)
Had a leak under the sink in our kitchen. I don’t want to imagine what this would have meant on a larger scale, compared the 20€ I spent on the sensor and the 10mins it costed me to install it.
A door opening contact sensor.
Aqara have good door and window sensors too
You can buy these at around the same price or less, so no point in doing that