On eBay, Alibaba or other web sites, you can find cheap Window sensors that will send a data packet to inform you about the state of your windows. They often costs less than 10$ per unit, which makes it easy to test them.
They look like this:
But what’s inside? Let’s have a look. The part that is mounted on the window itself is basically just a magnet. You might already know what will be on the other side: a reed relay. But what else:
The electronic circuit is based on the HS1527. The nice thing about this chip is the 1uA standby current. Even with a small battery, the battery life will be relatively high. It is powered by a small 12V battery – I haven’t seen this battery type before.
The good thing about the chip is that you can use a lot of cheap 433MHz receivers connected to a Raspberry Pi or Arduino to receive the signals.
However, there is one major downside: This sensor sends only a data packet when you open the window, not when you close it. This is ok if you want to use it to detect break-ins, but it won’t work if you want to detect if a window is open (e.g. if you want to turn off the heating).
Therefore I can’t recommend this sensor type for home automation use.