I use this...
»www.softperfect.com/products/net ··· scanner/
It will tell you what is currently connected to your network but I don't know if it will tell you, as you put it "if and when a device connects". It will tell you during a scan in other words, but I don't know if it logs so that later on it will tell you what connected during a certain time frame if that is what you are looking for.
If you are looking for something to tell you what is connected at any given time though, it works great.