There are moments when you need to know where exactly are the mains lines inside a wall. It’s not a pleasant experience to drill holes and hit the hot wire, not to mention that usually you’re doing that on a ladder, a few meters off the ground and serious accidents can result. A simple way to detect he power lines route inside a wall, is building this Mains hum sensor. A led will light when the device is brought close to the power lines. So all you need to do is move the detector along your walls and see where the lines are. Here’s the schematics:

The input pin can be a small cooper/aluminum plate. It is also sensitive to your touch, so get used with this detector before relying on it.

I am not to held responsible for any actions resulting from the use of this device. It is for experimenting purposes only.