The idea came looking at other Instructable:

I have chose the design with the 555 because I had several 555 around and I like build small projects, like this other credit card sized project.

Step 1: The Circuit and Components

This is a screenshot from the project of :

The Component are:

Tools:

Step 2: First Prototype

As you can see in the photo, pretty ugly.

But it was working, so I started to work to the final version.

As you can see in the video it is working and with good sensitivity, actually I had to keep the antenna very short due to the high sensitivity.

I used a 1kΩ resistor instead of 220Ωto reduce the sound of the buzzer .