Use an Arduino microcontroller to sense invisible electromagnetic fields using wire, a resistor, and an LED. Aaron ALAI's EMF detector project is awesomely simple to make and fun to use. An upgraded version can be built using an LED bargraph for more 'meter-like' functionality. original project can be found here - http://www.aaronalai.com/emf-detector additional info, and Arduino sketch - http://blog.makezine.com/