This is my favourite project, its too simple and very interesting because it does not require any voltage source 😀 it converts RF frequency waves from cell phone (whenever you call or send a text) to little current to flash a LED.
Actually this project is also called as LED power meter, it is used to test RF equipments. It can detect output power of our FM transmitters, by simply connecting voltmeter in the place of the load(LED) of this circuit.
The lead’s length is very important it must be exactly what told, otherwise it might produce a lower reading.
A simple description is that IN34 or IN34A RF Germanium Diodes for Crystal Radio can take “Ambient” or “Radio Waves” to turns them into DC energy.This is the simplest detector! without any batteries or hardships circuit, only with IN34 diodes and Ether to Earth!.. See https://www.youtube.com/watch?v=DtIoUX725Jk
The quality of LED is very important factor, must buy a good quality LED. The one i have used is a super bright LED I bought it for a little more than an ordinary LED
Bread board Arrangement:
Here is the link to see this project made by an Indian guy and his research on it: http://rookieelectronics.com/rf-detector-rocks/