The sensor circuitry is simple circuit, which detects if the mobile phone receives any message signal.
---------
this is required, as the phone has to be charge as long as user is talking.
---------
thus a simple F to V convertor would serve our purpose.
-----
In India the operating frequency of mobile phone operators is generally 900 MHz od 1800 MHz for the GSM system for mobile communication.
----
thus the use of simple F to V would act as a switches to trigger the rectenna circuit to on.
---
LM-2907- a simple n F to V converter use as it operates between this specified rang.
---
Thus on the reception of the signal the sensor circuitry directs the rectenna circuit to ON and the mobile phone begins to charge using the microwave power.