Newsgroups: sci.electronics.misc
Subject: Re: cellular phone ring notification
References: <[email protected]>
"Denis Digula"  writes:

> I saw hand clock whit LED's which flash when cellular phone is about to =
> ring or ringing. I'm very interested about that ring notification so if =
> anyone have idea how that works, or have schematic of device I would be =
> grateful for that information.
> [email protected]

This kind of gadgets generally work using the following principle:
- those gadgets contains (more or less frequency selective) RF receiver
  circuit which detects when cellular phone is transmitting something
- cellular phones transmit when you have a connection on, phone is
  trying to start a connection or otherwise communicates with  
  the network (when phone is moved from one cell to another,
  when the network tell to the phone to start ringing etc.).

-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/