ok,the small things is called I-SPOT,produced by a US company,but i do not know the web site.i seen it in a magzine.anyone interesting can search it himself.
if anybody want design such a thing,i think at least 3 methods:
1:just a very sample FM senter in baby,the can sent out a low level FM signal,such as 300m-400MHZ FM signal,the you receive it ,if the signal is lost ,you will know the baby is far away from you.in order to save power,you can sent 10-100ms every 1 second.when it is work,it only takes about 0.5ma 3v,and can be receivesd about 50 meter.the higher sensitivity are ,the longer distance can be get.so the device will consume only about 0.15mW in one second.
2:use a data RX/TX chip to design,and maybe a 8 pin cpu is needed.use the cpu to control the RX/TX and you can do more things than method one.
3:use a bluetooth modle,it is a easy way,also need a cpu.maybe it will have many functions but will be more expensive.
8 pin cpu can be found in:
**broken link removed**
http://www.microchip.com
use a li-ion ,which is 3v 200mah,size like a coin,can continuous drive such a device about 2000-4000 hours,about 3-6 months.
you can get some info from
**broken link removed**
and some of RX/TX chips can get from:
http://www.infineon.com/
**broken link removed**