Building Home Made Radio Locator

First, I know nothing at all about this topic, which is why I'm looking for help. I'm interested in building a radio transmitter/receiver like a child locator at home. The difference would be I'd like the receiver to beep rather than the transmitter, if that's even the correct language to use. For example, if your kid (or cell phone, briefcase, etc.) strays too far, your transmitter automatically beeps and/or vibrates at a certain distance. I'm trying to find out the following questions: 2. How to have the receiver both vibrate and beep. 3. Would it be possible to have the transmitter be adjustable to various distances. 4. How small could the receiver be? 5. How much it would cost to build this at home, using the smallest units for each .Essentially I'd like the receiver to be able to fit on the back of a phone, key ring, etc., and the transmitter to be similar in size. I understand this is a lot of information, so any help is greatly appreciated, I'm at square one right now. Thanks in advance.

These might give you an idea and they run from $30 to $350.

http://www.brickhousesecurity.com/category/gp...clid=CLfM-NC6xbICFad7Qgod-h0AEA&ad=4871686103

Hello,

I might be able to help answer your questions.

1) It can be done, the level of complexity is only limited to how much effort/money you want to put into the project.

2) Easy, vibration and sound are both easy add ons once you establish your other boundaries.

3) Various distances is ambiguous. I think this could be a very easy project if you wanted to keep the distance under 30 feet. Going beyond that might require GPS and exponentially increase your complexity.

4) Just playing around, the transmitter could fit on a key chain as well as the receiver(in a simple design).

5) That is entirely up to you. A cheap xmtr/rcvr on a super small range could be fairly cheap, maybe $50-75. A decent setup could run $100-150. A versatile application could easily be $200 in hardware but take 20-40 hours programming(depends on your level of skill).

6) Suggestions:
So there's a couple things your talking about here that could be broken up to make things easier:

Communications between the receiver and transmitter: May I suggest XBee's. Simple, easy to pick up, and can be found pretty cheap. Range is decent, 300 ft. outdoors maybe 100 indoors.

Sensors: cell phone vibration motors are a dime a dozen and buzzers/speakers are even cheaper. Sparkfun.com can get you started.

Data processing: This is the tricky part. What part of your system will tell you when you've reached your boundaries? Typically, one would use a microcontroller. May I suggest the Basic Stamp 2: parallax.com has multiple options with support for XBees, sensors, and many projects/code to get you started.

Power supply: Good to think about up front. Form factor wise a slim battery would work best, but limit your range/complexity. I think a 9V would be a good idea, I know the Basic Stamp can be ran off of it as well as most other applications. Just thinking from a size perspective.

Okay, so I hope that helps. Some other places to look might be Arduino's microcontrollers, instructables.com, and other great forums like this one.