Electronic – Measuring the distance between two points electrically

I have read somewhere (I think the Arduino forums) that sharp no longer manufactures these sensors (though that can be hearsay). I do know however that they are quite expensive compared to much simpler solutions.

The simplest would be a IR LED - IR phototransistor combination:

Phototransistor on left, led on right

You would power the IR LED (remeber the limiting resistor!) and then use the phototransistor to pick it up. The voltage drop over the phototransistor will be proportional to the intensity of the IR light it picks up which is proportional to the distance the emitter and detector are from the reflecting surface.

Pointers (Personal Experience):

• It will take some experimentation and calibration to find the correct voltage - distance relationship
• The voltage will vary depending on the reflectivity of the reflector to IR light.
• The detector and reflector will need to be shielded from each other in order to gain a accurate measurement.
• This type of sensor is only reasonaly accurate. Keep it in mind.
• The detector is susceptible to outside sources of IR light, including the sun, incandescent light and florescent light bulbs.

There are many links on the internet on how to use said components. Take a look here: http://letsmakerobots.com/node/2907

Lastly, this method (I have found) is only accurate over small distances (not more than 20cm I'd say). For longer ranges you should consider ultrasonics or those fancy Sharp sensors.

This idea might be patented, so it might not be suitable for a commercial project, but you can actually measure the position and orientation of one electronic device relative to another, with reasonable accuracy, using magnetic fields. This is how Polhemus and Ascension trackers work. They are used in VR motion tracking, and in surgery for tracking the position of surgical instruments during operations.

The basic concept is to have one set of coils transmitting, and another receiving. The transmitter coils emit audio frequency alternating magnetic fields, and the receiver coils then measure the amplitude of the fields in the three receiver coils.

There is some code available online for doing these calculations. You might also take a look at the guy's project page: Open source electromagnetic trackers using OpenIGTLink.

This may not be quite the system you're looking for, as it's fairly complex, and is giving you much more info than you wanted. However, a simpler algorithm could be used which just gave you distance.

A company called Sixense make a gaming controller with a 6DOF sensor in it. I don't know how easy it would be to integrate this technology into your project though.

Update:

Now that I know what your application is, I have been thinking of a very similar application. My suggestion would be this:

Use the three orthogonal coils approach. Both the mother and child have a set of coils. The child would be the transmitter. Every few seconds, the child module would transmit an acoustic frequency magnetic field on each coil in turn. The mother module would measure the amplitude of the voltage induced in its coils. If the amplitude was too low, or if no signal was heard for more than a few seconds, then the alarm sounds.