Remote distance alert

[JSUP]

I'd like to tag two paired objects with a wireless sender and receiver so that one object will beep if the two objects become more than twenty feet apart from one another. The electronic components must be no larger than a wristwatch and preferable waterproof, or at least be able to be made waterproof by being encased in a waterproof housing that won't interfer with operation.
Any suggestions?

Thanks in advance.

 

[poor mystic]

Hi JSUP
The first technology that came to mind for this work involves synchronising a pair of radio signals at 2 separate frequencies to an atomic clock. Such signals can be used in determining distance using a mathematical relationship I do not propose to discuss here. Some variations on this technique are lower power than others.

The IC for signals synchronisation has been developed for an arcane application and relies on the GPS satellite transmissions which are themselves readily available and are sufficiently precise.

It happens that I slightly know the man who developed the technology I have described in the 2nd paragraph and I'm sure he could unite it with the technology of the first paragraph for a mere half million or less. Or perhaps a lot more.

The second technology that comes to my mind is a pair of wristwatch GPS units with a radio link between them and an integrated microprocessor at the detector end. This is once again a job for "A", and once again his mission will cost you a lot more than you can pay.

These technologies might not even be feasible in such a small package.

In short I think your requirements are unrealistic.

 

[duke37]

An alternative is to use sound which travels much slower than radio waves. You would need to produce a very short pulse which the other "watch" would send back. The time difference would indicate the separation distance. This is the principle of SONAR.
Making a small waterpoof device would be difficult.

 

[poor mystic]

Still, what's the application please JSUP? How much precision do you need?

 

[JSUP]

I don't need to know distance, location, or accuracy. I just need to be alerted by one object when the two paired objects are no longer in close proximity to one another.

 

[poor mystic]

Hooray for that, JSUP!
That greatly simplifies matters.
Use a weak radio transceiver pair such as can be found in childrens' toy cars, and arrange matters so that an alarm is prevented from sounding for as long as a signal is received. Presto!
Nearly "presto!" anyway. At least it's cheap and practicable. It would be a pretty large wristwatch though, possibly more like a small alarm clock.

 

[JSUP]

Thanks poor mystic!
I'll start shopping around and see what I can come up with.