Out of range detection

[siriustech]

I need to develop an out of range detection device. When the reciver/transmitter moves out of range from a "base station" it would trigger an event. The range would need to be variable from 3 ft to 15 ft (+/- 20%) and any "line of sight" option will not work. I researched RFID however since this system needs to be portable, and battery operated, the power requirements were to great and the range was limited. Any other ideas?

 

[VEBill]

The real problem is to measure the range (as specified), everything else is trivial.

The speed of light (RF) is a bit fast for 3 feet. How about ultrasonic sonar pings as the basic ranging technology ?

 

[BrianG]

For the mobile unit, how about a differential GPS unit like surveyors use these days? (I'm not sure of the precise accuracy, but it must be fairly good for surveyors to use it, and they are battery powered.)

The GPS data could be transmitted by the mobile unit to the base station, which could then compute the difference, and decide if the mobile unit was out of range.

 

[buzzp]

Whats wrong with using the loss of communications betweeen the base and transmitter to say your out of range? You did say when it "moves out of range". I would think this would be a satisfactory solution unless what you really want to know is when your approaching the out of range border.

 

[itsmoked]

buzzp is right! And the didtance you are talking would be an FCC slam dunk.

 

[Higgler]

It would be tricky for higher frequency RF to provide accurate distance via loss of communications/amplitude detection. Antenna patterns vary too much for most higher frequency antennas based on surroundings, 20 dB change minimum in a small distance.
Very low frequencies are more stable and the components are cheaper and easier to put together. If you could separate the two units by 3 feet, then hit a calibrate button and measure the changes in power. Outdoors it may work well, indoors and around complicated outdoor structures may be iffy. You'll need frequencies in the 1-20 Mhz range probably, essentially RFID.

Try it with an AM radio and an RF generator first.
kch

 

[buzzp]

The poster said out of range, not really measuring the distance. Maybe he does want to measure the actual distance but thats not how I read his post. Guess will wait to see.

 

[Higgler]

Going from in range to out of range needs to be adjustable in the 3 to 15 foot distance, hence it seems like its a crude distance measurement device. Measuring signal/no signal near the noise floor of a receiver is always difficult due to the nature of noise, which is spiky. This sounds like a device to tell you when a pet or child is straying.
Maybe just an audio tone generator and detector could be used, like the old tuning fork tv channel changers. Use different signal levels for each distance
kch

 

[siriustech]

Thanks guys,

All your ideas are interesting. Higgler you're right in the application, it's not a measuring device but one that will just let me know when something has left a predetermined area. I'm an electrical engineer by occupation so my knowledge in RF is limited but I'm willing to do some research if I knew better which direction to look into. Higgler, I liked your idea of measuring the power received could this easily (and cheaply) be done in a portable devise? Anyone know of any "single chip" solutions out there that can do this.

 

[itsmoked]

THere is much on the web as Hams need to do this all the
time. There are chips to do this since every single cell phone requires this feature to function in a cell.

Here's a simple explaination:

http://engphys.mcmaster.ca/~elmer101/rfpower/rfpower.html

Here's a chip:

http://www.analog.com/en/prod/0,2877,AD8364,00.html