Wireless Comm: Establishing Distant Tracking Between 2 Objects

[EEstudentNAU]

I have to wonder if we are going overboard a bit.

We are trying to locate a roving module which will be in plain sight all the time by spending hundreds (possibly thousands) of dollars on electronics.
Unless it is on Mars there must be a more realistic way of doing it.

What about marking a 30 M radius circle (with a piece of string if you like) and get some clear flexible plastic tubing and lay it out around the circumference of the circle?
Use tent pegs to hold it in position.

It would take 189 metres of plastic tubing to do this.
Then you put a plug in one end of the tube and a pressure sensor at the other end and ring a bell if something runs over the pipe.

What is the actual application?

http://www.linxtechnologies.com/

The have some cheap TX/RX modules that can output the RSSI. Can this be compared to a reference voltage and if its lower, then it is outside the allowable range and sets off an alert? Because information is being exchanged, will I need a TX and RX on both devices, or are there devices that can do both (transmit and receive)?

The application is for me to learn something about wireless communication. Eventually I want to be able to have a robot follow me or another robot using simple RF communication.

 

[skeptic2]

Those modules have frequency ranges from 260 - 470 MHz with wavelengths from about 1.15 to 0.64 meters. At these frequencies the signal may bounce off the ground and arrive either in phase or out of phase with the direct signal, thus the RSSI would indicate a higher or lower reading than would be expected from distance alone. In addition a signal at these frequencies can be affected by the proximity of a human body.

In contrast, by using frequencies in the LF band (below 490 kHz), the wavelengths, instead of being about 1 meter, would be closer to 1 km. At 30 meters you would see no multipath nor will the proximity of a human body have much effect on the signal strength. In addition 30 meters will be within the near field region in which the signal level falls off more rapidly than the inverse square law giving you more repeatable results than at higher frequencies.

It should be fairly simple to build transmitters and receivers in the LF band. The received signal strength can be compared to an adjustable reference voltage that you would adjust by going 30 meters away and setting it so that it just barely turns on. If you don't put a small amount of hysteresis in the comparator, it may "chatter" a little right at 30 meters. You also need to keep good conductors (metals) away from the ferrite antennas as they will tend to detune them.

Lastly these frequencies are covered in part 15 of the FCC regulations which means you don't need a license to operate as long as you don't use excessive power.

 

[EEstudentNAU]

Are LF transmitters and receivers readily available or as you said is this something I will have to build myself? Size is sort of an issue (smaller the better).

 

[skeptic2]

Are LF transmitters and receivers readily available or as you said is this something I will have to build myself? Size is sort of an issue (smaller the better).

I know of none that are commercially available so you can have the fun of building it yourself. For the transmitter you'll want an oscillator, perhaps tunable, and a buffer amplifier driving a tuned ferrite rod.

The receiver will take the signal from the ferrite rod and amplify it, rectify it and compare it to your set point. The easy way is to use an IF integrated circuit with an RSSI output such as the MC3372. If you need more sensitivity you can always add a low noise, high frequency opamp at the input.

 

[berkeman]

The receiver will take the signal from the ferrite rod and amplify it, rectify it and compare it to your set point. The easy way is to use an IF integrated circuit with an RSSI output such as the MC3372. If you need more sensitivity you can always add a low noise, high frequency opamp at the input.

And what else needs to be part of the ferrite rod antenna and first gain stage circuit...?