electronic thither

[dudyyy]

my project is to design 2 AM tranceiver circuits one with me and with a child when the child cross a specific range say 20 meters say outside the house a counter start measuring the distance he crosses and displayed to me on an LCD screen can you plz answer me even if you joke at me i know its not that big project

 

[JimB]

Yes, dead easy!

The fixed transmitter sends a pulse, the "child" equipment receives the pulse and sends a pulse back to the fixed receiver.

Measure the time between sending the pulse and the return pulse, allow for the propagation delay through the child equipment, and you can the calculate how far away the child is.

I believe that aircraft DME (Distance Measuring Equipment) works this way.

Good luck getting it to work at such short range.

JimB

 

[dknguyen]

But the timing circuitry needed to measure light-speed fast radio waves is VERY expensive and hard to make, especially at such low ranges (to the point I doubt anyone on this forum has built such circuitry). If it were easy to time the speed of light as easy as it is to time the speed of sound, you would have homebuilt radars everywhere, but you don't.

 

[panic mode]

why AM? why test it on a child? to see how big antena child can really carry? if you make it big and heavy enough kid won't be able to get away with it - even if it wants to. compact ferrite antena as used in AM radios is directional and you can easily loose signal. how important is to know if child is still nearby? it's not the same thing to try this in a bussy mall or inside house or own back yard...
it is much easier to measure signal strength instead of signal travel time.

 

[zkt_PiratesDen]

u need gps receiver and rf transceiver chips
not such an easy thing
and I dont readily see any other way of doiing it
and now that I think about it, gps doesnt have that kind of accuracy (except military)
timing radio freqs for distance determination is out of the question
signal strength might be your best bet but will effected by surroundings
ultrasound pulses are timable but gonna need a good filter to eliminate noise and a microcontroller for timing
ahh I got it !!!!
nordic semiconductor makes a multi band uhf transceiver chip that that has digitally adjustable power output and an onboard 8051 microcontroller
calibrate the output power to correspond to distance, vary the output power of the kid`s, feed back a received not-received signal from yours and display the distance.
gonna take some time
maybe the kid can help if you send him to engineering school
remember the KISS principle -(keep it simple stupid)
hope you find this helpfull
zkt

 

[dknguyen]

KISS dictates that a leash would be more effective. Not only is it simpler and more reliable, it is a preventative measure of the kid from gettng away.

 

[Oznog]

Calibration of power to distance is not very reliable.
For one, few people have an installation that would have no obstructions or reflecting surfaces in between the expected tx and rx locations. Even if there were, the child can still turn and put his/her body between the two antennas.
Second, the tx antenna's orientation will substantially affect the amplitude received.
Third, without a regulating circuit, battery voltage will affect the amplitude too.

It'll work and all- just know that its accuracy is going to be generally "vague". As in maybe next door, maybe at the end of the block.

 

[stevez]

Read a little on time domain reflectometer (TDR). The concept is fairly easy and while the speed of light seems almost infinite one can see how common instruments can use reflected pulses at even fairly short distances. With that said, a portable package that's automatic would seem to be quite the challenge.

Light travels 60 ft in 0.06 microseconds (if I didn't screw up the math). A 20 mHz waveform has a period shorter than that. When you look at it that way then the speed of light starts feeling like it might be managable.

 

[zkt_PiratesDen]

your math is good
but its gonna take a pretty fast processor to time intervals that short
a pic at 4 mhz takes 1us/instruction
maybe 10 instructions for the timing thing
hell, maybe a pentium1 or 2 might could do it
another project on the back burner !

zkt

 

[dudyyy]

to jimB

ok but can i measure the distance if the antenna were no longer in range plz clarify

 

[dudyyy]

hey guys what about a step counter

 

[JimB]

ok but can i measure the distance if the antenna were no longer in range plz clarify

I am not sure what you are asking here.

If the units are "out of range", that is, not communicating with each other, then they wont work.

JimB

 

[Hero999]

Oh no not another clueless person asking about a project that's way beyond their current level of understanding.

Sorry, I wasn't meaning to flame, but it sounds like you don't have enough experiance to attempt this, perhapps you should attempt something simpler.

 

[philba]

why not just have the mobile unit send a "heartbeat" pulse at, say, 1 hz or so. When the receiver stops getting the pulse, raise the "kid's gone" alarm. reduce the transmitted power for a shorter "e-leash".

Of course there will be lots of ways to fool it and the kid will very quickly learn a new game called "jerk mommy and daddy's chain". endless hours of entertainment. I'd probably PAY to watch that one...

 

[dudyyy]

thanx philp i malready implement your idea but my problem is i need to measure the distance the child moved inside the house at any time by the use of counter butting in my mind the average velocity of the kid . and thank again and to our fellow hero 2 for nothing

 

[dudyyy]

hey am sorry that i ask too much but another idea came up to my mind how about the circuit contains a power meter , so i make a reference point if the child moves the power of the signal is changed so i can measure the range
what do you think

 

[philba]

the problem is that stepping behind wall or a bush or what ever will be indistinguishable from moving father away. For example, if the kid opens the fridge door, the metal in the door will diminish the transmitted strength if it comes between the child and the receiver. You would think he is a world class sprinter in that case.

sorry, the idea is pretty much unworkable and it takes only a small amout of understanding about radio to know this. basically, if it were that easy, you would see lots of really cheap products out there all ready. hero may have been mean-spirited but he wasn't wrong.