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Detecting several different signals within a certain distance?

Hi guys, 

I essentially need a device that emits a signal, while simultaneously tracking whether another device emitting the same signal enters a certain distance. If another device enters said distance then it will alert the wearer. I think this is possible, but haven't quite got the knowledge on whether it is possible.

I thought about using GPS to relay wearers position to a server, which can calculate whether another device is in the vicinity and to send a reply back to the wearer, but to send data would be a continuous cost, something I'm trying to eliminate. 

Radio frequencies may be another avenue to go down, but again, not sure. 

In my mind I would have say 10 devices attached to different people in a rural area, and each person would be able to tell if there was another person (and device) in say 100m. 

Anyone got any ideas?

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rickharris17 days ago

Ok think about this:

Possibilities:

Light

Infra Red

high freq sound

Light

Radio

1. You need a very good range so line of sight is out delete IR, Ultrasound, light, microwaves. high frequency radio (line of sight only)

2. You need to detect:

If a signal enters the detection zone

so your devise must transmit and receive

3. A radio wave can be omnidirectional, they can be detected by a suitable tunes receiver However they are not easily zone limited, ie. they spread out often a long way.

4. Radio systems are notoriously hard to build and get working for the average amateur. You need some specalised/rather expensive equipment to be able to test and set them up. AFAIK there isn't an easy commercial solution beyond the GPS option you offer and even then you have to devise a system to monitor the position and send out a signal to an individual to say someone is close (prisoner tags work this way - reporting back a position if the wearer goes beyond a given limit (not cheap).

I think your asking a lot - If you describe why you want this there may be a better solution available.

You can devise position from a standard cell phone by triangulating from the cell towers it can reach.

You could set up a radio triangulation system that could monitor several radio signals and know where they were then send out a warning - this is going to be very expensive.

kief54 (author)  rickharris15 days ago

Cheers Rick.

You have outlined my brainstorming already, hence the hail mary question here.
I understand that while my question may seem basic, there (as you said) is a lot in it, which is probably why there isn't anything out there for my intended purposes just yet.
I wanted something to be worn on your wrist. A very small LED screen, that would light up red if there was another device in the vicinity or stay green if there was no one. To compound the complexity, I envisaged these to work in dense bush. The idea behind this is for safety control while hunting. If there was another hunter in your vicinity you would be alerted, and hence you would have to be damn sure that it was an animal you would be looking at down your scope, not a human.
This is to hopefully limit the amount of accidental shootings in hunting.

Like I said, a GPS based system would work, however the continuous costs associated with it would probably mean that no one would want to invest in their safety.

In that case I guess a short range radio transmitter is your best bet. The zone will vary though according to the local terrain, battery condition, position of the antenna, and several other variables.

Except over ranges too short for what you want this would be astonishingly hard to do.

Example: The gps system works, (simplistically), by the ground unit sending a request to one or more satellites, the satellite responds back the time delay between reception and response gives a line of sight distance from the satellite do it with other satellites and you pinpoint your position . To make this work the satellite has an atomic clock on board synced with an atomic ground clock.

A radio signal could be generated that spreads out in all directions, once a receiver enters the signal it sounds an alarm. that isn't too hard, what is hard is to define and limit the range. Radio waves like to spread out.

A small 433Mhz transmitter https://www.rapidonline.com/Catalogue/Search?tier=...

will give a flat ground range of a few 100 yards. but is very affected by walls, trees, bushes, all the things you need to be transparent!

Sound might work, a blip every 30 sec or so but animals can hear too!

I think other than using outside triangulation your pretty much stuffed with this idea.

kief54 (author)  rickharris14 days ago

What about looking higher frequency sound? Something outside human and animal audible range? A sensor to pick up a pulse of ultrasonic sound? Just brainstorming here. I would imagine the signal would face similar penetration problems as radio waves.

David_Lin18 days ago

what's you need is to emitte radio.

Some what like the FM frequency

And get receiption from Radio receiver will done.

Emitte from pi zero. Maybe

And received from digital radio.

iceng18 days ago

RF signals do not confer distance.. Yes, strength of signal varies with distance from a transmitter but weather, obstructions, ionospheric reflections and geology affect signal strength so as to make it impossible to gauge if a desired respondent is in a certain range.. Triangulation of three or more direction receivers using sophisticated time of arrival is the way to locate an individual..

A Japanese and US dating cell like service, may simply use the first clear ping attributes and the following first successful data exchange as an indication of another person in the vicinity..

Obviously the person on high ground will reach more people further away then someone in a basement..

iceng iceng18 days ago

BTW there is a neat ible that fits in a breast pocket that searches for other members with a similar gadget and lights LEDs to announce nearness with extra emphasis when three or more are detected simultaneously..

kief54 (author)  iceng18 days ago

Cheers iceng.
The gadget you mentioned, have you got a link for me to read up on it? Thanks a bunch