RF Proximity Detector

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I need to find a way for two atmega chips to detect when they've been brought within about 10 metres or so of each other, and approximately how close they are together. The ability to exchange small amounts of data would be a plus. the units would have to run on battery power for 10-12 hours, so low power is also a requirement.
I was thinking of using a couple of bluetooth modules, and approximating distance from the signal strength, but I can't find microprocessor friendly bluetooth modules that can also report signal strength. Does anyone have any suggestions?
I'm looking for omnidirectional response that can give distance, bearing doesn't matter. Maximum range of about 10 metres, resolution of a metre or two is fine.

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10-12 hours is not much for a micro and some C cells. Thats only 50-60 ma-hr. BT would be a a challenge, I suspect.

Are there any relative orientation constraints? OR, would the relative orientation be effectively random?

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Need more info.

Indoors, outdoors, fixed location, or moving them to a new location all the time? Walls?, Ambient lighting?

There have been several Threads discussing indoor position sensing.

Note that depending upon the Area-of-Operation it might be easier to have a third, Master Station, that determines the location of the Remotes, and then notifies them if they are within a certain radius of each other.

In this case putting a (UV?) blinking LED on the remotes and doing visual processing becomes possible.

In a fixed environment on can also implant a grid of wires in the floor. The Remotes sense crossing various wires, which are scanned and encoded, much like scanning a 7-Seg display.

Each device then knows roughly which grid square it is in.

This doesn't work if you need to deploy the devices to random locations.

JC

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relative orientation is essentially random. I'm thinking of making some wearable electronics to let you know when your friend is near you in a crowd :)

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Quote:
I was thinking of using a couple of bluetooth modules, and approximating distance from the signal strength

My feeling is that would be VERY approximate.

Four legs good, two legs bad, three legs stable.

Last Edited: Mon. Mar 25, 2013 - 06:10 PM
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There is of course the low tech method:

JC

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Here is a big part of the problem with RF at any frequency.

Suppose that you have the gizmo in your shirt pocket and a "friend" with a compatible one walks up behind you. Signal strength would be VERY different compared to same friend directly in front.

If you are willing to put that little problem aside, there is a device known as a "ferrite loopstick". It is basically a bar of ferrite with a coil wound on it. If you have two with the long axes parallel, the coupling is pretty good, even at a distance. That means if you orient them all with the axes vertical, then they are essentially non-directional.

am thinking of a way that might operate at a few hundred KHz, max, Maybe 125KHz since that is an RFID frequency. Each device "pings" the resonant circuit with a short pulse and it rings, transmitting at its natural resonant frequency. May be in a simple pulse modulation to contain an "address".

If the resonant frequency is low enough and the CPU clock is high enough (16MHz, maybe), you ought to be able to detect individual cycles with a built-in comparator. This won't give any distance information beyond a simple threshold.

Once the devices own transmission is over, the resonant circuit would be quiet and open to receiving other signals. Say that it transmits for a millisecond every second. That gives a 999ms window for listening. Ping rates could be a little different for each so that if two transmitted at the same time, the next time, they would not collide.

it could be simple to make. Do you have access to an oscilloscope?

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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I did some simulation "experiments" last night, and I think it can be done. Not sure what the range would be - that is hard to quantify in a simulator. But I would not be surprised at a few meters. No active parts except the micro. Uses the comparator input for the receiver and pulses one of the comparator pins to "transmit". Timing is compatible with a 16MHz device, and maybe slower (possibly down to 4MHz).

I will post circuit details in about 10 hours.

Jim

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Jim,

I too was thinking something along the lines of very low frequency inexpensive TRF setup akin to the old ZN414 thinking.

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Here is my idea.

Micro that is set up with one side of comparator internally connected to Vref (and Vref label on LTspice schematic, below). Other side of comparator is connected to point AIN. With the AIN I/O pin disconnected (using DIDRx register), both inputs are very close to Vref.

The resonant circuit is tuned to 125KHz (RFID frequency). The I/O pin is enabled and set high for 2us, then turned off. The circuit rings at 125KHz with a modest amplitude.

The second resonant circuit represents a near-by similar device. The K1 statement defines the coupling coefficient between the two circuits (very difficult to correlate distance to coupling coefficient. Both inductors are 788uh "ferrite loopsticks". A detectable voltage is induced in the second inductor, easily exceeding the possible comparator offset.

Devices can identify themselves with some sort of pulse code modulation (position or other pattern).

The attached file Pulse-RF.asc.zip contains an LTspice circuit that can be run directly with just simulation control settings. After it runs, click the schematic then used the cursor to "probe" circuit points. It shows one pulse; Viewing 20ms of t race should be good. Change the last number in the K statement to vary the coupling. Some "interesting" things happen. The switch and 5v source and the pulsed voltage source represent the output pin that is normally disabled.

Jim

Attachment(s): 

Jim Wagner Oregon Research Electronics, Consulting Div. Tangent, OR, USA http://www.orelectronics.net

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Some Bluetooth modules have in their command set an RSSI function, or an analog output. YOu will have to look around, but IIRC they all say that the function/signal is not accurate as the module(s) automatically compensate.

The Linx technologies does have an RSSI pin that is usable though the module is not Bluetooth:
https://www.linxtechnologies.com...

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Those Linx modules look very interesting. I'll have to check into them.

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Quote:

I'm thinking of making some wearable electronics to let you know when your friend is near you in a crowd

Smartphone. Has GPS bilt in.

Write an app (or two, one for Android and one for iPhone). Set up a server that all can register on, and where they can become friends with each other. App sends location to server. Server matches friends and distances, sends out notifications.

Give the apps away for free. Charge for traffic/notifications from the server.

All bussinesses today strive to tie customers up long term selling "services" to them, rather that just do a one-time sale of a gadget.

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You can just imagine it.. walking past your favourite cafe... the iPad yells out "Come in Ross... free coffee today... then past the dentist... hey Ross time for a checkup... the bank... Hey Ross, want a loan... the old girlfriend... hey Ross ........ let's not go there!

Ross McKenzie ValuSoft Melbourne Australia

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