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Topic: Proximity approximation using RFM22B possible? (Read 313 times) previous topic - next topic

scott_fx

Nov 30, 2013, 09:59 am Last Edit: Nov 30, 2013, 10:25 am by scott_fx Reason: 1
I have a project that I need to approximate a secure/cost effective active RFID system.  The active RFID solutions out there are either too expensive, not secure or bit arduino friendly.  I was wondering if it was possible to use a pair 433MHz RFM22B boards and calculate an approximate distance between the two? (Assuming that any obstructions of the signal line of sight will be constant).

PaulS

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Assuming that any obstructions of the signal line of sight will be constant

Right off the bat, you make an invalid assumption.

The RFM22B appears to have RSSI capabilities. The signal strength is not an accurate measure of distance. It is, in fact, influenced by many things, with distance being a less important factor than things like power supply voltage, signal interference caused by noise, and solar radiation.

If you have COMPLETE control over the environment in which the generated signal and the received signal exist, then RSSI will give you a rough measure of distance (rough being near, not too near, a long ways away, etc.; not 1 inch, 4 inches, 24 inches).

scott_fx

Well. What I need to figure out is then I'm within a ~1.5m of the transceiver or less. Is there another way of figuring that out?

PaulS

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Is there another way of figuring that out?

That depends in whether the two devices are line-of-sight and whether or not they both participate in determining the distance.

There is no easy way, using radios alone, to measure distance. There isn't even a hard way.

scott_fx

Maybe I'm asking for the wrong thing. Is there a way of dialing back the effective range of that module to only transmit at a distance of ~1.5m

PaulS

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Maybe I'm asking for the wrong thing. Is there a way of dialing back the effective range of that module to only transmit at a distance of ~1.5m

By using an appropriate (poorly designed, by intent) antenna, you could ensure that the radios could not transmit and receive while more than some distance apart. The radios would have to be sending, and receiving, continuously, in order for you to detect a loss of signal. Even then, you can't assume that the other device moved out of range. Its overworked battery might have simply died.

scott_fx

Thank you for the help.   To prolong life I was thinking about only having it transmit data every 500ms until it was in range. Then after that it could wait every 5000ms to verify if it still was in range. I think that should extend the life of the battery.

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