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Within Range Sensor

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poe

New Member
I was thinking about constructing a simple system to detect when a subject was within a certain distance; it's basically the opposite of an "out-of-range" sensor used for child location and the like.

Ideally, I'd like to have a USB-powered dongle that would receive the low-power signal transmitted by the device attached to a mobile target. When plugged-in the USB dongle would send out a signal and if the receiver is in range it would return a different signal letting the dongle know it was within range.

I have a degree in Computer Engineering, but I don't work in that field anymore, so I really need some high-level guidance here. I'm not 100% sure even where to start.

Thanks in advance.
~poe
 

dknguyen

Well-Known Member
Most Helpful Member
Passive and active RFID does what you want. Passive requires no power on the mobile unit- it absorb the transmitted RF energy, modifies it with it's tag/data and reflects it back. Active requires a power source. Passive has a much more limited range- a few cm to a meter at best. Active has a range of a few hundred meters.

I am not sure whether active RFID tags are always transmitting, or only transmit in response to the correct incoming signal. Passive, obviously has no choice.

But what range do you need? Line of sight? What kind of interface do you expect on the PC? Because in general native USB is a PITA to work with. Virtual COM ports are much easier. Do you really need a transmission to trigger the mobile unit?

Oh yeah:
http://www.mobia.com.tw/content/pro03-2.htm
(but it's for passive RFID).

EDIT: It appears active RFID is always transmitting. But "battery assisted passive" requires an external signal to wake it up and has improved range over passive.
 
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poe

New Member
I should have stressed that being USB-integrated was definitely a "nice to have." I need a range of at least 150 feet, but preferably somewhere around 500 feet; that's why I originally ruled out RFID, but if it's possible now to get a good reading/signal from that distance then I'd certainly look that direction.

The receiver will be positioned inside a building while the mobile device will be outside the building. For instance, say I needed to detect when a certain known vehicle approached the building; that's basically the situation I'm looking for.

[edit] As long as the reader is strong enough, the mobile device does not NEED to transmit anything; I just assumed that because of the distance requirement the communication would need to go both ways.
 
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dknguyen

Well-Known Member
Most Helpful Member
Did you rule out all RFID? Or just passive RFID?

Passive RFID is what you normally see because the tags are cheaper and smaller and require no power and the range is much more localized. It's what most people think of when they think RFID.

Active or battery assisted passive RFID sounds like what you need then because it has massive range.
 
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poe

New Member
Thanks for the advice. If active RFID has a big enough range, I may just be able to buy a pre-built package.
 
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