# RFID alarm: try this challenge electrical enthusiasts & engineers

Here's a technical challenge for the electrical engineers and enthusiasts:

How would one reduce the size of a passive RFID alarm to a 3cm diameter but still maintaining the alarming at a maximum distance of 1metre?

Quite simply use a UHF RFID system.

School assignment ...

// Per.

Can a UHF RFID reader sound an alarm when the RFID tag reaches a certain distance apart (maximum 1 metre) from the reader?

Or must it only be that the reader will sound whenever it detects a tag?

I am looking for a device which does the following above.

Grumpy_Mike: Quite simply use a UHF RFID system.

Can a UHF RFID reader sound an alarm when the RFID tag reaches a certain distance apart (maximum 1 metre) from the reader?

Or must it only be that the reader will sound whenever it detects a tag?

I am looking for a device which does the following above.

RF energy reflects and interacts with all the conductive objects in its field. So it makes a very unreliable way to judge distances, except by RADAR techniques (disambiguation of the signal by imaging).

instant double post http://forum.arduino.cc/index.php?topic=403310.0

nerdmia: Can a UHF RFID reader sound an alarm when the RFID tag reaches a certain distance apart (maximum 1 metre) from the reader?

No RFID system can do that.

Now what exactly are you trying to do. Hint Google X-Y problem.

Grumpy_Mike: No RFID system can do that.

Now what exactly are you trying to do. Hint Google X-Y problem.

I'm wanting to use 2 RFID tags- one sticking on to each earring- so that when one falls off, the reader picks up that that earring distance is out of radius and sounds off an alarm. Therefore, you would need 2 of the smallest tags- one for each earring

Normal RFID will not read more than one tag in the field at any one time. But a UHF reader will. The snag is that a UHF reader is not a wearable item, more of a very heavy handbag item.

Grumpy_Mike: Normal RFID will not read more than one tag in the field at any one time. But a UHF reader will. The snag is that a UHF reader is not a wearable item, more of a very heavy handbag item.

Yeah, your right. Would you know of any alternative option outside RFID which can do it? Thank you so much for the help so far grumpy mike

Sorry I can't think of one.