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I'm looking for some help. Looking for a simple circuit. Proximity sensor not IR based. Not line-of-site? Answered

I would like to have two devices(Circuits) know when they are apart from eachother.

Example device A is in a fixed location. When device B moves away from device A an LED flashes.
I'm looking for a distance of 10 feet total.
I don't want to use a line-of-site circuit.

Any advice/guidance would be great!

Thank you.


Is device A powered? Use an RFID system. Put an RFID tag in or on device B (doesn't need power!). Add a very simple "reader" circuit to A -- just a ping receiver, no data query. Use a 555 to trigger the reader at regular intervals (every few seconds if B moves around a lot, or every few minutes if B is "supposed" to be stationary). If the reader fails to get a response, use that to trigger the LED.

10 feet is still a long way even for RFID

It depends very strongly on the technology used:

Active tags broadcast a signal, so they have a much longer read range—300 feet or more—than passive tags. [...] low-frequency and high-frequency tags tags are read from within three feet (1 meter) and UHF tags are read from 10 to 20 feet. Readers with phased array antennas can increase the read range of passive tags to 60 feet or more.

Ok didn't know re powered RFID only seen passive ones.

Me, too. That comment about using phased-array antennas on passive tags is quite interesting, though!

10 feet is a long way for a capacitive proxy sensor.

not LOS means radio or sound -

this isn't going to be easy you could offer more details see here