RF proximity sensor around a perimeter

Question

I'm trying to build a proximity sensor that triggers on nearness of the sensor to a perimeter antenna, sort of like an underground invisible fence. I want to feed that input into a microcontroller which will respond when it approaches the perimeter. Most of what I'm finding online is dealing with RFID, which is not exactly what I'm looking for.

Are there components that I can use that will transmit/receive simple signals? If I need to build this with discrete components, are there schematics? The sensor should be sensitive only over a short range (~1m)? Ideally this could be configured by increasing or decreasing the power of the transmitter.

I understand that the question is a bit vague, but I'm not looking for super detailed responses, just a nudge in the right direction.

Answer 1

You can use simple magnetic induction from a perimeter wire to a portable coil. If you used an audio frequency such as 10kHz to drive a current thru the perimeter wire, maybe 100mA RMS, this should easily be picked up on a portable but also small multi turn coil. The level may only be a few millivolts at 1m from the perimeter but this can easily be amplified and filtered and converted to a dc level that approximates to received signal level. Because the orientation of the receive coil can cause a null in the signal you might want to use two receive coils at right angles to each other to get full coverage at any angle.

I would, on the receiver want to try and filter out unwanted frequencies such as power AC so a bandpass filter set at the operating frequency would be ideal. This can be built around an op-amp and the op-amp can provide gain although it's probably best not to try and use the same op-amp for filtering and gain. Use a rail-to-rail op amp to make maximum use of possibly a limited battery supply.