Simpler than RFID?!?

[Luhan Monat]

I assume that you must totally shut the souce down fast, listen for harmonic
then?

[snip]

Only if you're listening for fundamental. A harmonic receiver would
allow for continuous transmission, although it might be easier to
decode an event if the transmitter were pulsed, and that pulsing acted
as system clock.

...Jim Thompson

Hi,

I tried this once, 'ham radio' days. Two tuned circuits with a diode
between them. Listening for the second harmonic on a shortwave
receiver, I got a definite signal. Maybe 1 or 2 foot range.

 

[Brian]

sorry, unfamiliar with top post.

How do you keep from picking up harmonics from the reader itself?

 

[GPG]

Worked on passive tag some years ago. 2 tx, 1 at uhf, 1 at lf, rx at
uhf using uhf tx as lo, if =lf. Tag is signal diode causing mixing of
tx's producing output at rx freq. Times have moved on and I would
guess there are modules available that would make this easy now.

 

[YD]

OK, experimenting with this...

Would like an RFID-like system that can read a passive as small as possible
tag. However, it only needs to know its prescence, not an id or any data. It
must now it arrives, that is is still there, and that it has left.

Reader could read directly up, some side lobing perhaps, and know the object
with tag is with approx 30 inches.

I thought metal detection type circuitry, but how would I discriminate other
objects except tag?

Any thoughts?

Look up how those anti-theft tags work.

- YD.