Very Short-Range Carrier Wave AM or FM Transmitter Help

[Joe G.]

Hi:

I am trying to find or design a circuit that would transmit a carrier
wave (at any fixed audio frequency) radio signal on the AM or FM band
only a few inches (anywhere from 2 to 12 inches). The AM or FM
frequency must be able to be changed across the band by changing a
resistor, capacitor or ? It also needs to as small and simple as
possible. Audio quality isn't important. Audio frequency it transmits
isn't important.

What I am trying to do is something like RFID they are using in
inventory control, except mine must transmit less than 12 inches and
just transmit a carrier wave at a fixed audio frequency. Since I have
less than 16 objects I have to identify, I planned to tune each of the
16 units to a different AM or FM frequency. Then I could identify
which of the 16 units I had based on the frequency.

I am also open to other ideas that would accomplish the same thing. I
am trying to keep the costs low, so I didn't think standard RFID
equipment would be an option.

If you have any suggestions, even just a word or two of sdvice, it
would be greatly appreciated.

Thanks a lot,

Joe G.
Reno, PA

my email is:
ru4him at
gmail dot com

 

[Luhan Monat]

Hi:

I am trying to find or design a circuit that would transmit a carrier
wave (at any fixed audio frequency) radio signal on the AM or FM band
only a few inches (anywhere from 2 to 12 inches). The AM or FM
frequency must be able to be changed across the band by changing a
resistor, capacitor or ? It also needs to as small and simple as
possible. Audio quality isn't important. Audio frequency it transmits
isn't important.

What I am trying to do is something like RFID they are using in
inventory control, except mine must transmit less than 12 inches and
just transmit a carrier wave at a fixed audio frequency. Since I have
less than 16 objects I have to identify, I planned to tune each of the
16 units to a different AM or FM frequency. Then I could identify
which of the 16 units I had based on the frequency.

I am also open to other ideas that would accomplish the same thing. I
am trying to keep the costs low, so I didn't think standard RFID
equipment would be an option.

If you have any suggestions, even just a word or two of sdvice, it
would be greatly appreciated.

Thanks a lot,

Joe G.
Reno, PA

my email is:
ru4him at
gmail dot com

Do you need the device to tranmit continuously? That would make battery
life a problem. Comercial RFID's are passive devices powered by the
unit scanning them.

If you transmit on different frequencies, you would need a separate
reciever for each one. Better, to have a unique tone on each but
tranmit all of them at a common frequency.

 

[Joe G \(Home\)]

You can do that with Long Range RFID systems but the RFID readers are
$3k~$5k USD

Look up Alien RFID uses the 900MHz band

The tags are cheep but the Min order qty for tags is 12k pcs

JG

 

[Le Chaud Lapin]

Do you really need to use RF, or can you go with Infrared?. If so,
infrared would be a lot simpler. You could build a pulse-width
modulator, have it demodulated at receiver, feed into poor man's A/D
converter, and get one of 16 outputs.

If have to use RF, you're going to have to build transmitter/receiver
pairs, and while that's not terribly difficult, especially at such
short distances, it's not something you want to do in a weekend.
First, you should look up ring oscillators on Google. Connect an odd
number of inverters (3 or more) in a ring and think about what signal
looks like. This will give you cheap oscillation. You can put low
pass filter/amp on output to get nice clean sine wave.

If you don't mind going straight analog, you might try Colpitts
oscillator. There are several variations. Pick the one that is least
intimidating. Are you can purchases off-the-shell crystal oscialltors
that generate very stable fixed frequencies. Then you will only have
to worry about the receivers.

For each receiver, you will need a tuned tank circuit at the antenna,
followed by an amplifier, followed another signal-conditioning stage
(high input impedance, low output impedance - look up emitter follower
on Google), followed by half-wave rectifier, then level detector that
will indicate whether carrier is present or not.

IMO, making 4 transceiver pairs this way just to do what you're trying
to do is a bit much. But if you want to use the same circuit for each,
you will have to modulate the signal somehow. In other words, if
you're going to make these circuits yourself, a little math is
unavoidable.

You can also call up companies that make key fob transmitters and see
how they do it.

-Chaud Lapin-

 

[Joe G.]

Thanks for the input.

Your comments shed new light onto the project. I am not an expert at
these things, and your experience really helps.

Thanks again!

Joe