RFID or better alternative??

[G30RG3]

I want to be able to transmit and receive a signal from one device to another; the devices need to be very small and very inexpensive.. i'm hoping for it to work over quite a long distance; definitely more than just 10m.. also if a battery or energy source is required thats fine, i can work to sort that out..

any suggestions? rfid seems to be quite useful; but what about bluetooth and any others? could i also not just use a simple radio wave?

i dont need to transmit any data, only a signal that can be sent and receivedd

[fss]

RFID is the smart version of what you want (or so it seems) in that it usually transmits some kind of data (hence the "ID" moniker). Does the signal have to be omnidirectional? It seems like you could achieve the same thing with a light source and circuitry to tell whether it is "on" or "off."

[G30RG3]

primarily the signal needs to be transmitted wirelessly; hence rfid or an alternative is preferable
for the purpose of easier explanation; call the transmitter "A" and the receiver "B"

We don't know the location of "B" but we are holding "A"; I want to be able to stand in a room and find out the location of "B". It could be anywhere.
The location could be determined either by a signal sent form "A" to "B" and then a signal released from "B" (eg. sound); or the signal could be transmitted from "A" to "B" back to "A" and thus determining the location in this method. Simply, it is which ever is more efficient.
The receiver device is required to be very small and very cheap; the transmission does not require any data transfer, rather that when the signal is received at "B" it can cause a sound to be emitted; ie. like an on/ off switch yet activated with RFID (or alternative).

[berkeman]

primarily the signal needs to be transmitted wirelessly; hence rfid or an alternative is preferable
for the purpose of easier explanation; call the transmitter "A" and the receiver "B"

We don't know the location of "B" but we are holding "A"; I want to be able to stand in a room and find out the location of "B". It could be anywhere.
The location could be determined either by a signal sent form "A" to "B" and then a signal released from "B" (eg. sound); or the signal could be transmitted from "A" to "B" back to "A" and thus determining the location in this method. Simply, it is which ever is more efficient.
The receiver device is required to be very small and very cheap; the transmission does not require any data transfer, rather that when the signal is received at "B" it can cause a sound to be emitted; ie. like an on/ off switch yet activated with RFID (or alternative).

There used to be a product that let you find your lost keys. It was a fob that you attached to the keys, and when it received an RF signal, it beeped. Clever idea, but it never caught on. I suppose because you always had to be able to find the transmitter box, and if you couldn't remember where you kept leaving your keys, well then where did you leave the transmitter box?

You might look at garage door opener technology. The RF part is pre-licensed for you, and I would think that the receiver circuits are pretty jellybean and inexpensive. Beeping at the receiver is the right idea, if you are trying to find the receiver.

[NobodySpecial]

RFID generally needs a large antennae because it's also radiating the power for the transmitter.
Another possible approach is zigbee - a low power alternative to bluetooth, it also has a simple to implement 'device present' and 'device wakeup' protocol that you could use.