# 2 way RF switch

#### Bear91

Joined Dec 28, 2011
3
Hello Everyone!

It's my first post on the forum so take it a little easy on me eh?

I am trying to come up with a circuit using Radio Frequencies for 2 items to essentially ping each other. In essence, 2 keychains with an RF circuit (that i dont have) that activates a speaker on each one. I know how to wire the speaker, but is it possible to use the output of the RF on a transistor to act as a switch for the speaker to turn on?

Currently i am thinking of using http://rentron.com/Files/rf.pdf as the transmitter and receivers... but i really dont know a whole lot on how they work or even if they are the right part.

I do have some electrical knowledge... mostly in transistor logic, PLC and some digital logic for robotics, but idk how well it would translate onto this.

EDIT: Also was thinking, is it possible to modify the frequency Via dip switch? That way if i make multiples of the circuit, it doesnt ping them all at once and make someone deaf? =D

~Bryan E. A. Rosenstock~

Joined May 28, 2009
514
Basically you want to build a miniature walkie-talkie. A transistor can be used as a switch. As long as it gets base bias, the speaker could stay on. Do you have a schematic, a block diagram or a conceptual drawing you could post?

#### Bear91

Joined Dec 28, 2011
3
What im trying to build is actually something more akin to a garage door opener from what my old mentor says. It would be lovely to build it without a 32 pin micro controller, and ive seen the insides of a garage door opener from the keychain side... pretty tiny 8 pin chip in there.

To get a small mental image, imagine 2 keychains, A and B. Now lose Keychain B!

Press/hold a momentary switch on Keychain A, to send the signal out to Keychain B
Keychain B picks up the signal, and the output of the receiver is used to open a transistor that is acting as a switch in a circuit to power a piezoelectric speaker to a really loud and annoying tone.

congratz, keep doing that and youll eventually find Keychain B.
optimal range indoor, 35m+ for signal strength.

i'll try and draw up a conceptual drawing shortly if that was confusing

#### praondevou

Joined Jul 9, 2011
2,942
Keychain B picks up the signal,
That means while idle it will always have to listen, i.e. the receiver is always ON. Your battery will be dead in a very short time, not comparable to the very long battery life keychain transmitters normally have if you choose a common receiver module... It would need to be one that has extreme low power consumption. There are some on the market though.

You can also build a receiver module that is turned on for example every 10 seconds, checks if something is being transmitted and then goes to sleep again. Current consumption drops considerably (still higher than a normal keyfob though)

#### Bear91

Joined Dec 28, 2011
3
I think i see the problem you are referring to...

Please feel free to correct me if i am wrong, as i am very new to radio communication.

the Transmitter (A in the example), would only tap power on the press of the momentary switch, where as Receiver (B) would have to always be aware, if and when A transmits, even if it is never actually used.

Which would explain why receivers are generally sold as bricks (and here i thought it was that bulky relay).

I guess there are really only two solutions to this, at least that i can see.

Solution one, Find an ultra low power receiver that isnt the most bulky thing on earth.

Solution two, Go through a boatload of trouble to create a wake-up circuit as described http://www.atmel.com/dyn/resources/prod_documents/doc4694.pdf <- there. Unfortunate problem there.... it's all greek to me X_X (actually looking over it a second time... not all greek, but not at all easy i think)

Though one would think it isnt so hard to solve such a seemingly simple task >.<

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