# How to make a simple RF TX/RX circuit for radius tracking

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#### zonemikel

##### New Member
Hello everyone,
Nice to see a good forums related to electronics.

I'm wanting to make a small circuit that transmits a signal. This signal will be received by a receiver and when the receiver is out of range of the signal the receiver should be able to drive a transistor to start another circuit.

All the circuits and stuff i read on this type of thing are for transmitting audio or data. My plan is to have a peizo buzzer and some leds on a cats collar and when/if he escapes the buzzer will sound and the lights will flash so I can find him. So the circuit needs to be pretty small.

I'm thinking I will need to build a oscillator of some sort to transmit the signal.

Can someone recommend some reading and/or suggest a circuit ?

#### audioguru

##### Well-Known Member
They are already made for children. But the child has the transmitter and the parent has the receiver with the "low signal level" buzzer. When the child wanders too far away the receiver receives a weak signal and sounds the alarm. But if the child wanders behind some metal shelves the alarm also sounds.

#### zonemikel

##### New Member
They are already made for children. But the child has the transmitter and the parent has the receiver with the "low signal level" buzzer. When the child wanders too far away the receiver receives a weak signal and sounds the alarm. But if the child wanders behind some metal shelves the alarm also sounds.

Any idea how they make them ? Basically how do i dumb down am transmitters so that they only have a very small amount of components and simply transmit a signal (not audio or data) and how would i make a receiver (rf probe) for just that frequency ?

#### audioguru

##### Well-Known Member
A dumb transmitter that transmits all the time would drain a battery quickly so either it needs a huge and heavy battery or it needs to have some "smarts" that pulses the transmission with many no-power pauses in between.
The receiver might be purchased since it is tricky to design and make a radio.

#### HarveyH42

Might look into a wireless doorbell, usually under $10.00, shop around, some have some fairly small transmitters. Mine uses a 12v battery, about the size of a AAA battery. You would need to add a circuit to pulse the transmitter in place of the button, maybe a 555 timer could handle this. On the receiver end, you would just need to detect when you miss the pulse, another 555 circuit might work. I haven't had great luck with RF circuits built from scratch, very little range, drift, and interference. Seems like kind of touchy stuff, placement of parts is sometimes important. Just so much easier to use something ready made, and add to it. Cats don't generally like collars, or anything attached. Might have some trouble with that. He might slip out of the collar, or strangle himself trying to remove it. Cats are pretty quick, if he wants to wander there isn't much you can do. Hate to suggest it, but with male cats it's a good idea... get his nuts clipped (ouch)... Not much reason to wander, if you provide food and water, too embarrassing to seek a mate. They also don't spray piss (musk) all over everything, which I'm sure has cost more than a few their lives. Not only does it stink, but its corrosive, nobody appreciates it. Basically, you are going to outlive quite a few pets in your lifetime, no matter how much work and money you sink into prolonging your time together. Cats are great, they rarely die at home, if they can avoid it. They leave you with a mystery, their end is left to your imagination. #### zonemikel ##### New Member Thanks for the responses ! His nuts are clipped, bugger keeps going out anyway. He actually has a bad habit of disappearing and all i hear is him fighting with other (cats?) at night. One time he disappeared for about a week when he came back his tail was horribly mangled and cost me$500 to have amputated. So money spent on this project (and most things in my life) are pretty small in comparison. But this is a little off topic. Oh and the collars are break away ... so i dont want to make it too expensive, but it wont strangle him ...

I have transmitters and could easily design a module that does what i want it to do digitally, but the receiver is about the size of triple A battery (rcr-433), then i would need some sort of mcu and all the other circuitry which would result in a circuit thats kinda big. I was hoping for a simple analog solution.

My cat dont mind collars, he is pretty big. I guess we could even go with a harness if i needed to go the digital route.

My original plan was to use infared transmitter and receiver then create a wall of infared transmission that is just outside my door, once/if he runs through it the circuit would be activated. This solution is looking better all the time, the circuit would be about the size of my thumb past the joint (1/2" cubed) and i could run it off of 4 AAA or maybe two coin cells.

Thanks for the ideas, I'm sure ill figure something out.

How do you transmit anything ? If i were to create a oscillating circuit and hook it up to a antenna is that going to transmit ?

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