Small animal tracking device.

I would like to build a tracking device that will help me track my grandfathers ferrets when he is working them.

I have seen loads of fm transmitters but I cannot find any whose transmitters would be small enough or have components that are not obsolete. The transmitter would need to fit on a collor so would need to be about the size of a thumbnail.

The range would only need to be around 15 foot though the transmitter wll be underground.

The receiver could be handheld and i would like it to beep in a higher pitch the closer you get to the transmitter.

Does any one have any circuit diagrams that would satisfy this?

or can you point me to a web page that might be able to help me.

thank you
charlie

try this site
Spy Circuits

I thanks for that link. i like the look of the some of the circuits that are used on that site.
the author is using the FM transmitter to transmit sound picked up from a microphone.
How could I get the receiever to change in pitch depending on how close to the transmitter it is?
Would the transmitter need to transmitt beeps?

Looking about i am led to beleive that it may be better to transmit AM rather than FM. I understand AM is used by search and rescue as the signal is less idstorted by being burried.
How would i transmitt of AM?

Unfortunately AM (I assume you are saying 545 - 1605 kHz) needs a much larger antenna than higher frequencies.

You need my ferret-finder. It is about 100kHz AM and the receiver detects the depth of the ferret.

Colin55, would you happen to have a circuit diagram for that?