Sensitive audio detector
I don't know if they still do, but Radio Shack used to carry a tiny 500:8 audio transformer that'd work VERY well for this sort of project. I'm sure there are all sorts of similar transformers available still.

I think the Radio Shack ones were even center-tapped on the 500-ohm side so they could be used in experimental/prototype push-pull amplifiers.

EDIT: I'm NOT sure how available Radio Shack parts are in Montenegro, though. If all else fails, you COULD wind your own.

How's availability of mail-order electronic parts there? Can you freely buy things through eBay and Amazon?