This would require some additional research on my part, because they are not listed on a public space. I know for sure of at least four functioning ones, three of which are close to each other, but of different design. They are near the city of Stara Zagora (if you have a Bulgarian map handy, it's the middle of the country). I will see what I can do to provide some details, there was some media coverage on the issue.

Be careful! Is it classified?