For a few years now we have had bat which regularly flies round the garden, Googling about bats I came across a bat detector circuit from Tony Messina.
Tony’s site is packed with interesting information and a link to the UK where you can buy the printed Circuit Board if you didn’t want to use veroboard.
The PCB is from Lee Rogers (mail: firstname.lastname@example.org) in the UK for £5 including p&p, the majority of the parts are available from Rapid Electronics , things out of stock at Rapid can easily be found on eBay or Maplin.
The testing and adjustment was done by pointing at running water or rattling keys and tweaking the volume to a comfortable level as you go.
The total cost of the project was £28 and took about an hour to construct.