Ready built simple detector

In the page Simple heterodyne / FD detector you can see different designs for detectors that uses quite simple technique. 

I had a bunch of PCB's made for the frequency devider detectors and heterodyne (arduino) detectors. 

I sell these as ready built detectors. 

The FD uses very sensitive ultrasonic transducers and needs no display, the HET (heterodyne) uses a mems microphone element and a oled display

Choose the technique you like best!