I purchased a few kits of EBay as fillers when I am waiting on gear to turn up. This metal detector kit was a couple of dollars so my expectations weren't high. Its designed to detect nails in studs where plasterboard is fixed so you can find the wall studs.
It went together Ok but while it seemed to work Ok right away, after a bit of fiddling around it seemed to stop working so I had to do some fault finding to get the thing working again.
The kit was at this url: https://www.ebay.com.au/itm/Metal-Detector-Kit-Electronic-Kit-DC-3V-5V-60mm-Non-contact-Sensor-DIY-Kit/263373872700
You can checkout the build video on YouTube at: https://youtu.be/GBPGocxWc8s