Ed Paradis has put some thought into a couple of hacks for the Hot Wheels toy radar gun:
The Hot Wheels Radar Gun is a real radar gun currently (Feb 2007) available from Walmart and other retailers for about $30 (USD). It is a real radar gun, operating at 10.525 GHz capable of clocking cars, people, pets, and toys.
But before you can play with it, you’ve got to get the thing open! Here are some pictures of me taking the thing apart.
You can use the radar to detect the speed of moving objects, but Ed also posted some semi-successful (and some not-so, but worth reading) experiments with using the device in a custom circuit to detect the distance of objects for robotic projects. It’s also worth noting that the LCD can be repurposed as a display output for your other electronic gizmos. For 30 bucks, it’s a pretty good collection of useful robot parts.
Hacking the Hot Wheels Radar Gun – Link