To make a metal detector, take an AM radio and tune it the highest end of the AM radio frequency, but not on a station that is broadcasting. Turn the volume up as loud as possible. The only noise heard should be static. Then, tape a calculator to the radio.
Turn the calculator on and place it near the AM radio until a loud tone emanates from the device. Tape the radio and calculator together in this exact position. To test, take any metal object, such as a fork, and place the metal detector near it. The device should begin to beep as it nears the metal object. The closer to the metal object the detector is, the faster it beeps.
An alternative method to building a metal detector involves a CD case instead of tape. Remove the inserts from the CD case, and using Velcro mount the radio, face-up, on one panel of the case on the inside. Mount the calculator, face-up, in a similar manner on the other panel of the CD case. When closing the CD case, the AM radio emits a loud tone. To test, open the CD case until the tone is barely audible, then place the device close to a metal object. As the device nears the metal object, the tone becomes louder.
This device works because the electronic circuit in the calculator produces an AM frequency signal that the radio picks up.