This circuit is from an old Popular Electronics, but it can be easily implemented nowadays, even adapted to act as a shield. The circuit has the frequency given by C1 and adjusted in R1.
The sensor diode used in this circuit is the common type of germanium. Experiments may be made...
This small transmitter that we found in a 1988 documentation, has a range that depends on the...
This power supply has a resolution of 10 pA in the current range from -100 to +100 nA. The circuit is...