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 zener diode determines the triggering of the circuit and R3 must be a resistor that will be...
In the figure we have the way to control the frequency of an oscillator with a NE602 using a...
This circuit was found in a 1977 Radio Shack documentation. The unijunction transistor can be...