Found in a 90's EDN this circuit can be used as a shield. The transistor is a general-purpose type and the frequency of the emitted sound is determined by R1 / C1.
This 1968 EC&G circuit generates high voltage pulses for firing a gas LASER of the time. The...
With the precision given by the crystal this circuit provides signals of several frequencies for the...
This circuit can be used as a shield for microcontrollers such as the Arduino. Frequency is given...