Although most common piezoelectric transducers have a reduction in performance at frequencies above 10 kHz, the capsules found in piezoelectric tweeters can reach 20,000 Hz or more. The idea of the circuit shown in figure 7 is to use only the cap of a piezoelectric tweeter, from which the small internal transformer has been removed, to generate ultrasound at around 20 kHz. The frequency is basically determined by C2 and adjusted in P1. Just turn P1 until the high whistle produced by the transducer disappears. If this is not achieved, decrease the C2 value. The circuit can be supplied with voltages from 6 to 12 V and the consumption is quite low, even with the continuous emission of the com (something around 10 mA). Note that a counterphase output is used so that two 4093 act as digital amplifiers, thus obtaining a good performance for the circuit. One application for this circuit is as a rat-scare, since rodents are quite uncomfortable with the continuous production of ultrasound. However, never use the circuit in places with people or pets. The circuit can also be used as a "dog flame", remembering that these animals can hear sounds of frequencies that we do not hear, such as those produced by this oscillator. To use it, simply condition the animal, teaching it to respond to the signals emitted.
