This circuit uses two transistors and one IC (555 timer IC) to produce a pulsating ultrasonic frequency. Transistors Ql and Q2 are connected in a direct-coupled oscillator. The frequency of that oscillator is set by capacitor CI. The oscillator output is taken from the emitter of Q2 to pin 7 of IC1. Transistor Ql is an npn transistor, and Q2 is a pnp transistor. The signal of pin 7 on IC1 causes the output signal appearing on pin 3 to be modulated or varied by the audio frequency developed by Ql and Q2. The IC itself is connected as a stable multivibrator with a frequency that is determined by C3.
Capacitor C3 sets the basic frequency to be well above the human hearing range (ultrasonic). The combined modulated ultrasonic frequency appears on pin 3 of IC1, where it is coupled by capacitor C4 to the piezoelectric transducer.
