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NE555 Ultrasonic Pest Repeller Circuit

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.

NE555 Ultrasonic Sound Source

Using two NE555 timer IC devices, this circuit generates either pulsed or continuous ultrasonic signals. The values of CT for both pulse rate and ultrasonic frequencies can be calculated this way. SPKR is a small hi-fi tweeter.