The 4046 PPL is used as the heart of a tunable ultrasonic receiver that can be used to locate unheard ultrasonic sounds. The receiver might also be used, along with a simple ultrasonic generator, to send and receive Morse code. The incoming ultrasonic signal is picked up by piezo speaker SPKR1, and amplified by transistors Q1 and Q2. The output is fed to the phase comparator input of U1 at pin 14. Tile chip`s interval VCO is tuned by turning potentiometer R9.
If a 20-kHz signal is picked up by SPKR1 and the VCO is tuned to produce a 19-kHz signal the difference output at pin 2 will be 1 kHz. That 1-kHz signal is amplified by Q3 and coupled through Tl to a pair of headphones. If the received frequency increases to 22kHz, a 3-kHz tone is heard in the headphones. With the values given in the parts list for C1, R1, and R9, the VCO can be tuned from 12 to well over 42kHz, which should cover just about anything the piezo sensor can respond to.