https://radiokot.ru/circuit/digital/security/43/
The ultrasonic sensor is a resonant piezoelectric ceramic transmitter TR2516T1, shunted by resistor R1 to expand the band of receiving frequencies.
Blocking capacitors C1, C3 and capacitors in the emitters of transistors C2 and C4 are used to shape the frequency response of the amplifier (above 20 kHz), in the ultrasonic frequency range.
The device does not respond to audio frequencies.
The generator works great, but the receiver doesn't respond for some reason.
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