Ultrasonic blind pathfinder circuit

Under normal circumstances, when a blind person uses ultrasonic waves to find a path, a fixed sound is emitted, indicating that the road is flat and can continue to pass. When encountering obstacles
and being unable to move forward, the ultrasonic waves will emit different sounds, allowing blind people to take a detour.
    Its circuit is shown in the figure. When encountering an obstacle, button SB is turned on. VT1 and VT2 work to make the ultrasonic transmitting head send ultrasonic
waves. When the receiver receives the sent ultrasonic waves , they are amplified by VT3 and VT4 and output to LM567 for processing. They are output by the VT5 emitter, and then
output by IC2 (NE555) ③ pin, and then by VT6 Amplify, push the speaker to produce sound.
When a blind person hears the sound of an obstacle, he will take a detour.