works on the sonar principle, sending an ultrasound burst and listening
for first echo. The other outputs (1 to 9) are scanned sequentially
following burst generation, until an echo strikes back the RX receiver.
The monostable stops temporarly the scanning, and a led corresponding to
the obstacle distance appears as continuously lit. The buzzer bleeps
when the first led (minimum distance) il lit.
If no echo is received, the scanning never stops and all the leds are slightly lit.
Set TR2 for maximum sensitivity (usually 40 kHz for most commercially available ultrasonic transducer pairs).
Set TR1 for your preferred range. Setting it to minimum resistance shortens the distance for each led (minimum range).
Pin 2 of U10 goes to pin 16 of 4017 AND to pin 14 of 40106 (Vcc) ,
pin 3 of U10 goes to pin 8 of 4017 AND to pin 7 of 40106 (GND). Then
adjust TR1 and TR2 using a rigid surface (for example a metal sheet)
placed in front of sensors, at a variable distance.