It is an ultrasonic parking sonar. Connecting it to the reverse gear lights, it switches on automatically and shows you the distance to the nearest obstacle (according to his beam) on a led scale. When the last led lights, a buzzer is also activated telling you to stop immediately.
It works on the sonar principle, sending an ultrasound burst and listening for first echo. The burst generated by the oscillator built around U4D (you must set the frequency using TR2 to have 40 kHz or the maximum sensitivity), U4E buffers the output and U4F boost the signal doubling the voltage span across the TX piezo transducer .
A new burst is generated each time the decade counter (4017 in the circuit diagram) is in its reset state, that is output 0 is selected. The other outputs (1 to 9) are scanned sequentially following burst generation, until an echo strikes back the RX receiver. It is then amplified by the transistor input stage, triggering the monostable built around U4A – U4B. The monostable stops temporarily 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.
When the monostable expires, scanning is resumed and restarting the send-and-listen sequence. If no echo is received, the scanning never stops and all the leds are slightly lit.
I enjoyed the sonar for many years. It was installed below the car’s rear bumper in a plastic case. Do not choose an enclosure too small: always separate the transducers by 7-10 cm and plenty of sound-absorbing material, otherwise the receiver will reveal the direct sound instead of the reflected one. The same applies if the sound travels through a rigid fixture, so it is a good idea to fix them with separated supports.
And remember not to mount the transducers exposed to direct sunlight nor rain.
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