Haris Andrianakis sent this in! Via his personal blog Candrian:
It has been two years since my last post as i can see from the date but finally it seems i found the time to come back. The idea of reverse engineering the car parking sensors came when i replaced my car parking sensors and i was curious to find out how this thing really works.
Generally, i had a picture in my mind on how they should work. A sounder that sends the sounding and a microphone/receiver that receives it back (as radar works), calculating the time, the signal takes to travel from the sounder and back to the microphone from the obstacle reflection. Having 4 sensors on the car bumper pointing to the same direction, is challenging for the processor to recognize which sounding comes from which sensor. A quick thought i made is that it may use a different operating frequencies for each sensor or a different timeslot for each sensor keeping the same frequency for that purpose. But we will find out later on, while powering things on.
So having this picture in mind we move on.
First interesting thing that got my attention while i was removing the sensors from the bumper was the sensors themselves. They had only two wires each so the picture in my mind with a separate microphone and sounder on each sensor should change to one device which may switch from sounder to microphone and vice versa (operating both as a sounder and microphone). All this because there are only two wires so there could not be two different components such as a sounder and a microphone. If that was true there should be at least 3 wires, 2 wires one for each component (sounder & microphone) and a shared wire for the common ground.