Electromagnetic Parking Sensor Teardown / Reverse Engineering

I bought a car parking sensor from Aliexpress which is a typical china made Electromagnetic parking sensor. The Curiosity in purchasing this was to find the mystery behind it. Tested the sensor when i got it and found working like charm.....The thing I noticed is that it sense only movement, just as PIR modules unlike the ultrasonic parking sensor.
Surely the working is something related to sensing the interference in the Electromagnetic field and with a frequency counter or phase detection method it is sensing the movement of object in the field. Lets find out.....

I have started opening the small electronic housing, inside its just a small circuit consist of a LC oscillator and 89c2051 controller and 5V power supply.

Since the LC oscillator is fully SMD components, to understand we need a circuit. I have traced the circuit and found its a simple oscillator wired around 2N3904 with a supply of +12V. I'm sorry for hand drawn circuit, didn't got time to draw the schematic in software.

The output of oscillator given to a Schmitt trigger with LM393. This output is given to the INT0 and T0 of the controller. I didn't got the idea what exactly the code inside the controller. But analyzing the signal variation when we move object across antenna with a scope and observing the FFT of the signal its clear that the frequency of the signal is changed when there is a disturbance in the field. The variation in the frequency might be calculated and threshold in the controller. The signal below is the signal transmitted by the antenna. It shows the variation in the frequency when there is a movement near the field.

How ever i will shortly get the hex code of the controller and will try to find the exact logic applied. With this circuit anybody can clone this device by getting the hex code. Hex code is available here. The signal below is the output of the shcmitt trigger which is going to the controller.

Here the frequency of the signal is in 350-450Khz which is found not fix every time because its dependent of the impedance of the antenna, since there is not proper matching circuit with the antenna in this the frequency will not be constant always. In this scenario we don't need that stability also since we are only measuring the shift in frequency. With this basic principle of working i think if we increase the frequency to MHz and making little more complex circuit to measure the change based on movement we could make a gesture sensing also. Anyway enjoyed playing with this, got something to learn......