I want to simulate this circuit in ltspice:

http://www.circuitstoday.com/parking-sensor-circuit

So at the transmitter part. The 555 is wired as a bistable. Thus, the LED will flash with a constant frequency dictated by the 555 and its RC network. This I understand.

Then these flashes of light are reflected by the obstacle and the receiving diode sees them.

My question. The smaller the distance, the higher the intensity of light that the receiving diode sees? And thus, the higher the voltage that it generates at its terminals?

I know that the intensity of light decreases with distance, but I did not know that the receiving LED can be so sensible so that it can detect very small changes in intensity.

Also. I do not see a photodiode model in ltspice. How can I do it? An other way could be to just put a voltage source there and pretend its a photodiode.

 

An other way could be to just put a voltage source there and pretend its a photodiode.

That's how I'd do it. Check the datasheets of typical photodiodes (or research for LED voltages when used as sensors) to get an idea of likely voltages. Bear in mind that, depending on the light source beam angle, the sensor output will be a non-linear function of distance as well as being dependent on surface reflectance (a dirty car probably won't reflect much!).

 

That's how I'd do it. Check the datasheets of typical photodiodes (or research for LED voltages when used as sensors) to get an idea of likely voltages. Bear in mind that, depending on the light source beam angle, the sensor output will be a non-linear function of distance as well as being dependent on surface reflectance (a dirty car probably won't reflect much!).

I've done exactly that.
But I don't think I will get into details like non-linearity or surface reflectance.