Does anyone know a manufacturer of this kind of systems? (I'm looking for a simple one, just needs to send an impulse when he detects a car, not a complex one, like 3M, with time-stamping etc.....)
Thanks in advance
P.S: it should be able to detect both stoped and moving vehicles.
Look for an Inductive Loop Vehicle Sensor. Its basically just a LC multivibrator that uses the inductive variability of a loop of wire on the road. When a vehicle passes over or stops over the loop the frequency of the multivibrator shifts and a microcontroller can sense this, setting a flag. There is plenty of information on building the loops and circuitry on the net.
If you're looking for the type of detector that is planted beside the road, they are fluxgate-based units that tend to be expensive, maybe 250 euro.