need a circuit diagram for monitoring of parking lot

the concept of my project is about monitoring the availability of a parking lot in a certain establishment. my problem is i dont have a circuit diagram and a trouble finding a kind of metal sensor. can i ask some assistance where to search a circuit diagram about my project and could anyone give me some idea what kind of metal sensor will i use. what am i looking for is a circuit diagram that is to count and display the number of vehicles that enters a parking lot and the number of vehicles that exits..hope you could help me with this project.. thank you!

I would imagine a pair of sensor coils in the ground, similar metal detector, they would trigger sequentially. the output could increment a counter for cars coming 'in' and decrement for cars going 'out' so theoretically, the number displayed should be the number of cars in the parking lot?

This assumes the cars are entering and leaving by the same route?

This is probably an ideal project for a small PIC

The same could be done with pairs of IR beam interruptors, as the coil version means burying a ribbon cable in the ground (alternately connected to form a single coil) Although the IR version would trigger on pedestrians, so probably the coil is best?

Just a few thoughts

Steve

Theres a few ways to tackle this, the three most used methods are :

Pressure sensitive wire, it's used at about every intersection I have ever been thru ( US) If your a yank it's the little lines you see in the asphalt .
The second most popular is called an inductive loop..similer to the pressure wire but more like a metal detector like crofty said.

The third ( and prolly most expensive) is a pressure SWITCH ( **broken link removed**) which indicates transit of an auto across it's surface.

The first and third example can be interfaced to a simple counter ( caveat both are divide by 2, two sets of tires right? )
Any circuit is dependant on the type of sensor you use.

what about a push switch to operate a barrier in/out, which is connected to a counter?