hi Colin

regarding the diagram on http://home.cogeco.ca/~rpaisley4/ATDetLong.GIF

tat's an example for setting the IR to send data out?

can i know wat's the basic h/w i need for it?

That's all that's needed for the sensor. The IR LED is on constantly. The output is taken from the collector of the phototransistor, which acts as a switch. When an object blocks the IR light, the phototransistor is off and the output is HIGH. When there is nothing in between, it's on and the output is LOW.

Indoor or outdoor parking lot? Sunlight washes out infrared!

Feel like digging up a parking lot, then halleffect sensors. A single loop will detect motion of magnetic metal. A double loop acts as a metal detector.

Pressure sensors.

Infrared - washed out by sun.

Ultrasonic - Can detect distance to ground, if short then being blocked then highly likely a car parked there.

Camera - Will need pattern matching to not shading difference to detect motion, etc. (Think MPEG MacroBlocks). Are pixel blocks being disrupted into a parking stall without going out? And what about those overline parkers?

First problem is the sensor. Work on it first!

Hint: There's a reason automated parking lots use choke points with vehicle counters.

If it's just a proof of concept project, and you're not going to be near a car park. Then IR is fine.

ya is just a proof of concept idea

so here's what i will do

a IR transitor and one receiver tat's constantly on, once a object block it, the receiver will send out the signal right?

here's the problem.. how should i capture the signal and send it to the pc? as far as i know the FTDI link and USB thingy given by Colin seems bit complicated is it?

my idea is to add in an IC with calculating the counter programmed in it and send it to the PC

i planed to use rs232 to send the signal to the PC, but the problem is wat type of signal is it? how do i determine the type of signal?

as far as i understand the signal will be determine by the programmed in IC is it?

The phototransistor is just a switch, the output is either the positive supply or ground. You could program a microcontroller to read the outputs and send serial data to the PC's serial port.

The FTDI devices are just a simple way of creating a USB interface.

ya tat's what i m planning to do.. i haven figure out what should i do when the signal is transfer to the PC.. mayb VB or C++ to program a calculation and display

i m trying to get the h/w and try to get the signal to the pc now, tat's the stage i m now

RS-422 or RS-485 would be an appropriate serial connection. Handles distance. Since it sounds like you're putting a processor (may I recommend a PIC) directly attached to the sensor(s) you may consider putting an LED on an output driver as well. You can self test by toggling the LED on and off and monitoring the input sensor.

So you've experimented on a breadboard and observed a working design; first, before taking the leap of hooking it up to the PC?

Have you thought of using a photo cell(or a solar cell) and a laser(visible or invisible)? It can also be used to detect movements. If it suits your needs, then programming and other things can be so much easier!

Just a thought.