hey:
am working on my school project trying to build a sensor circuit that senses an object approaching premises/house with LED lighting according to the distance of the approaching object. Now i want to connect this to bcd display that shows the distance of the object and the time at that particular distance. pliz am out of time i need help. thanks

 

For an easily available sensor - http://en.wikipedia.org/wiki/Passive_infrared_sensor

These sensors are not really intended to measure distance. It might be possible to arrange two of them with some collimation so they only detect at a very narrow angle.

This gets elaborate, but with another wide angle sensor doing the initial detection, it might be possible to swivel the narrow angle sensors until they both see the object. The angle would give distance. That would require a microprocessor to control all that stuff and translate the angle to distance.

Outside of that, you can consider a series of beams that get interrupted by the passage of a person. Distance is given by whih beam is blocked.

 

You may also consider ultrasonic detectors, such as the Ping))) Ultrasonic Sensor. It has a range of up to 3m. It would also require a microprocessor. There are many articles available online on how to use it if you search for "ping ultrasonic sensor".

 

What is the maximum distance you want to detect?

I've used "tremblers", small seismic sensors in a lawn for perimeter detection, they work pretty good at catching footsteps within about 6-8 feet. Any higher threshold and wind sets them off.

If you are trying to measure distance of something over 15 feet away, you'd need to use ultrasonic ranging instead of IR, or have sensors at known distances in a grid.