Interactive Leds array. best motion track sensor?

Hi everyone. I am new to circuit making and I am looking for any kind suggestions, regarding the building of an led display which can register motion. I have an idea where an array of ldr's and an equally sized array of Leds could be used. Thanks for reading :)

paralax ultrasonic range detector, or using a low ma laser shined onto a photo diode (when walked past the beam will be cut off and u can use the readings and intergrading it to the leds using shift register chips etc

hey, thanks chris for your reply. I'm thinking of maybe making this a wearable display. sorry, I should have mentioned. Would this mean i would need an array of lazers to control each Led individually. I'm trying to track the outline of a someone approaching. The photo diodes seem a good idea but they would work in reverse. I want the area within the shadow to be represented as lit Leds rather than the other way about. With Lrds and photo diodes, they both increase current with light, not in the absence of it. Do you think there is any way around that. Thanks again for your suggestions, youv'e given me alot of new ideas.

Do you think there is any way around that.

Since you're posting in a microcontroller forum: It's called "software".

Or, if you prefer to do it in hardware (which negates the advantages of using a microcrontroller) then it's called an inverting amplifier and consists of a transistor and maybe a couple of resistors which may include your sensor.

Actually, you could do without either... depending on how you arrange the photodetector and resistor, you can make the voltage at the analog input go up or down with increasing light level.

+5V ---resistor--- analog input pin --- light sensor --- gnd Will give an analog reading that decreases with increasing light.

+5V ---light sensor--- analog input pin --- resistor --- gnd Will give an analog reading that increases with increasing light.

Either of which can be inverted in software by subtracting the reading from the max possible reading (1023).

int x, y;
x = analogRead(pin);
y = 1023 - x;

thanks dilbert, ill look into that. Still at the brief stage of my project so i am still learning a lot about sensors and circuitry. Ill keep posting as I my understanding develops. Thanks again