I was working with the simulation model for intelligent vehicle last night using just basic components like the infrared LEDs and LDRs to developed a program that would avoid the rear end collisions in car. Using just one OpAmp in comparator mode, I linked the car models with the computer program developed on VB and able to get 2 detecting ranges to keep track of the two cars' distance. However, I found out something so strange about using parallel port that is it works differently (more precise I guess) if you join its ground pin to the ground of the external circuit. I tried not joining them; it gave almost the same result of the input reading but in a not very stable manner... I kinda repeat it several times to confirm this phenomena, and yes it is. Does my printer port has something wrong in it?

Can anyone suggest me the site where I can find more detail about some simple or homemade kind of sonar? Love to have it. thanks kittipat_0541(at)yahoo.com