Thanks for your suggestions, Optikon & I really appreciate it. I would consider ultrasonic sensor as IR may not be able to detect the model car. If I am not wrong, IR dectects heat emitting from the human, having a skin temperature of about 93 degrees F, radiate infrared energy with a wavelength between 9 and 10 micrometers. Therefore, the sensors are typically sensitive in the range of 8 to 12 micrometers. I am using it to detect a model car and I don't think that it may radiate any infrared engery, so IR may not be applicable. Also, u suggested inductive loop for the real implementation & I think it is a very good suggestion. But I am not sure whether any vendor is selling it.