Originally Posted by
SweetLou
Not quite true, the inductive loop trigger senses a change in the frequency of the magnetic field. A car with enough conductive material will trip the sensor, a bike, doesn't have enough conductive material. A powerful enough magnet will trip the sensor.
There is an instructable on how to make one. It was intended for a motorcycle, buy you could easily make one for a bicycle.
Traffic Light Trigger
Er, as an electrical engineer, I don't like the terms you used- very confusing. The loop is the inductive part of a resonant circuit that changes frequency when a piece of metal comes near- changes the inductance of the loop. Kind of like a metal dectector. The relationship of your bike to the loop determines how much the frequency changes, and therefore whether the traffic signal is triggered. A rule of thumb is to stop your front or rear tire on top of and parallel to one edge of the loop, which will give maximum coupling of your rim to the electromagnetic field of the loop. Stopping right in the center of the loop is actually quite a bit worse- you want to maximize the amount of magnetic flux flowing through the loop of your wheel (or your frame if it's not carbon), and the field is coming straight out of the ground at the center of the loop.
Magnets won't do any good these days. The old style loops that detected the change in magnetic field due to the motion of the residual magnetic field in the car's frame are obsolete and never really worked that well anyway.