How do Traffic Lights Notice Bicyclists?
There are two basic types of intersection detection systems out there:


Induction loop coils:
These are basically a metal loop or figure 8 embedded in the pavement, and will detect anything that conducts electricity. Much like a metal detector, such as you might walk through at the airport. Sometimes you can see lines carved into the road surface. Getting your wheel over the lines will help you get picked up.
Cameras: You may be able to see the cameras, even.  These are generally black and white cameras that are programmed with areas they pick up for each lane and do some sampling things and if the samples look different than the "baseline", that's a detection. If your headlamp is helping, this is the type of detector in use at that intersection. A bright light will definitely be picked up at night. Light clothes that contrast with the color of the road surface will also help.