Modulator Technical Page.
Modulators are a little controversial. They are authorized by an act of congress and also a Canadian law. In fact, we recommend you visit the printable version of this page and keeping a copy on your bike. Riders with modulators have been known to be stopped by police and hassled about the modulator.
Some people think they are obnoxious and hard on the eyes. That's as may be, but especially with running lights and headlight upgrades, they are very noticeable.
Some modulators require cutting and soldering, or tapping the harness, and we suggest you avoid them. You can get several brands of modulator which install by unplugging the headlight from the harness and inserting the modulator between the headlight and it's socket.
You will have to set a DIP switch or jumper to tell the modulator to trigger on the low beam or high beam.
You can also get modulators which come with a relay, which you can wire directly to the battery, and get the additional benefit of bypassing any voltage drop in the wiring harness. This is still a pretty easy install requiring no cutting or soldering of wires.
You should get the type of modulator that matches your headlight bulb configuration, and dual versions are also available for dual-headlight bikes.
Modulators are, by law, fitted with a photocell to cut off the modulator when it is dark. You should not be running the modulator after dark. The sensor needs to be placed where it can see the light, which might mean tying it to a cable housing. If you close up the sensor inside a fairing or light housing, the modulator won't work.
Modulators are required to be fail safe, which means that they are designed to the lights will return to normal operation if the modulator breaks.