I ride alot at night in the city and got tired of not being able to use my blinker, hold the brake with the brake light on, have a headlight on and honk all at the same time, while idling at some hilly SF intersection with insane SUV drivers on cellphones trying to kill me.
This circuit allows you to use a small battery as a buffer for times when you drop below minmum voltage needed to light your 'ped. Works just like a car circuit. When the 'ped is revving, it powers the light and charges the battery. When the 'ped is under-revving, it runs in reverse and the battery powers the light. No more flicker-flicker and no more dim-dim. The battery I used (1.3amp-hour) should give you about 20+ minutes of lighting backup at slow idle, which is far more than you need for a 2-3 minute long traffic light.
The rectifiers isolate each circuit and allow you to use a relay to automatically turn the light on and off when the 'ped is running or off. Otherwise, you'd have to use a switch to turn the light on and off. This might result in forgetting to turn on the light during the day, or forgetting to turn OFF the light during the day, which would drain your battery down to zero, and ultimately kill the cells.
The second rectifier circuit powers a SPST switch which basically closes the circuit between the battery and the light when voltage is sensed (i.e. the 'ped is running). When the 'ped is off, the switch opens, the light turns off and the battery stays charged. If you did this whole project with just one rectifier circuit, it would work, but the battery would remain connected to the coil of the switch when the 'ped is off, and might ultimately drain down if it's parked for a while. It's just a few extra wires and connectors and maybe $3 more, and I always like to avoid deep cycling of lead-acid batterys as much as possible.
I attached the circuit diagram for a super cheap and super quick fix. It works for my 2009 Tomos ST, which is 12V AC with chassis ground and is already regulated coming off the coil. Of course it might work for tons of other 'peds too. I am also sure there are 20 ways to do this mod and not everyone will think it's cool to want to use your horn/signals etc... Whatevs, for those who do, here it is.
Basically the idea is to avoid murking with the existing headlight/headlight switch circuit which could be frought with calamity if you clip the wrong wire or even look at one of the harnesses funny. Just ignore that circuit and go with a single beam (i.e. no high/low) lamp circuit.
2 Radio Shack 50v 25amp full wave bridge rectifiers (like $3 each)
Some 12/14 ga primary wire
Bunch of 1/4" male and female fasteners, and twist-top wire connectors
1.3 amp-hour lead-acid rechargable battery (weighs 1.5lbs, cost about $15 from BatteryMart.com)
SPST switch from any auto place (maybe $5)
1) Tap into one of the leads coming out of the 12V AC regulator, which comes off the lighting coil. Run this to wherever you are going to store the guts of your assembly. I put mine under the seat, since I long ago chucked the oil injector.
2) Split this 12 V AC tap into each of your two rectifiers. Connect the other AC terminal on the rectifier to the chassis.
3) One rectifier will be the switch circuit. Wire the +/- out of the rectifier to the coil terminals on the SPST switch.
4) The other rectifier will be the light/battery circuit. Run the + from the rectifier to the battery and to the light (parallel), and run the - to the battery and to one of the two switched terminals on the SPST switch (parallel).
5) Connect the other light terminal to the other switched terminal on the SPST.
6) I ran the wire from the regulator and the two wires to the headlight through the oil level checker hole in the seat pan, then under the left side cover and up the conduit under the gas tank to the triple tree, and then into the headlight pod. Wire ties/tape etc... yadda yadda yadda.
7) Get a blindingly bright xenon light / halogen / tractor bulb and blind everyone while you finally can see the road in front of you.