I've wanted a Sturmey Archer Dynohub on my Raleigh Chopper since the mid 1970s. Now, I've finally decided to treat myself to a hub dynamo.
After some research -- and much resistance of my lust for the multi-centibuck machined aluminum alternatives -- I bought a B&M Lumotec IQ Fly Plus Senso headlight. Here are a couple of exterior photos:
In the rear view, note the white spot beside the switch: that is the opening for the light sensor. Note also the two pairs of spade terminals, and the built-in power wire. Here is one more shot showing how the emitter faces backward and downward against the reflector:
After some messing around, I decided that this must be the effective circuit for the thing:
Specifically: the switch is single-pole on the power side; the standlight capacitor does not appear to drain back into the spade terminals when the power is turned off; and the spade terminals bypass the switch.
This last fact is highly annoying. It means that, to use a hub dynamo, you must use their supplied wire. Which happens to be too short to work with my recumbent bicycle. But the wire is built in and cannot easily be replaced -- either to make it longer or, as is likely down the line, to fix it if it starts to wear from road vibration.
What I really wanted to do was to cut the provided cord short and attach connectors of some sort, then use these to attach my own wiring. But I was worried about what would happen if I messed up and needed to redo the connection. Should I cut the cord longer than I would like just in case? Even more annoying!
It's usually easy to convince me to take something apart; this was the excuse I was waiting for.
To open the thing, I pried off the front cap (which holds the reflector) with a flat screwdriver. This does mar the finish a bit and also slightly damages the clips that hold the cap in place, and will I'm sure Void Your Warranty™, so proceed at your own risk:
Here is a view of the opened unit looking at the circuit board from the front:
The circuit board is retained inside the back case by the two small screws in the picture. Note that B&M did not bother to put strain relief in the power cord. Tsk tsk. One fortunate fact, though, is that the points where the cord is soldered onto the circuit board are pretty easily accessible should the cord need to be replaced. Happiness. But while we're having fun taking things apart, why stop here...?
This is the back of the circuit board, revealing the standlight capacitor:
As you can see, the light sensor on the board is the little square white dot to the left of the switch. Finally, here are two shots of the innards of the actual beam unit:
In the second image, you can see the emitter; the formed aluminum heatsink; the reflector unit; and a retainer spring that holds the heatsink against the emitter when assembled. I expected thermal grease between emitter and heatsink, but there was none.