I have a Bridgestone Tentoumushi dynamo hub that I built into a 27" wheel some years ago. It came with an ancient headlight which, as I recall, worked ok with streetlights around, but not at all in pitch black. It has a single pole, and uses the frame for ground. My question is: Is there some way to wire this thing so it can be used with modern led lighting? Can anyone point me in the direction of someone who might be knowledgeable in these things?
Thanks in advance

I posted this, briefly in the lighting and gadgets forum and then thought better of it.
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Yes, it's not terribly difficult. But it depends....

An LED requires D/C while the dynamo puts out A/C. A simple device called a bridge rectifier, which any caveman with a soldering iron can put together from four diodes, converts the A/C to D/C.

If your LED headlight is intended to run off a dynamo, it has a bridge rectifier built in. [Though I may admit: I can't be sure of that and don't know how to check.]
Assuming so, then all you have to do is connect the wire from the dynamo to one connection point on the headlight, and run a wire from the frame (or fork or any place that connects it, such as the headset or brake) to the other connection on the headlight, and you should be ready to go.

Thanks. I'll give it a whirl.

You can also buy bridge rectifiers as a complete unit.

Since a light bulb will run happily on AC, it's unlikely your headlight has any sort of rectifier built into it. You'd be best off doing something a little fancier than a basic bridge rectifier. Some capacitance will smooth things out. Here's one example, there's lots of others on the web:

Dynamo Powered LED Bike Lights: 7 Steps

I'd imagine that someone must sell some sort of LED unit with a built in regulator that could be shoehorned into your old light housing. That would be the easiest way to go.

It would be a good idea to determine your dynamo's voltage and current output before proceeding. Got a multimeter?

First, your dynamo should output 3 Watts of power, or close to that. Second, if you buy just about any modern LED headlight it will have a rectifier in it. Most B&M headlights will do the trick.

The web says 6v, 3w, but nothing more that I can find. I'm reticent to connect it directly to my $80 B & M light, but I do have some spare bulbs around. I'll see if they can be shoehorned and check the results.

6v and 3w is standard, though there are also 6v and 2.4w dynamos. In theory the 3w dynamo powers a 2.4w headlight and a 0.6w taillight, and the 2.4w one just powers a headlight, but it doesn't really matter. The rated current is produced by some specific wheel size at some specific speed, such as 12 mph, and if you go 24 mph that voltage will double. The electrical engineers at B&M understand this and they've figured out what to do about it.

So I'm a hundred percent confident your $80 B&M headlight has the circuitry to convert the variable A/C current from your dynamo hub to the reasonably steady D/C the LEDs prefer, including over-voltage etc.

I've been running a B&M lumotec cyo senso-plus premium (or some such gibberish) on my randonneuring bike for a couple years, occasionally hitting over 40 mph, and nothing has gone wrong with the light. It has an on/off switch that I never use, I just leave the light on all the time, well into the thousands of miles now.

If you want to assemble your own headlight from all the individual components, then there's a lot more to this than I can explain; but that's not the situation here so let's not worry about it.

+1 The B&M or similar headlight should work just fine. Go for the easy solution, unless of course you enjoy soldering stuff together.

Dynamos haven't really changed. 6V 3W is pretty standard.

If LED stands for light emitting DIODE, and a bridge rectifier is four diodes soldered together, couldn't you just solder four LEDs together in a bridge configuration to work with a dynamo? Or am I forgetting something from electronics school? It has been a while and the only LEDs we had were little red ones.

I have to say I didn't think it would be so simple. I've been sitting on this for years now, wondering where exactly to ask. Now it appears I'm forced to actually try it out. Soon.

Yes.

The real caveman solution (my specialty) is to attach one wire from the dynamo to the "+" of the headlight and the "-" of the taillight; and attach the other wire to the "-" of the headlight and the "+" of the taillight. That way the alternating current goes to the headlight and taillight alternately, giving each one full power half of the time. No on/off switch, no capacitor, nothing fancy.

It works, I know 'cuz I've done it! But my $80 B&M headlight works so much better, I can't recommend anything else.

I was impressed and intrigued when I saw Rudi's caveman solutions, but I've used B&M (and Philips) lights for a while now, and they really are terrific lights. They're only slightly expensive, but they work so damned well that they are a good value. Just hook it up, and it works. The Philips Saferide dynamo light is out of production, so forget that one. I've used a few B&M headlights, and they are all terrific. They have more than adequate overvoltage protection. The light is adequately bright, and the beam shapes are ingenious. The optics are engineered, arguably over-engineered. I wish German law allowed higher power, because the one thing that would be nice is more brightness, but I'm getting by fine. They are not as bright as good battery-powered lights. I prefer the B&M lights, because I prefer the beam shape so strongly. I also prefer dynamo power over battery power.

If I am understanding your explanation correctly, that would be a full wave rectifier. Adding a capacitor would help smooth out the voltage peaks to reduce the flicker at lower wheel speeds. At higher speeds, the flicker is fast enough to not be noticeable.