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Back in the mid 20th century, Raleigh sold bikes with Sturmey-Archer's "Dynoluxe" lighting setup. This was a combination of dynamo (generator) and battery powered lighting, and would automatically swap the lights between the two power sources depending on how fast you were going. The effect was that your lights were as bright as a dynamo, but stayed lit even when stopped, and the batteries lasted for years.
This system was never improved upon, but it was eventually discontinued. Sales of bicycles collapsed in the second half of the century, and Raleigh survived by abandoning commuters in favor of recreational and sporting riders who neither understood nor wanted that kind of feature.
As a fan of being able to see in the dark, and unwilling to put modern lights on my very old bike, I've put in some halfhearted attempt at reproducing it.
my version
Circuit is as the original, except that there's a zener diode D5 added as a crude voltage regulator. I thought it best to give modern LED bulbs some protection.
It works with more or less any diodes, but for maximum brightness you want ones with the lowest voltage drop you can find. I picked 1N5818 out of convenience and they work well.
The original circuit was a stack of discs inside a specially made battery holder, and I can't be bothered making the battery holder. Instead went for a little board that can tuck neatly by the rear axle.
There's plenty of room for improvement. I've used a D-Cell battery holder similar to the original, but it'd work just as well with AAAs, which could potentially let you hide the entire assembly inside the headlight. Alternatively it could be miniaturized with surface mount components, and a close replica battery tube made with a mix of 3d printing and plain PVC pipe. Or you could go in a radically different direction, and with a suitable smoothing capacitor and a low dropout regulator, remove the disposable batteries and use it to charge a USB power bank.
I'm happy with it already, so probably won't bother.
download files, acknowledgements
The original device was patented by Sturmey-Archer, a subsidiary of Raleigh Industries circa 1951.
I'd like to thank recent ko-fi contributors for keeping me solvent through a difficult period, and particularly Dex The Dragon, who's long term patronage can't go unacknowledged.