Battery power for dynamo lights
Useful battery lights for bikes cost too much money and need recharging every few hours. Dynamo lights work whenever but only when you're moving, which can be dangerous with stupid drivers about, and no help at all if you have to fix something in the dark.
Turns out the solution has existed for years and years but for some reason nobody makes it anymore. Here is a circuit that combines both battery and dynamo power and switches between them automatically.
It's a copy of what Sturmey Archer used to make from the 40s to the 80s, for the more expensive Raleigh bikes.
All it does is convert the Dynamo's power to DC so that it doesn't damage the batteries. The batteries are three D Cells, and they last a very long time because they stop doing anything whenever the dynamo is spinning.
There's nothing there to switch between the battery or the dynamo, no fancy IC logic, no relays, or anything at all. They're just wired in parallel. This works because the lamps will only draw power from the one with the bigger voltage. So when the dynamo spins up and gives it's normal 6V, the 4.5V battery is totally ignored.
On battery your lights will be noticably dimmer. I recommend upgrading to regulated LED bulbs as this will solve that problem, and also they're fifty times brighter.