The order was clear: make it look like an old light set for Favorit bike, run on batteries, and do the "to be seen" job. Not supposed to be used for long night rides in the wild, it's just a safety light to get you home when darkness falls unexpectedly. No dynamo, no caring for accumulators, just something simple, light and stylish to be used a couple times per year. This is how it ended up:
Four AA batteries in series. Three of them would be enough to light the LEDs up, but due to their threshold voltage the lights would go dark long before the batteries were fully depleted. Four cells can be drained more or less completely. Accumulators would get damaged by such a deep discharge, so only use the circuit with one-shot alkaline cells.
Battery holders are readily available in electronic hardware stores, some of them have practical snap-on contacts shaped like those of a 9V battery. Just be sure to test the holder before using: the contact springs must be strong enough to overcome all friction of plastic clips and reliably push the batteries all the way to their second contact points, which they tend to bounce away from due to vibrations and bumps.
Seven white LEDs are placed in the front light, four reds at the rear. Everything is simply parallel, the excess voltage gets burnt in resistors. Of course, a switching constant current regulator would be more efficient, but who would want to bother with one for such a small and simple light. Maximum current consumption is about 216 mA. Batteries are "harder" power source than a dynamo, their voltage doesn't change much with the load. So it's theoretically possible to add more parallel branches to the circuit, or remove some.
(something||something means resistors in parallel, something+something is series)
The front light is a mix of various white LEDs I had lying around, paired together by voltage. If yours are all the same, you don't have to bother with so many different resistors. My resistors were recycled from electronic junk, hence the wild serial/parallel combinations. Current values for every single LED are 19.something mA, so their lifetime should be close to unlimited.
D1 diodes serve two functions. First, they protect the circuit against polarity reversal, which is necessary because the battery and rear light use unkeyed connectors (marked with little white circles in the plan above). If something is reversed, nothing gets damaged, it just won't light up. The second purpose of the diodes is voltage reduction. By how much? Datasheets show some plots, but they're not detailed enough and they say something about pulse mode, so I measured the continuous DC characteristics myself to be sure:
1N4004 looks about the same, but the respective plot (ruined by my quirky breadboard) is not publishable.
The circuit boards were drawn mostly by hand. The front one could be smaller, but not much because it's glued to the cut-off end of the original reflective dish and the components on it must fit the narrow space around the perimeter. There is some aluminium tape stuck to the board under the LEDs to add shine and hide the butchery. The board is further secured in place by two pieces of wire and some hot glue (not yet present on the following photos).
Rear part of front light housing contains the switch and connector board for battery and rear light. The steel sheet contact plate which originally reached all the way to the light bulb was shortened with a dremel and bent to make a hook for the hot glue blob holding the connectors in place.
Rear light's tiny motherboard was tricky to design, but in the end all the components fit on it. It is hot-glued to the original light bulb socket, there was no place for a screw. The LEDs are aimed so that they cover around 60° horizontally and 30° vertically, one of them shining through the original lens as a long-distance beam.
Batteries are hidden in a small leather saddlebag, cables run inside the frame.
Modern yet discreet light set in a vintage shell - a perfect fit for a lovingly restored vintage bike. Rear light is a lot brighter than the original bulb, front one is about the same. Nothing for me though, I prefer power sources that never run out of juice.