Unfortunately dynamos seem to be forgotten by many modern cyclists. I'm not going deeper into reasons of this situation - instead I'm going to show You bicycle LED light that works better than many of commercially available battery (an dynamo) powered ligts.

The advantages of dynamo powered bicycle LED light:
- lots of light!
- always available - You can not forget to take it with You
- unlimited burn time
- no cells, batteries, chargers (think of costs and environment)
- unattractive to thieves.

I use this light for daily bike commuting since one year. The instructable misses some photos, because I didn't took many when I was building the light :) The circuit idea comes from the page http://pilom.com/BicycleElectronics/DynamoCircuits.htm (highly recommended!!)

Step 1: Electrical Schema

The bicycle dynamo works as a current source - it always "tries" to deliver its typical 500mA of current. This is an ideal source for light emitting diodes, which are current driven. The 500mA is way too much for single diodes, but it is just good for modern power LED (such as Luxeon, SSC, Cree etc). Power LEDs are delicate when it comes to reverse voltages, so we will rectify the dynamo current to power them.

The circuit is super simple - the graetz bridge rectifer, some smoothing capacitors and power LED diode.

Red LED diodes for the tail light are put as one branch of the rectifier. 13 diodes are connected in parallel - that multiplication gives more light and splits the current on more diodes (as You know, one diode can take only 20 ~ 25 mA current).

Please note, that the bicycle frame is usually used as the ground in AC dynamo circuit. The rectifier on the AC side is connected with the dynamo and with bike frame. The DC circuit needs 2 wire cable for its connections - there mustn't be any electrical contact with the frame.

Step 2: Mechanical Schema

Mechanical

The lamp body is made of plastic tubing (1 inch inner diameter) - we need one connector, and one cap. The power LED (on "star" pcb) will be glued (using heat conducting glue) to a long bolt which will be fastened to the plastic cap with two nuts. The bolt will keep the LED in centered position and will serve as a heat sink. Power LEDs must be operated with heatsinks - otherwise their life won't be long.

The light emmission characteristics of power LEDs alone are not very good, so we have to use special optics to form desired light beam. I used a 30 degrees collimator. The collimator comes on top of LED's star PCB. My collimator has exactly the diameter of the plastic tube I used - it fits perfectly with some glue from heat glue gun between tubing and collimator.

In my case, the rectifier was too big to fit it in the lamp body, so I used a separate 35mm film canister. The diodes and capacitors are connected using an wire connecting rail (which fits the canister), but they can be connected on an universal PCB board or simply soldered together.

Step 3: Part List

- 1W power led (luxeon, SSC P4, Cree or something similar) on star PCB - it must withstand at least 500mA current.
(it looks like [http://commons.wikimedia.org/wiki/Image:2007-07-24_High-power_light_emiting_diodes_(Luxeon,_Lumiled).jpg that])

- Collimator or reflector suited for above power LED with 10 - 30 degrees beam width (10 degrees has visible light-spot in longer range; 30 deg. puts more light to the sides and is more visible for others)

- 3 (or 4 when You don't make the tail light) Shottky diodes (for example 1N5818) for rectifying (faster, lower voltage drop). the normal silicone diodes will also do.

- 13 - 15 red diodes for the tail lamp

- a base for tail light (universal PCB is very good)

- Capacitor C1 - 2200uF 4V

- Capacitors C2 and C3 470uF 63V

- Resistor R1 - 47kOhm

- 1 Longer Bolt and 2 nuts for LED mount/heatsink

- Plastic tubing for the casing

- some casing for tail light (I used some clear plastic rail in which integrated circuits are transported)

- some length of 2 wire cable for connecting dynamo, rectifier, head and tail light

Step 4: Building the Circuit

1. Build the rectifier according to the schema in step 1 (think of polarity of diodes and capacitors :-)).
2. Solder leds in parallel for tail light (this can be a little boring)
3. Solder wires from the rectifier to the pads on the power LED's PCB
4. Check the polarities and connections once more (a multimeter can be helpful)

When everything seems to be ok, then You can connect the circuit to the dynamo and spin the wheel gently. The diodes should flash immediately. When not, then check the connections once more...

Warning: when You disconnect the power LED from capacitors in rectifier then ALWAYS discharge the capacitors before connecting it again. The load in capacitors can destroy the expensive diode.

Step 5: Front Light

1. Drill a hole in the rear part of the headlight casing, put the bolt through it and fasten it using 2 nuts.
2. Drill a hole for cable.
3. Glue the power LED to the nut head (use heat conducting glue).
4. Put the collimator onto the led (usually the collimator legs fit into the cuts in LED's star PCB)
5. Put the front part of the casing on and fix it to the rest (glue or duct tape).
6. Optionally, You may want to fix the collimator to the tubing with a little of hot glue and/or put some clear plastic on the front of the headlight.
7. Fasten the headlight to the handlebar (I used 2 hose clamps)

Step 6: Tail Light

1. Build and connect the tail light.
2. Fasten the taillight (I glued mine under the rear reflector light)
3. Fasten the power supply cable to the bike frame (for example using zip ties) or pull it throuh the frame in place of old wire.

Step 7: Ready!

You are ready for test ride!

The LEDs are so efficient that the generated light is useful even when You walk with Your bike. The full power is reached at speeds from 5 to 8 km/h.

As I mentioned, I use this system since 1 year (I go to work by bike even when it snows or rains) and I am very happy with it. I use old bottle dynamo and slipping and clogging was never a big problem for me. Just fire and forget ;-)