# Dynamo Powered LED Bike Lights

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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!!)

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## 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 ;-)

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## 45 Discussions

I don't get the purpose of R1, C2, C3...why connect the rectifier input and output like that? If it was smoothing why not put a cap across the inputs? If you just got rid of R1, C2, C3, wouldn't it also work fairly well?

Would it be a problem to keep the rear light as a tungsten bulb? Will a resistive parallel load affect this circuit?

Great idea. Would appreciate a video of it in action ?

i haven't read the entire instructable or all of the comments, so if it was mentioned i'm sorry..

You should increase C1's size or increase the count, this way you would insure having the light stay on after stopping for a couple of seconds or slowing down

How are you attaching the Dynamo to the bike? And what part of the bike rotates it? (the wheel or the sprocket?)

HI,

how much all together that cost you? the materials you bought and the time you spent to make that. thank you.

So where is a good place online to get these electronic components? I found the 1w LED but the site says its 350mA. I wonder if it will take 500? I am working on a project to teach kids about the differences between LED and Incandescent bulbs.

2 replies

Maybe try to find the datasheet for this diode - usually you can find the maximum ratings there...

What is your reasoning for using the tail light as part of the bridge rectifier instead of powering it in parallel with the front?

1 reply

Red and white leds have different voltages and I'm not sure what happens when You connect them in parallel :-)

You can power it serial with front, but then it would get all the current the front gets (and You would need more leds or red power led).

When it's in rectifier branch it gets only half of main led current.

HI guys,

very good and easy tutorial. I have just few question. Your heighpower diode should be 1W and withstand 500mA. I am not a professional, but in this tutorial, the author uses 3,8W 1000mA. What is the difference?

Btw, I am asking, because in my country it is hard to find 1W diode which can withstand at least 500mA.

Thanks very much!

(i will use dynamo 6V 3W)

1 reply

There's no difference - the 3,8W diode will just not reach its maximum current rating and probably will not be as bright as it would be with 1A supply (but you would have to check the current/lumen characteristics to confirm that)

You should not use diode which cannot withstand current lower than 0,5A.

If you live in a metro area with those rental bike kiosks (like Boston), You may notice the leds on the rental bikes flash while in motion. I believe they work just exactly the way this author describes only more simply. Just a 12V dynamo and sets of 4 LED's in series front and back that flash as the dynamo's AC is rectified through them.

Maybe figure a way to use a couple capacitors in their or add a small charging circuit for a set of rechargeable batteries so you have light at stops.

I am aware of the issues some battery chemistry's have so doing the rechargable dynamo powered led light might be up in the air.

1 reply

Amen, Brother!
Or just some high capacity caps that take a minute or so to discharge while at a stop and forget the batteries altogether.

Hi,
the heat conducting layer, must have to isolate or can be conductive? Thanks.