LED Cycle Lights

The objective of this project was to create some lights for my commuting bicycle, using up a load of LEDs that were spare. It evolved to be something a bit bigger! It started off as an extension to our bicycle iPhone charger but we have now separated the two projects again.

Design

We started this project on the assumption that our existing bottle dynamo would provide the main power for my lights or phone charger but, part way through this project we upgraded the bottle dynamo (which is noisy and runs on the side of the tyre) to a hub dynamo. Typically these output approximately 6V and have a power output of about 3W. The output voltage generally ramps up to 6V (at around 5-7mph) and slowly increases as the bicycle goes faster. Some dynamos generate an ac output (like our new hub dynamo) and some provide dc output voltage (like our old bottle dynamo).

Schmidt SON28 Hub Dynamo

The reasons for choosing this dynamo hub over many other (cheaper) ones were:

  1. It is fantastic quality and construction and comes with a 5-year warranty.
  2. It offers lower resistance than many other makes.
  3. It is available in disc brake form, to fit my current commute bike and my planned, new mountain bike.
  4. It provides a higher output voltage than most for any given bicycle speed.
  5. It reaches working voltage/power at lower speeds than most other dynamo hubs.
  6. Is provides an ac output and doesn't use bike frame as earth. It has two isolated output tabs.

SON28 disc hub dynamo in black
There is a very good guide to Schmidt SON dynamos here. These hub dynamos are relatively expensive, but are of superb quality and have a fantastic reputation. I only ever plan to buy one in this lifetime! There are also some useful wiring instructions available. The SON28 dynamo has two output tabs, providing an ac output voltage. It has two tabs because the output is floating and not earthed to the frame.

SON28 disc hub wheel
I bought my SON hub as part of a complete wheel deal. On my commute bike I'm using Schwalbe Silento II tyres because they have a low rolling resistance and a kevlar liner to reduce punctures. This is the disc brake version as I'm also using this project as an opportunity to upgrade my front brake. This wheel should also last a long time, without brake pads wearing away the wheel rim.

Energy Efficiency

Whilst using a regulator to drop the voltage down to 5V provides a stable and known voltage to which you can connect LEDs, this inherently introduces some losses due within the regulator. This means that not all the energy produced by the dynamo is converted into light output.

Worse still, is connecting a 91Ω resistor in series with each white LED. P = I² × R, so the power dissipated by the resistor as heat is 0.0364W, which represents 36.4% of the power being supplied to each resistor / LED pair (P = V × I = 5 × 0.02 = 0.1W).

Since V = I × R, then 20mA through a 91Ω resistor represents 1.82V dropped across it. Put another way, 5V - 1.82V = 3.18V is the forward voltage across each white LED. If we were to put two LEDs in series and put the rectified 6V dc across a pair of LEDs, then we would need no resistors and all of the electrical energy from the dynamo would be used in the LEDs to produce light.

Testing two white LEDs in series shows that the current flowing through them when supplied with 6V dc is 8.5mA, which actually results in quite a lot of light. As the voltage increases, the current and brightness of the LEDs also increases. The problem with this solution is that we have no way of controlling the dynamo voltage and hence the maximum current through the LEDs. The absolute maximum is 30mA but, this could be exceeded if the bicycle is going very fast.

Voltage Rectifier

As the SON28 dynamo outputs an ac voltage, a bridge rectifier is needed to convert this to a dc voltage supply required for our USB charger and to drive LED lighting.

The diodes in a bridge rectifier introduce a voltage drop (0.7V for ordinary silicon p-n-junction diodes and 0.3V for Schottky diodes). Because the dynamo output ramps up with speed, it is key that we don't introduce too big a voltage drop with the diodes in the rectifier circuit. A bigger the voltage drop, means the bicycle will have to go faster to reach full output.

Diodes
We are am using the 1N5822 3A Schottky diode (Maplin part no. AL08J) because it has a low forward voltage drop (0.3V) and can handle currents up to 3A. These are also readily available on eBay.

Capacitor

The rectified output voltage needs to be smoothed out to a more consistent dc voltage for the phone charger but this is not an issue with LED lighting. Even when using a full-wave rectifier, the LED front light flickers noticeably at low speeds.

The placement of the capacitor in the circuit is important. If left connected to the dynamo with the load not connected, it will charge up to a high voltage (>10V) and then discharge into the load when switched on. This is not good as the current surge will damage the lights. The solution is to have the capacitor connected to the load side of the switch, so that it discharges when the switch is off.

Voltage Regulator

Tests showsed that the output voltage ramps up rapidly from low wheel speeds and the output voltage exceeds 16V dc with no load. On stopping the wheel, a charged capacitor will hold at more than 10V dc. When a load is added though, the voltage output from the dynamo is much lower. With our first lamp attached to the dynamo and using only a rectifier, the voltage across it was less than 5V when spinning at a decent speed. This implies that the voltage output from the dynamo drops off as greater loads are applied. This means lamps need to be designed to operate at 6V and draw 3W power (500mA).

The initial lights constructed for this project were designed to get a regulated 5V dc supply. This is because each LED has a 91Ω resistor in series to keep the current flow at around 20mA. The regulator means that the lights can handle the varying voltage coming from the dynamo or any other source of power used. Later tests showed that with a well designed lighting load, the regulator is not required though as the dynamo output voltage is limited by the attached load.

For the USB charging port a regulator is required though. Like the diodes in the bridge rectifier, voltage regulators also introduce a voltage drop. Typically this can be 2-2.5V and this is simply too big for this application. Fortunately, you can get 'Low Drop Out' or LDO voltage regulators, which introduce a much lower drop into the circuit.

regulator circuit
For the 5V rectifier, we are using a Micrel Semiconductor MIC29300-5.0WT (datasheet). This is a 3A fixed voltage (+5Vdc) with a very low drop-out voltage. These were purchased from Farnell.

Wiring

The currents involved in this application are quite low, so there is no special wiring required.

Apple USB port wiring
Typical of Apple, they have 'modified' the USB ports on their chargers to 'signal' back to Apple devices how much current they can draw from the charging source. This is why some Apple devices tell you they can't charge from certain docks, in-car head units, etc. The fix is quite easy and requires a simple network of resistors as described here.

You cannot use this design with most hub dynamos though, because 1A at 5Vdc is a 5W load and more than most hub dynamos can provide. This means the iPhone 4 cannot be charged in 'fast charge' mode.

Apple USB port wiring for 500mA charging
This is the a similar circuit and resistor network that signals to the iPhone to only draw 500mA current. This is what we have used in our design. It means the iPhone takes longer to charge but it does charge. This also makes life easier for the voltage regulator and means we don't need a particularly high current one.

Battery Pack

Quad AA battery pack
With a very simple modification it is possible to add the ability for my current lights to work from a battery pack. The advantage of doing this is that they can then also work when the bicycle is stationary.

We have chosen AA size NiCd and NiMH as the default rechargeable battery in our home as they are used in so many devices like Wii Motes, torches, etc. It just so happens that a set of four AA NiMH batteries has an open output voltage of around 5.6V. Under load, this drops to about 5.2V which is well within the specification for the lamps we have built. We can't connect a regulator in-line because the voltage drop introduced would be too great (and we are not using one for the lights anyway) but, a battery pack like this can be connected directly to our lights by using a two-way switch.

We have also tested the final lamps with Alkaline AA batteries and the voltages and currents are also within spec of the lamps as designed. These provide more light than when using rechargeable batteries.

Progress

Front Light (No. 1)

This is my first attempt at building an LED front light using a load of old white LEDs I had lying around. It uses 35 clear white LEDs, each one in series with a 91Ω resistor. You will have noticed that 35 × 20mA = 700mA, more than we said we could draw from the dynamo hub. The reason behind doing this was to test just how much power the dynamo can really supply.

The LEDs are first mounted into a circle of 1.5mm plastic sheet. The layout is captured in Powerpoint slide, which is stuck to the sheet and then used to mark out where to drill all the 5mm holes. Each hole is marked with a pin-point, then enlarged with a punch and drilled out using 1mm, then 3mm, then 5mm drill bits.

Before all the LEDs are mounted into this sheet, a 91Ω resistor is soldered to the positive pin of each one, as close as possible to the body. The LEDs are then pushed into the holes, starting with the inner ones and working out. For each circle of LEDs, the negative pin is orientated towards the outside. This means that a circle of wire can be run around the outside and then soldered to all the negative pins. don't forget to link each ring of wire, linking all the negative pins to one single point. When all of the LEDs have been inserted and the negative pins all connected up, the LEDs are then glued in place, to stop them falling out. The resistors are then all connected together to provide a single positive power connection point. The plan is to keep all of the resistors and wiring as low profile as possible.

Before mounting this array of LEDs into an enclosure it was tested with a 5V dc power supply.

Front light case + mount
For the enclosure we used the lid from a can of spray paint as it has a nice rounded shape, it strong black plastic and is the right diameter (~62mm in this case). We had to cut it down so that it was about 20mm deep and put a 5mm bolt through the back to act as a mounting point. We used stainless steel fittings that won't rust.

Front light painted
On this first lamp, we sealed the LEDs by painting the from of the lamp with the LEDs in place. This was not a good move as we got paint on the LEDs and then had to clean it off, once dry. It is far better to paint the front (if required) before the LEDs are inserted and to use the glue from behind to seal the front of the lamp.

As soon as I'd super-glued the front in place, I noticed a dodgy connection on one of the LEDs :-( Must be a dry solder joint.

Rear Light

I am only planning to do do one rear light! Based on my learning from building the above front light, I've adopted a slightly different strategy with the rear light. It doesn't need to light up the road, so the focus is more on making me more visible to other road users. The two main factors that improve visibility are brightness and the size of the surface area being lit up. Big and bright is good!

Rear light plate painted
The most efficient way to increase the lit surface area is to use reflected light, so my planned rear light is based upon a ring of 16 LEDs around, red reflector in the middle. This is the mounting plate made from 1.5mm thick aluminium sheet, drilled and painted red.

Rear light
This is the completed front part of the rear light with LEDs fixed in place and central reflector. This is mounted in the enclosure via the central bolt. The enclosure is another plastic lid which is readily available from supermarkets, so I've 'standardised' on this one for all my lights. It supports a larger, 65mm front plate, onto which the LEDs are mounted.

The LEDs used here are water-clear red LEDs, simply because they are brighter and look better. The colour of an LED depends on the frequency output by the emitter chip and not the plastic case.

The forward voltage on the red LEDs is lower, so with a 5V supply, two can be used in series with a low value resistor. The other approach I'm looking at it to use three in series, with no resistor. This improves efficiency but I need to test the voltage from the dynamo is not going to be too high.

One other thing I've learnt, is to make this front mounting plate removeable, so that I can fix things if they go wrong. This is achieved by using a 12mm hex spacer in the centre and a 3mm stainless-steel, button-head bolt to fix through the front and back.

Still constructing and writing this up. Before I wire up the LEDs and complete the light, I want to test the voltage from the dynamo, to see if I can wire them up without resistors.

Front Light (No. 2)

I learnt quite a lot from building the first front light so, I'm having a second go. The main things to improve were:

  • Change the way I solder the LEDs and resistors, so that it could fit in a more shallow enclosure. I'm aiming to use one that is less than 15mm deep this time.
  • To see if I can do away with the resistors completely. This involves testing the maximum voltage output from the dynamo and seeing if I can drive two or three LEDs in series.
  • I really like the way the light looks when viewed from distance. The large area filled with LEDs make it very visible. With this second attempt, I aim to make the lit surface area even bigger by removing some of the central rings of LEDs and adding a further outer one.
  • Like the above rear light, I plan to make the new front light in way that can be dismantled if required.

Front lamp mounting plate
For my final front light, I decided to avoid using resistors and to use a mixture of 5mm and 10mm white LEDs. This is the unpainted mounting plate.

The 5mm clear white LEDs typically operate at 20mA forward current, with the maximum value being 30mA. If you put two in series across a 6V dc supply, they use 12mA and are pretty bright. On this basis, I'm not using any resistors within my front light and have four central 5mm LEDs, in two pairs in series. This ring of LEDs draw 48mA in total at 6V.

Outside of these, I have a ring of eight clear white 10mm LEDs. These things are bright! At typical 100mA forward current they are rated at 285,000mCd output. My testing shows that two in series, across a 6V dc supply draw 55mA. With this current they are seriously bright. So this ring of 8 LEDs draw 220mA at 6V.

Outside of this ring, I then have another ring of 16 clear white 5mm LEDs, wired in series pairs. This ring of LEDs draws 106mA at 6V dc.

If you add up the three rings above, you get 374mA current draw from the front light at 6V but the LEDs are rated such that they can double this. With the rear lamp drawing another 120mA at 6V, the total comes to 494mA, almost exactly the rated 500mA of the dynamo. My tests have shown that as you draw more current from the dynamo, its output voltage drops, so the configuration is effectively self regulating.

Front lamp
This is the final front light.

Housing

iPhone holder
Because my lamps are just lamps, I need somewhere to put the rectifier, regulator(s) and switches. I'm currently building something that mounts on the handlebars of my commute bike and also provides a mounting point for my iPhone 4. This is fixed to the handle bars using clips. This is the cardboard template.

The standard method I've used for mounting my iPhone (and many other items) on bicycles and in my kit car is based upon using lots of Velcro! I've got a cheap iPhone rubber case with the back covered in the soft 'loop' part of the Velcro. The other 'hook' half of the Velcro is stuck to any flat surface I want to mount my iPhone on.

Lip on which front light will be mounted
Whilst I was designing this bit, it also occurred to me to use it as a mounting bracket for the front light, so I've extended the front lip to enable this. This also raises the front light up, to improve visibility and simplifies the wiring required.

Folded aluminium bracket
Using the above cardboard templates, I cut out the right right shape from 1.5mm thick aluminum plate and bent it over a 25mm rod, to give a nice smooth finish.

The spare brake cable you can see is because I'm also converting my front rim brake to a disc brake.

Folded aluminium housing an iPhone mount
This is the painted housing with switches mounted and Velcro for the iPhone.

Finished
This is the final front light.

My testing has shown that the beam output from this light is a symetrical cone shape, with a main beam angle of about 30º. Outside of this concentrated beam, there is quite a lot of light provided within a secondary beam over 120º angle. Whilst this symetry is not optimal for a bicycle light (a lot of light is wasted upwards and I can light up bridges and signs overhead), the level of light output is huge. Reflective signs over ½ mile a way can be lit up and the road ahead is also well lit up for about 20m. The light beam is also wide enough for off-road use. I've seen brighter pin-point lights on bicycles but, I've not seen a light that also provides wide coverage like this.

Switches

Switches
There is a one on-off-on single-pole switch, to switch power to the lights or the USB socket, with centre being neither. There is also and on-on single-pole switch, to switch the lights between dynamo and battery power. The plan was to use standard 10A toggle switches with 6mm spade terminals but, these are a bit too big, so I'm using mini-toggle switches (rated at 5A). These are Maplin part no.s FH00A and FH10B and are show here with the additional rubber boots to make them fully waterproof.

Conclusions

  • The ac output frequency causes LED lighting to flicker at low speeds but, anything much above walking speed and this is no longer noticeable. On this basis, I'm not bothering with a capacitor in my circuit.
  • Full front light output is acheived from quite low speeds (<7 mph).
  • The SON28 dynamo hub seems to be voltage self-regulating when a 3W (6V @ 500mA) load is connected. This means that you can safely connect LEDs in series if their forward voltage is high enough.
  • The light output from my front light is huge!

Updates

January 2011

I've been using these lights for my daily commute for many weeks now and they are working brilliantly. I tend to leave them on during the day (it's still fairly dark cycling to work in the winter mornings) as there is no noticeable extra effort required to cycle and they improve my visibility a lot. I've had a lot of positive comments about the size and brightness of the front light.

February 2011

GT Avalanche 2 mountain bike
Bought myself a new bike, an Avalance GT 2 mountain bike. Because this has big, chunky off-road tyres on it, I've had a new rear wheel made up to match my front wheel with the SON dynamo hub. These are now my road wheels and have road tyres (Schwalbe Silento II) on them. Because the handlerbar design is different, I'm no longer using the aluminium plate for mounting all the parts. I've simplified things and I'm now using a front light that is mounted in isolation.

The iPhone recharging module is now smaller and can be easily detached. The iPhone mount is also separate now. This has made things lighter and is a more flexible solution overall. Pictures to follow very soon ...

GT Avalanche 2 mountain bike with front light
This is what the front light looks like now.

GT Avalanche 2 mountain bike with rear light
This is what the rear light looks like now.

April 2011

iPhone 4 mount
Took a more simply approach to mounting the iPhone 4 on my bike. Using a simple and light-weight bracket fixed using the bolts that clamp the handbars. This provides a very stable and flat plate, which is covered in 3M Velcro. This stuff is amazing! Once stuck down it won't come off and the grip between the Velcro halves is incredible too. I like this appraoch because I could basically mount anything on this plate.

iPhone 4 mounted
To hold the iPhone secure 4, I use my old silicon case with 3M Velcro stuck to the back face. This is the approach and case I also use in my kitcar.

November 2012

With winter arriving, I've been using the lights daily. I tend to use them on the cycle in to work in the morning regardless of the weather. With the low sun and dappled shade, they greatly improve my visibility to others. The hub dynamo is so efficient, that I can't feel any extra resistance, with the lights switched on. These lights have also been 100% reliable since I built them and I've yet to see a brighter front bicycle light in my travels.

I've also bought an iPhone 5, which has required a new charging lead to be used on my bike. Unfortunately, there are no 15cm long lighning charging leads yet :-(

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