Bike Taillight With a Twist




About: Slightly Dorky High Nerd - You might find some of my stuff on Tindie.

Let's face it. Taillights are boring.

At best they go 'blink blink - look at me! I'm blinking - woohoo' all the time. And they're always red. Very creative. We can do better than that, maybe not much, but still better than just 'blink blink'. I was riding my bike during new year celebrations and people liked it, and not all of them were drunk ;-)

The rest is pretty straight forward: 2x AA cells, boost converter for 5V, some RGB LEDs, the obligatory micro controller, custom printed circuit boards from BatchPCB, perfboard and the usual soldering gear.

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Step 1: Main Schematic

Really nothing special. If you know how to wire up an AVR chip on a breadboard or an Arduino on a breadboard, if you like that better, you won't have any problems with this one. I used KICAD for designing the schematic and the printed circuit boards. KICAD is open source and as opposed to eagle, which has a free (as in free of charge) version as well, there are absolutely no limitations in the size of boards you can make. You too get gerber files that work with any fab house you want. E.g. BatchPCB had no problems with them.

In the schematic you'll just find the cpu, the LEDs, a few resistors and capacitors. That's all. There's a few headers too. The boards have an ICSP header for flashing a bootloader and a 6pin header for convenient serial upload. The last 2 headers are mirrored and contain power, I2C and two more GPIO/ADC pins.

3 GPIO pins with 3 current limiting resistors are used to supply current to all 8 anodes of a single color. Individual LEDs are turned on or off using 8 GPIO pins to drive the cathodes. Depending on the type of operation the LEDs are either multiplexed (PWM for more colors) or fully on (higher brightness).

Some info on the packages I used for this board:

- ATmega168-20AU: TQFP32 SMD
- LED: PLCC6 5050 SMD
- Resistors: 0805 SMD
- Capacitors: 0805 SMD, 1206 SMD

Step 2: Dealing With the LEDs

I won't go into great detail here, as this has been covered elsewhere numerous times. You just have to make sure you don't exceed the micro controller's maximum output current per pin (about 35mA or so for AVRs). The same is true for the LEDs current. As you can guess from the picture, I used one of the SMD LEDs to figure out the resistor ratio to get well balanced white light. There are three 2k something potentiometers on the other side. That's all. In this case I ended up with resistors ranging from 90 to 110Ω, but that depends on the kind of LED you get. Just use a standard multimeter to determine the LED's forward voltages V_led and you're in business.

Using Ohm's Law, you can calculate the values for current limiting resistors for small LEDs like so:

R = ( V_bat - V_led ) / I_led

I_led should not exceed any current limit of the parts you use. Also this approach is only good for low current applications (maybe up to 100mA) and should not be used for Luxeon or CREE LEDs! The current through LEDs is temperature dependent and a constant current driver should be used.

If you need more info on that topic, wikipedia will have some information. Searching for electrical conductivity of semiconductors (low/high doping etc.) or negative temperature coefficient may be helpful.

I've used 6pin SMD RGB LEDs without commond anything. If you google for them, you'll get many results. The magic words are "SMD, RGB, LED, PLCC6 5050". 5050 are metric dimension for x and y in units of 0.1mm. On ebay you'll find them as well for as low as 50¢ per piece for high volume orders. Packs of 10 currently sell for about 10 bucks. I'd get at least 50 ;-)

Step 3: Backplane & Power Source

The backplane provides power and a common I2C bus to both boards. Each board has 8 RGB LEDs and an ATmega168 mcu running with its internal oscillator at 8MHz. The latter requires synchronization between the boards and/or recalibration of the oscillators. This issue will show up in the code section again.

The schematic for the 5V boost converter was taken from the Maxim MAX756's datasheet without any modification. You can use any other chip you find suitable that can provide about 200mA at 5V. Just make sure the external part count is low. Typically you'll need at least 2 electrolytic capacitors, a Schottky diode and an inductor. The reference design in the datasheet has all the numbers.

I used high quality FR4 (fiber glass) boards for this job. The cheaper rosin based boards may work as well, but they break too easily. I don't want the boards to disintegrate on a bumpy ride.

If you already own a 'MintyBoost', you can use that as well if you can make it fit on your bike.

Step 4: You've Got to Have Some Code!

In high brightness mode the board supports 6 different colors + white. The color is chosen by setting 3 GPIO pins to high or low. That way all eight LEDs can be fully on, but only show the same color.

In PWM mode the color is set by applying a pulse width modulated signal the the 3 GPIO pins and multiplexing the 8 LEDs. This reduces overall brightness, but now individual color control is possible. This is done in the background by an interrupt routine. Basic functions are available for setting the LEDs a certain color value, either using an RGB triplet or a HUE value.

The device is programmed in C using the Arduino IDE for convenience. I've attached the current code I use. Up to date versions are available on my blog. You can browse the GIT repository using the gitweb interface. Many stupid programming mistakes will show up, I'm sure of it ;-)

The second figure illustrates PWM generation. A hardware counter counts from BOTTOM to TOP. Once the counter is larger than a certain number representing a desired color, the output is toggled. Once the counter has reached its TOP value, everything is reset. The perceived brightness of the LED is somewhat proportional to the on-time of the signal. Strictly speaking that's a lie, but easier to understand.

Step 5: See It in Action

Just some preliminary tests. Yes it can do full RGB colors too ;-)

Real world testing. Yes we had some snow, but that was before xmas. Now we have some snow again. But, as usual, during the xmas holidays and new year celebrations all we had was rain. Please ignore me groaning at about mid-video, I'm getting old so squatting gets a bit hard.

Finally some slightly improved effects.

Mission accomplished. Geeky taillights, and illegal where I live too ;-) I'm pretty sure I won't be ignored by sleepy or ignorant motorists anymore. By tuning the timings a bit, you can create pretty annoying effects that are good eye catchers. Especially at night.

As there are 4 GPIO/ADC pins on the boards (2 may be used for building a small I2C network), it should be easy to hook up a push button to trigger all sorts of effects. Hooking up a CdSe photo resistor would work as well.

Total material cost is about 50$. The biggest chunk went into the printed circuit boards. Low volume order penalty as usual.

In analogy to a once widely spread TV commercial for a cell phone company in the US, let me ask you this:

"Can you SEE me now? - Good."

Step 6: Updated Design

I've changed a few things here and there.

Most notably is the addition of a low drop voltage regulator. Now the board can run with anything from 4 to 14V DC. I've also changed the PCB color to yellow and added jumpers to disable auto reset and to bypass the voltage regulator if it's not needed.

Demo code for grabs and assembly instructions. You'll find KiCAD files and a schematic there as well. In case you want one, you can find more info on my blog.

Step 7: Supersized

Next thing on the list: Tic Tac Toe

Step 8: More Light Hack

By adding 3 wires and 3 more resistors the brightness can be doubled. Now two GPIO pins per color are used for sourcing current.

Step 9: More Updates

So I've finally switched from 'dumb' interrupt driven PWM to BCM (Binary Code Modulation). This drastically cuts down the cpu time spent twiddling the LED pins and increases brightness quite a lot. The all improved code can be found on github.

The first few seconds of the video show the improvement in the left board.

Until the next hardware revision of this board is out (waiting for the boards to arrive), this will feed the need for 'more light' a bit. Looking at the new boards running full blast will be painful.

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    42 Discussions

    This is a great design! I've been looking for a good design for bicycle tail, brake and turn lights. I may borrow your idea if it's cool with you. Thanks for posting this great work!

    1 reply

    8 years ago on Introduction

    Great 'structable - thanks!

    Regarding the Legal vs. Illegal debate: Car versus Car is a fair fight. Car versus Bike is not. Without drinking a drop, a large percentage of drivers currently drive "impaired" - texting, talking on cell phones, Googling movie start times on their iPhones, trying to fish a Big Mac out of a bag on the passenger seat, etc.

    When I questioned a Belgian friend of mine living in New York about the massive dual headlight setup on his bike, he said "Drivers don't think about bikes, and consequently end up hitting bicyclists, because they're not perceived as dangerous. When they see me coming, I want them to believe they're looking at the landing lights of a Boeing 747.  I want them to FEAR me."

    When sharing the road with cars, bicyclists NEED to be noticed to stay alive.  If the choice comes down to "living within the law" and "staying alive", I'll always choose the latter.  And if the law is forcing you to make that choice, the law needs to be changed.


    1 reply

    Reply 8 years ago on Introduction

    Yes, I know exactly the type of drivers you're talking about. Sometimes I can literally feel their evil stare at my neck because I dare cycle as fast as them and they can't overtake me easily (speed limit) ;-) At the next best opportunity I can hear them rev up the engine and 'restore' the 'proper world order' by letting me inhale the fumes while admiring their taillights.

    The more light the better... maybe not blindingly bright, but quite intense nevertheless.


    9 years ago on Introduction

    >> BTW, it's the same thing as staring into rear fog lights. You get blinded as well.

    Like driving with rear fog lights on isn't illegal too in every civilized country. :^)

    Anyway, since you haven't updated this in ~7mo, you're either dead (bummer!) or distracted, hopefully with interesting things. But if you do get around to that next prototype, just wanted to say, we're still tagged up and lurking.

    Who knows? Maybe take a pin off an AVR and interface that mag sensor from your cyclocomputer so that when the bike is moving, it makes all LEDs a boring, ISO-certified vehicle tail light red? Then instead of getting schooled on worldwide laws regarding bike lights the comments would be just about praising your geekness. (shh! keep the secret override switch wiring off the instructable.)

    1 reply

    Reply 9 years ago on Introduction

    Civilized... hrmmm... doesn't exist ;-)

    And yes, I'm forced to devote most of my time to other unpleasant things.


    9 years ago on Step 5

    dude i've been staring at the third video for so long now i'm so mesmerized lol 

    3 replies
    madwormkee gee

    Reply 9 years ago on Introduction

    ...don't watch (stare) at the taillights...

    BTW, it's the same thing as staring into rear fog lights. You get blinded as well.


    9 years ago on Introduction

    Tail lights are red for a variety of reasons.  one of which is the red indicates that they are tail lights.  Headlights are white.  With white tail lights  you are going to have confused drivers thinking you going opposite directions.  It will also make it difficult for drivers to see other things around you (not behind you.)  In most places white headlights and red tail lights are required by law.   If there is ever an incident between your bicycle and a car  you will probably be found at fault for having misleading lights.
    Tail lights are not meant to be exiting, they are meant to be safe.  For that matter, the same could be said for anything related to driving or riding in traffic.
    You have a nice set up with the lights, but you need to take legal codes into consideration.
    I am also wondering how well that rides in the snow?  Have you found a way to keep it from slipping?

    1 reply

    Reply 9 years ago on Introduction

    First, just let me say that this taillight is not necessarily white. It just so happens to be white in this picture. I can't say how often I've been told that "You can't have a white..." already :-)

    If you want, just pretend you're looking at the front of the bike. But please don't now tell me that it can't be red. As this thing can display many colors, people can choose any permissible color they like.

    Not slipping in the snow is tricky. The best thing I know of is using off-road tires with metal spikes. But that's only worth it if you've got snow all year I guess. It also doesn't help if thick layers of snow are covered with a thin sheet of ice. As I hate changing tires, I stick with semi off-road tires all year and switch to biped mode if it gets too dangerous.


    9 years ago on Step 5

    hey whats up im really intrested in science and building stuff but i don't have access to all of the right materials cause im just a child and i spent all my money on christmas stuff but i really like your creation and would like on ... do you know of any where i can get it with alll the settings and stuff ordered to me on the cheap?

    1 reply

    Reply 9 years ago on Introduction

    Hmmm. Fully assembled, small quantity and cheap. That's a tricky one. Unless you have these things made in quantities (100s and up), there's no way I know of to make it any cheaper. You could have the prototype if you want it badly, but you're not going to like the price tag on that one. It might not even fit your bike.

    I'm thinking of having a small series of the circuit boards made, but that will take some more time. The next prototype is still a several weeks away.


    9 years ago on Introduction

    The color issues seems to be well covered in the comments so I'll pass on that specifically. 

    THIS IS GREAT!  Good job!!  I like the rotating ZOWIE look as an attention grabber.  I was sort of yawning until that came on.  Could make those counter rotating and then reversing?  Are there other patterns available? 

    If you did away with the multi colors, would that simplify the electronics? 

    4 replies

    Reply 9 years ago on Introduction

    YEAH! This is a good regular back-light! :D I want one for my jackets and two for the shoes!!! Stunning! :D


    Reply 9 years ago on Introduction

    I'm working on a new prototype right now. It will have a voltage regulator built in, so it can run with anything in the range of about 3V to 14V. That should make it a bit easier to use it.


    Reply 9 years ago on Introduction

    Oh, and I forgot: both LED rings could be driven by just a single micro in single color mode. That makes about -3$ in part cost. But you'd need 8 more wires on the perfboard... I'm not sure if I'd be willing to spend more time with soldering just to save 3 bucks. I like my boards neat and clean and having to many wires mushroom on there gives me a headache. YMMV.

    In terms of programming using just a single micro would make things easier. No worries about drifting clock frequencies and the patterns going out of sync. The drifting is due to the fact that I didn't use a crystal (yes, I was lazy and didn't want to route a complex board), but the internal RC oscillators. They are only good to within 10% or so without recalibrating them. And that needs more gear, frequency counter or even better a scope.