BBB (Bothersome Blinker for Bikes)

In my city, as I guess in many other, you'll always have to do your best to stay alive when you ride your bike, this is particularly true by night. This is the reason which brought me to design this circuit.
I anticipate worried drivers (I should be the first to complain) saying that my bike light hadn't become a stroboscopic floodlight which makes you blind, only catch your attention a little more than before, it will be more as looking at an old very used neon light.

Step 1: The Frequency

This blinker circuit runs at about 50 Hz, this means that it's very near to the frequency at which our eyes work. So you'll see an "almost" fixed light, the difference with a fixed light is that it's a little bothersome to see, and it keeps your attention at once. Indeed old TVs and PC monitors worked a few hertz more than this frequency, and after some hours in front of those, your eyes were tired. To work long periods in front of your monitor it's better this is 60 Hz or more (more or equal to 85 Hz with old crt monitors, LCD are less stressful).
This circuit is intended to work with any LED light with a power source of about 4.5-6V. If you have different voltage you have to review the design adding a voltage regulator (or booster) for the chip.

Step 2: The Simulation

Look at the working simulation of this circuit with Falstad applet. You see how changing potentiometer value you'll change behavior of LEDs.
Note that this circuit was not born to vary luminosity and consumption of your bike lights, although at the far scale of potentiometer it does also this function. Rather I tried to keep the more power I could, I designed this to make my light blinking at a particular frequency which makes your bike more visible. The potentiometer is to regulate this behavior from nothing to full visual "bother".
Here we can choose which potentiometer suits more on our desires. Indeed we can make this circuit blinking more effectively our lights choosing an higher potentiometer's max value. Look at the next table to choose that, I recommend 5 or 10 Kohm.

• 1 Kohm: min. frequency ~45Hz, only bothersome or fully light behavior
• 5 Kohm: min. frequency ~10Hz, a little more fun at low scale, then bothersome and full power
• 10 Kohm: min. frequency ~6Hz, a fast blink at low scale
• 20 Kohm: min. frequency ~3Hz, real blink at low scale, about three flashes every second

Varying the potentiometer you'll change frequency from minimum to about 120 Hz, when you have almost full power on your light. This means that at low value you have a 55-60% duty cycle, and turning the potentiometer you'll note a faster blinker, passing through bother, and at far end an increase of the light power, because the duty cycle change to 95% of efficiency.
Look also the trace left by a LED meanwhile I varied the pot.

Step 3: Choosing the Case

The cicuit box could be the same you use for batteries, because this circuit is very compact, and doesn't need a heat sink. Otherwise if your bike light already has batteries, you could find a nice little box at your choice. Based on it's measures go ahead to design your circuit. For my circuit I've found one of these transparent boxes from Muji. The shop has various sizes of these containers, and I've chosen a model which is the right size to keep 4 AA batteries inside, plus a little rectangular circuit.

Step 4: And Setting It Up

Unfortunately it has a fixed divider which I had to remove, together with the guides for the removable dividers. Anyway if you have right tools it's very fast. I used an utility knife and a chisel.

Step 5: The Pcb Design

In Fritzing you can design your circuit and pcb. This is a simple program, but you can choose to go through other more professional ones, as Eagle o DipTrace which lets you making a better customization. Note that this circuit is so simple that you could use a prototype board and with no effort solder your components and connect them with jumpers.
Another note, the circuit here is a little different from the one I etched, because after finished my project I've decided to add a switch and also some copper tracks to keep the possibility to use a 5 pin stereo potentiometer instead of the ordinary 3 pin one. These potentiometers were diffused in old music devices as walkman and transistor radio, so I've some of them. If contacts are not right I left some space to cut a trace and add a jumper on the right pin. My potentiometer works in this way, but your could be different.
I also created a page with some schemes and the components positions, so you can print it to etch the pcb.

For this circuit you'll need the following components.
Resistors:
5 Kohm (or different value) variable resistor (look schematic for a typical stereo 5 pins potentiometer)
47 ohm
1 kohm
220 ohm
Capacitors:
10 nF ceramic
25V (or 16V) 10 uF electrolytic
And also:
a switch
an IC NE555
an NPN transistor PN2222A

Step 6: [UPDATE] Adding a Cool Button Switch

[UPDATE]
I've redesigned the circuit in DipTrace, as you see in the image. The new circuit includes a button switch to turn on and off the lights. This switch is based on the cool tutorial "World’s Simplest Soft Latching Power Switch Circuit" by Dave. Sorry for the mess of traces on the board, placement could be improved. I upload the DipTrace file and the .pdf I've created with schematics to be printed. See the pdf for values and details of components. Note that this board needs a little better skill about soldering, because the traces are very thin. 

Step 7: Toner Transfer Method

Indeed now that you have your schematic, you need to etch your circuit. To do that I recommend to follow some good instructable which describes more accurately every step. There is a guide to choose the right instructable: pcb-methodes.
Anyway I'll show you rapidly the full process, just keep in mind I'm not a prof.
I would transfer my trace on copper board with toner transfer method. If you're lucky enough to find the right paper, you'll obtain a good results. Anyway this method is not good for pcb with very thin traces, but it's handy for this instructable.
To know if your paper is right you have to try it. They also sell some dedicate paper which should be perfect, but if you have to buy it maybe it's better you try with some magazines.
Place the print with toner against copper and keep it in position with two little tape pieces. With iron head pass on the back of the paper so that the toner acts as glue, then remove tape which otherwise should damage your iron surface. Now press full suface hardly with iron at max temperature, wait 2 or 3 seconds, and make it again, trying not to burn your cotton ironing board, or anything you've used as base.

Step 8: Removing the Paper

Now let everything cool down then, paying attention to not blend the paper sheet (which otherwise comes off taking toner with him), put everything in cold water and let it get soften. After some time, maybe 10-15 minutes, you could try to separate paper from copper. Toner (with some paper scraps) should stay on the board, and you could patiently remove last paper scraps with a sponge or scrubbing them with your fingertips.
Your copper board is ready for etching.

Step 9: Fixing the Traces

In case of this circuit I had to use an ordinary paper sheet, and the traces are not good, because paper has some filaments which remain attached to the toner and then they'll damage the border of traces. Anyway I've checked every track and I've seen some of them which were too thin, so I've filled these lines with a permanent pen.

Step 10: Etching

Etching using ferric chloride is very simple, shake every some minutes your plastic container with board inside to remix the fluid and accelerate the process. I've made some plastic supports to keep the pcb upside down and reduce the needed amount of ferric chloride to cover all the copper. Also with them you can handle and shake your pcb without touch the acid, which should colour your fingers.

Step 11: Drill Holes

When you've finished etching your pcb clean everything with kitchen paper and then with lot of water, then drill the holes.
If you see that traces are not perfect you have to check them with multimeter. In my case everything works good in both boards, also if they're ugly! 
Now your pcb is ready to take components! Remember the trick to solder easily, heat the surfaces, not the tin!

Step 12: Insert and Connect

Now that every component is soldered in place you can insert the circuit in the box and make the last measurements. I've cut a slit into the side of the container to reach the potentiometer, maybe you'll need to cut an aperture on top to reach the switch, or simply you could leave it inside, indeed this box has a practical clip cover. Then I glued the pcb with a thick double side tape, and the batteries holder with a thin one (depending on the space available).

Step 13: Happy (and Safe) Ride!

All right, add batteries and connections to see how your BBB works!
After connected the light, moving it fast you should see the flashing in a row, as short lines separated one from the others. Turning the potentiometer length and distance between lines should change, until the lines are very near themselves (full power, no bother).
[UPDATE]
The pictures are of the first version, but pcb schematics are updated with the soft-power switch version, I just realized that it still misses an additional LED on the board, which would show the state of the circuit when it's not connected to the lights.