Rechargeable Helmet Light (electronics)

I was tired of having clip-on bike lights stolen and decided to build a set of head and tail lights into my helmet.  The final helmet has bright white LED headlights, flashing LED taillights and runs off a rechargeable Li-Po battery.   For an extra challenge I decided to do the switching and flashing entirely with analog components and use SMD electronics where possible for a smaller form factor.

I'll describe the process by which I designed and built the electronics.  The plastic enclosure I built wasn't anything special and I would redesign it if I repeated the project.  Schematics, layout and SPICE simulation are in the attached ZIP file.

Parts are detailed in the schematic in the next step.

Tools & Supplies:
- Laser cutter (at the TechShop)
- Soldering iron
- Tweezers
- Helping hands
- Toaster oven
- Spackle knife
- Painters tape
- Fine (<#70) drill bits, chuck and drill
- Dremel (or other tool capable of cutting PCB)
- Wire snips
- Needle nosed pliers
- Black spray paint
- PCB etchant
- Solder paste
- Electronics solder
- Li-Po charger

The circuit was designed to accomplish a few things:
- Support on/off toggle via a single button
- Flash the taillights
- Allow battery charging in-circuit without damaging other components

Toggle is handled by a 3 transistor latch controlling an NFET.  It is based on this EDN article.  Note I used the NFET since I had it lying around, you could just as easily used another BJT.  Flashing is handled by a simple 2 transistor astable multivibrator.  You can find other examples here and here.  Neither circuit is sensitive to in-circuit charging.

I simulated the circuit using LTspice to adjust the values for the various components.  I would recommend adjusting the multivibrator values a bit to slow the flashing rate.  The switch might also be more responsive with a smaller capacitor in the latch section.

The final design is in the LTspice ASC file.  A slightly outdated version of the design (using different transistors) is included in the DCH file.

I did the PCB layout manually using DipTrace (see DIP file).  Note that instead of using two layers I used 24 gauge wire in a few places to join on the back plane.  I followed this Instructable for masking and etching the PCB using a laser cutter at the TechShop.  I used ferric chloride as my etchant -- be careful with etching chemicals!

I also used the laser cutter to make a solder paste mask for my SMD components.  I used the paste image (attached) to etch a sheet of thin clear mylar from TAP plastics.  The result was imperfect due to some melting but acceptable.  Unfortunately I wasn't able to get a good photo of the clear mylar stencil.

NOTE: Sending out for professional PCBs and stencils isn't particularly expensive and would save a lot of time / increase quality.

I attached the SMD components using reflow in my home toaster oven.  The Sparkfun tutorials on stenciling and reflow were invaluable here.  I taped my stencil to the PCB and used a spackle knife to spread solder paste evenly over the surface.  I then removed the stencil an placed the SMD components using tweezers.  Finally, I reflowed the board in my toaster oven.  It's worth testing the joints for continuity before continuing.

Next I attached the larger components (taillights, switch, jumper wires) to the board.  A decent soldering iron goes a long way to making this process easier.

Finally, I attached the off board components (headlights and battery) and fired it up!

I won't cover the case in detail since the design was relatively poor.  It consisted of glued together acrylic that was ugly and impossible to service.  Hopefully this can serve as a jumping off point for something much better!