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Rechargeable Helmet Light (electronics)

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Picture of 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
 
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Step 1: Circuit Design

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  A slightly outdated version of the design (using different transistors) is included in the DCH file.
anonymouse1979 months ago
Fantastic, and you've made it onto LifeHacker!

http://lifehacker.com/build-your-own-rechargeable-bike-helmet-light-812488040
tumidaj9 months ago
Thank you ! It's amazing
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