Here is how to add safety LEDs to your bike helmet using conductive paint. I was always losing those lights you clip on to your bike, and they're not cheap or good quality.

Materials:
conductive paint
conductive epoxy
5-minute epoxy (hot glue for grownups) or hot glue and gun
wooden sticks or plastic coffee stirrers for mixing and applying epoxies
9-volt battery
magnetic reed switch
magnet(ring-shaped for tethering)
LEDs(white and red if law in your state mandates, or your choice)
clear spray enamel(for coating circuit to protect from rain-shorts)
wire
solder
moist paper towel
bike helmet

Tools:
pencil
soldering iron
wire cutters/strippers
needlenose pliers
masking tape
paint brush
fine grit sandpaper or sanding block
solderless breadboard for prototyping (optional)

Flickr set available.

Project home page

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Step 1: Prep Your Materials

Prototype your circuit:
Test your LEDs to make sure they work, and work out what your circuit will look like. Since I'm using a 9v battery, I'm using three white LEDs for the front in series wired with four yellow in-series LEDs, and the two sets are wired in parallel (see circuit diagram). See what works best for you, and try it out on the breadboard before you commit to the design on your helmet.

Prep the helmet:
Lightly sand the surface to allow the paint to bond more easily. I also took this opportunity to sand off brand logos from my helmet. Follow the sanding with a thorough wipedown with a damp paper towel or cloth. I'd avoid using any cleaner here, since who knows what chemicals are in it and what they might do to your circuit. Water does just fine. Be sure the helmet is completely dry before doing any painting.

Step 2: Draw and Tape the Circuit

Using a pencil, draw your circuit directly on the helmet. Hold components (LEDs, battery) right up to it and trace around them. Mark positives and negatives for current direction. Using masking tape, mask out the area that won't get painted. Use your fingernail to really get a tight seal near where the paint will go.

Step 3: Paint the Conductive Traces

Pretty self explanatory:
Paint inside the stencil you've just made. Make sure to stir the paint thoroughly and often, as the particles have a tendency to settle. I applied two coats. Refer to manufacturer's instructions for optimal paint thickness.

Touch ups:
Let it dry a little, but not all the way, and peel the tape off carefully. I took the tape off before the paint was dry to make sure none had bled under the paint, shorting the circuit. I touched up any bleeds with my damp paper towel.

Let the paint dry for at least two hours before attaching components. My paint's instructions said that it reaches full dryness (and therefore highest conductivity) after 24 hours.

Step 4: Prepare Components

Solder leads from your battery. I already had a wired-up magnetic reed switch, but you really could use whatever switch you want. I chose not to use a toggle, as it would be difficult to waterproof. I solderd one regular wire and the wired switch to my battery since my switch was already wired, but you could just as easily work it into your conductive trace circuit.

Lay out your LEDs and trim their leads (but make sure you keep track of the cathode and anode orientation). Mix some 5-minute epoxy and affix the battery and switch to the helmet. Allow to dry. position wire ends on paint traces and lay out ALL LEDs before mixing the conductive epoxy (so as not to waste time once its mixed)

Step 5: Affix LEDs and Wires With Conductive Epoxy

Mix the conductive epoxy and attach LEDs and wires to the paint traces. Since the helmet is not a flat surface and I wanted to attach everything at once, I used a large piece of tape to gently provide support to the LEDs while the epoxy dried. Once it's dry (or mostly dry), test your circuit to make sure the LEDs light up. If they don't, you may just need to let it dry more. Let everything dry for 24 hours, then use some more 5-minute epoxy to reinforce the conductive epoxy and seal around the bottoms of the LEDs (for waterproofing and extra strength).

Step 6: Seal and Wear With Pride!

Take your helmet outside or to a well-ventilated area, and spray the outside with clear spray enamel. Tape off areas you don't want to cover like the strap and bulbs of the LEDs. This will protect your paint traces from wear and water.

I affixed my magnet to the strap with a cord, so it's always with the helmet. I simply reach up and touch it to the switch to turn it on!

Design note: I avoided placing traces or components on the top of the helmet, as that is the surface on which it rests when the helmet's not on my head, and I didn't want to risk scratching up my circuit or damaging the components.

Flickr set available.

Project home page on Sternlab.