A low profile neat way of adding lights to your bike helmet without damaging the helmet or permanently attaching anything to it. The mod uses a universal phone charger as a compact power supply that uses a single AA battery.

You will need:

Helmet
2 x 10mm 0.5W white LEDs
2 x 5mm high brightness red LEDs
3 x resistors (about 12 to 20Ohm)
Solid core wire
Heat shrink: various small sizes
1 x 5g pack of sugru
Polymorph
Switch
1AA type universal phone charger
Small piece of thin ply wood or tough plastic.
Total probably comes in under £10.

Tools:
Basic woodwork
Basic soldering / electronics tools

Step 1: The Circuit

Power: I chose to use a AA powered phone charger, it is a small and neat case which conveniently boosts the voltage of the AA cell (1.5V) to about 5V which is suitable for powering the LEDs. I have seen the phone chargers for between £3 and £5 in several shops (Tesco and Maplin).

Phone charger provides roughly 5v but when the LEDs drew current I found the voltage dropped a bit. To get the correct currents through the LEDs three 12 Ohm resistors worked well for me. Resistor values between 12 and 20 Ohm are likely to be suitable if you are using 0.5W white LEDs, but it is worth checking the currents with a multimeter if you can.

The full circuit is shown in the diagram provided.

Step 2: Assembly

Attach long lengths of single core wire to all the LEDs

Put heat shrink around one of the LED legs to stop the leads shorting if they are pressed together.

Heat shrink over both LED legs to keep things tidy.

Thread the wire through the hole on the helmet you want the LED in.

Test out the LED to make sure it is pointing in a sensible direction and that it is not obscured by the visor.

When you are happy with the LED placement wrap the LED wires around the plastic strapping inside the helmet to hold the LED in place. Make sure the wires are well flattened down and that the helmet is still comfortable to ware.

Once all the LEDs are attached cut the wires to length solder on the switch, resistors and battery pack connector. I encased the switch and resistors in Sugru to keep it protected and neat.

Step 3: Battery Holder

To make a bracket to hold the batter pack I made a back plate out of a small bit of plywood (painted black). In the back plate I drilled 6 holes: 2 large holes for attaching polymorph to and 4 small holes for threading wire through so I could tie the plate to the helmet strapping.

Polymorph clip: Polymorph is a thermoplastic that softens at low temperature and is great for making custom plastic parts. To get good results with polymorph takes a bit of practice but it is reusable so make some test parts before any ambitious builds.
To make the clip mould the plastic around the batter pack and push it onto the back plate, a bit of plastic should go through the large holes drilled in the plate, if you flatten the polymorph out where it has gone through the holes it will hold the polymorph to the plate (like riveting).

To make the clip mould the plastic around the batter pack and push it onto the back plate, a bit of plastic should go through the large holes drilled in the plate, if you flatten the polymorph out where it has gone through the holes it will hold the polymorph to the plate (like riveting).

The bracket was attached to the helmet by tying it on with small bits of wire wrapped around the plastic neck strap at the rear of the helmet.