Trickle Charging Auto-switching LED Helmet

I was inspired by calebjc's green helmet to build my own. I suspect the parts on mine were easier to come by (bike store and wherever solar powered yard lights are on sale). Another addition on mine is that closing the buckle switches on the helmet.

I needed to attach a block to the bottom front of my headlight to raise it, on first draft it lit right in front of the front tire unless I looked at the sky. I had checked the aim at home, but had not thought about how your head is held differently on a bike with relatively low handle bars.

- front and back bike lights
you may already have some, if so, use them. If you don't have any, choose some that appear "modifiable", e.g. screwed shut.
for simplicity, make sure both use the same number of batteries. Mine both use 2 "AA"s

- 4 solar panels (2 might do)
I got mine from cheap-o yard lights from a sale at Canadian Tire, ~3$ a pop. You might find some in the garbage, some people throw them out when the rechargeables die or after the get knocked over and break

- 1 diode
scavenged from the yard lights as well

- wires
I used some old phone wires, small guage will be fine

- double-sided tape
I also tried zip-ties but the tape seems to work better

- metal duct tape
not the plastic stuff... this will be used for circuits. Other thin metal sheets glued on would work too.

To be able to use the buckle switch (explained later) you will need to be able to switch your lights from a separate switch. To use the solar panels you will need to be able to access the "+" and "-" of the batteries.
This means you need to add a "+", a "-" and a "switch" terminal to the lights' circuits and wires to those to come out of the light's enclosure.
I don't have very good pictures of this, but it would depend on your light anyway.
Generally you can just attach the "-" wire in somewhere near the "-" of the battery compartment. The "+" and the "switch" are a bit more tricky, since you actually need to interrupt the circuit inside the light.
For the front light, I ended up making a disk with metal duct tape cut into appropriate shapes on each side. One side lines up with the battery terminals, and the other with the contacts on the lamp part. The "-" side goes around the disk to connect to both the lamp and the battery side. The "+" side lines up with the "+" of the battery. The "switch" lines up with the "+" contact on the lamp side that would usually line up with the "+" on the battery.
The back light I was able to take apart enough to solder the "-" right onto the battery cradle. I unsoldered the "+" side of the battery cradle from the lamp cicruit and soldered one wire onto the battery cradle and one onto the previously unsoldered wire.

In both cases, test the light after the circuit modification to make sure that the light will run on when the battery is attached while "+" and "switch" are touching. That is, make sure your light defaults to "on". If it does not, you may be able to mess with the circuit further, or you might need to get a different light. Or you could just skip the buckle part I will explain later and switch your light on as you did before.

If the lamps default to "on", find a way to get the wires out of the lamp cases while keeping the cases as waterproof as possible. I went through the rubber membrane that used to protect the switch on the back light. On the front light, where the lamp part screws onto the battery compartment, I filed a recess that the wires can sit in.

You'll need to see what works for your lights.

I am forgetful. I have drained the batteries on my bike lights many times because I forget to turn them off. Also, I wanted to make sure I don't forget to turn the lights on in the daytime to be seen better in traffic.
So, I decided to wire the switch right into the buckle. If the buckle is closed,
the lights will be on.


I added metal tape to the female end of the buckle, and put the "switch" and "+" wires each on one prong on the male side. All of it needs to line up so that the metal tape completes the circuit.

To finish things nicely, I stitched the wires onto the helmet strap.

In the yard lights they use one solar cell for one battery. In the bike lights there are two batteries in series. There are two bike lights. So, I use two pairs of two solar cells in series to charge the four batteries in the two bike lights.
My solar cells have their terminals on the back side, so I needed to wire them before gluing them down. Decide how you will lay out the solar cells on the helmet and wire two pairs where a solar cell "+" is connected to a solar cell "-". Also attach wires to the lone "+" and "-" terminals.

I forgot to take a picture of the back side of the solar cells before mounting them, but it's pretty straight forward.

Find a layout on your helmet so that everything fits and the lights are aimed correctly. I needed to jack up the front light so it would go ahead of the bike properly. If you test this aim standing at home, keep in mind that your head will likely be bent down more on your bike if your handle bars are fairly low.

Resist the temptation to modify your helmet itself to fit the lights as it will weaken the structure. If you use glue other than mounting tape, make sure it is compatible with the helmet material.

Attach all "-" wires to each other, i.e. from battery to solar cells.

Attach the "+" from the solar panel to the "+" from the batteries but protect the circuit with the diode. A diode will only allow the current through in one direction. Which one confuses me each time anew, so you will have to figure it out. (Sorry :-) )

Attach the "+" to the one wire to the buckle switch and the "switch" wire to the other.

I used heat-shrink tubing around all connections in the wiring and around the diode.

I used some sort of silicone tape that doesn't actually have glue to neaten up the wires once all works. The tape worked well enough on larger diameters, e.g. when looped through the helmet, but it did not work so well around just wires.