DIY bike light

A couple times this season I've gotten caught in the dark on my after work rides.  I've always wanted to try night biking, but my little 55 Lumen headlamp was awkward to strap to my helmet and barely lit up the trail.  Night biking sounds fun because it offers flexibility, it's a bit cooler in the summer, and the trails are less crowded!

A little research told me I was in trouble: A 700 lumen bike light can cost in excess of $300, which is more than I wanted to spend!

So, I'm going to test the limits of my electrical engineering skills and build a light myself. I'm borrowing a lot of ideas from this guy, although i think my system is going to be way more bad-ass (mainly because i'm using LED bulbs and mounting them to my helmet).

My final product included:

  • 500 lumen, 5 watt, 12 volt LED light ($10.50)
  • MR16 socket (to wire up the bulbs) ($1.50 each)
  • 10 AA 2950 mAh NiMH batteries ($20) Note: The higher the mAh rating, the longer the batteries should last.  If you already have batterys, i'd highly recommend buying new ones that are all the same type and brand.
  • 2 AA holder and an 8 AA holder ($1.5, $2.8). I wish i could have found a single 10 AA holder.  For a light I built for my friend I used a 6 and a 4.  Whatever you choose, you need 10 AAs in series to get the 12 volts to the LEDs.
  • 12 v smart charger ($21, but optional).  Chances are you already have a NiMH battery charger around the house for charging AA batteries, but this makes charging much easier because you don't have to remove all the batteries.
  • 1-1/2" CI, PL, or ST to 1-1/2" Tubular coupling ($8, from hardware store) for housing the bulb.  I'd recommend taking whatever bulb you order to the hardware store and seeing what fits well
  • A speaker cable, that I used to connect the battery pack to my light
  • a piece of webbing with a strap that i used to tie the light to the helmet, through the vents.
  • 1" PVC cap for the back of the housing.  I ended up leaving this off my final design.

To build, you'll need to wire everything up in parallel.  Here's my progress as it happened.  Note: Originally i was thinking of using 2 4 watt, 12 v, 360 lumen LED lights, however during the build, I decided one helment mounted bulb would be easier to work with.

All the parts are here! here's the light, it's the only light on in the room. Seems pretty bright! and that's only using one bulb.  You can see also the charger sitting on my desk.

5/14/12: 3 weeks after placing my order, I had almost lost hope; but, the parts I ordered from are finally trickling in (They have great prices and free shipping, but apparently the parts ship from hong-kong which takes forever).  I've hooked up the batteries to the charger, and connected a single 360 lumen bulb to see how bright it is.  I was plesantly suprised with the beam it shot out, and am thinking 1 bulb (vs my original plan of 2) might do me just fine.

Here's my first attempt at mounting the light.
The original prototype, housed in parts from the plumbing isle at the hardware store, and secured using a strap of webbing


Update (5/20/2012): I finally got the light assembled.  I'm going to take it out for a test ride before i soder all the parts together, but basically i used a 1-1/2" CI, PL, or ST to 1-1/2" Tubular coupling from home depot (just the rubber part) and a 1" PVC cap.  I also used an RCA cable I had laying around and a piece of webbing to strap it to my helmet through the vents.    The light fits perfectly snug in one end of the coupling (it even has little ridges that the light sits nicely in).  The PVC cap was a pretty tight fit in the other end, but i got it.  The really cool part about this setup is I think it will be water tight.  I'm thinking more and more that the single bulb helmet mount might be the way to go, but I also might place an order for a stronger bulb.

Me blinding Randy with my spotlight at the trailhead after a great ride. Randy was so impressed, he wants to build one too.

Update (5/22/12): I went night mountain biking last night, and it was fabulous! It was a new moon out, so all the stars were out in their full glory.  The light performed (almost) flawlessly (Right before we left the house, i accidentally ripped the wires out of the back because i hadn't soldered them in place.  I did a quick fix and ditched the PVC backing to give me a little more room to work). The light cast plenty of light for the trail we were on, and i barely noticed it on my helmet.  Although the light illuminated the trail just fine, i wouldn't mind a wider beam to keep me from getting tunnel vision.  I'm still considering the brighter bulb (although the 360 lumen one worked). I am wondering if i could mount some sort of diffuser on the front to cast a wider beam.


Update (7/11/12): I Finally got in my brighter bulb and wired it up and went for a ride.  The 500 lumen bulb is Killer bright, and I'm super satisfied with how it performed on the trail. In case you're wondering about battery life, My light has lasted 3 rides without a single recharge.  In theory, the 5 watt bulb should last 7 hours (12 V x 2.95 Ah  / 5 watts = 7.08 hours), and given my average ride is 2 hours, this makes sense!

For those of you like me, with no electrical knowledge, here are some things i learned along the way:

  • Lumen - How much light output a light has (This is measured in a Flux, which means it doesn't take into account how focused the light is, or what wavelength (color) the light is, just how much light it puts out there).
  • Watt - This measures how much power the light will use.  If you have 2 x 4 [w] lights connected together, they will drain 8 [w].
  • Volt - a measure of electric potential.  Most NiMh rechargeable AA batteries are 1.2 Volts.  You'll need to choose how many batteries you use based on the volt rating of your light.  In my case, I used a 12 [V] light, meaning I need 10 batteries (10 x 1.2) connected in series.  Also, I might decide to use 2 lights.  I must connect these to the battery in parallel so they each get 12 [V].
  • Amps - This is a measure of current. Think of it like saying a river is flowing at 300 cf/s.  Batteries are rated in amp-hours (or milliamp-hours [mAh]) and the higher the rating, the longer they will last.  Think of it as saying how big a reservoir is.
  • W=V x A - This is an important formula you can use to determine how long your light will last.  Example: I have a 12 [V], 2.6 [Ah] (or 2600 [mAh]) battery connected to a 4 [w] light. How long will it last?  Answer: 12 [v] x 2.6 [Ah] / 4 [w] = 7.8 hours.  If I use 2 lights (8 [w]) my light will last half as long.
  • Charging - You can use standard AA batteries and plug them into a battery charger, but I decided it'd be more convient to buy a charger i could hook directly up to my battery pack without taking out 10 batteries, plugging them into a charger, and re-installing them every time I go for a ride.  Chargers have V and A ratings.  If you have a 12 [v] system you need a 12 [v] charger.  The amp rating defines how quickly your pack will charge (E.g., if I have a 1.8 [A] charger and my batteries are rated at 2600 [mAh], then it will take 2.6 [Ah]/1.8 [A] = 1.44 [hours] to charge.