The perfect OSRAM spotlight just turned-up in my local DIY shop - so I have had a go. I am very pleased with the results. I recon I have made a bike light for around £20 that would retail for well over £200 from a bike shop.
Step 1: Buy The Bulb
This is a 240Vac 4.5 Watt replacement for 35 Watt GU10 halogen spotlights. It has a narrow 25 degree beam angle.
There may be better units to start with to be found if you scour the internet, for example 12V, or single LED rather than 3 LED array. I liked this OSRAM one because it is near watertight, comes apart with screws, and looks the part. (Unfortunately I didn't take any photos of the unmodifed spotlight - but you can imagine the GU10 terminals in the place of the switch and lead exit in the photo below, other wise it is unchanged externally.
Step 2: Dismantle Lamp
The lamp is made-up of 7 parts:
o Lens Clamp Ring - Aluminium
o Lens - Plastic
o LED Plate - Aluminium
o Heatsink/Shade - Aluminium
o Electronics Capsule - plastic
o Electronics - PCB
o Insulation cap - plastic
The lens clamp ring and lens are removed by simply unscrewing the 3 small allen key screws.
This exposes the LED plate with 3 LEDS and a few electronic components and terminals.
Removing the 3 phillips screws on the LED Plate releases the plate, heatsink and the electronics capsule. You need to scratch off the blobs of silicon rubber and unsolder the plate feed wires to completely release the 3 parts. As the whole thing is a heatsink you need a decent soldering iron to get enough heat. Also beware the the plate has heatsink jointing compound on the back. I carefully wiped this off, as it is usually toxic.
You need to be extremely careful of the LED surfaces - I kept them covered with a lint free cloth to ensure they didn't pick-up fingerprints.
the electronics capsule and cap look like a single unit, but you can just lift off the cap. At the other end, I heated the GU10 terminals until they melted the plastic and just pushed through. If you want to keep the terminals you need to drill out the clamping pin holes.
Once the cap is removed and the terminals pushed through, the electronics falls out. You should now have 7 parts and 6 screws on your bench!
Step 3: Designing a DC Supply
The electronics produced a variable ac supply of around 14Vac and 350 mA. to the LED Plate. This comes off the final output transformer. There is a lot of electronics driving the transformer, presumably ensuring the end result is a constant current at the LEDs. On the LED plate there is a rectifiying diode, a pair of capacitors and a resistors (and the LEDs in series). This gives a half wave rectifier and charge pump. This means without unsoldering the LEDs and monitoring with an osilliscope it is very difficult to be sure of the current and duty cycle actually at the LEDs. The only thing to go on was the resonably constant 9.6Vdc across the 3 LEDS.
Combined with the quoted 4.5 W this would give around 450 mA. As the LEDs may be pulsing (therefore lower heat disipation) I planned to apply around 350 mA as a constant DC supply.
At this point I stole a useful circuit from Dan - see http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/
I used a MosFET, NPN transistor, collector resistor (47Kohms) and a choice of current sensing resistors (1.5ohms, 4.1ohms). I just bought whatever the local shop had. These gave 370 mA and 180 mA through the LEDs to provide a high power mode and a battery saving mode.
For the battery, I found an old cordless drill with 14.4V rechargeable batteries. As it was knackered, it barely gave 12V, but still at 1.5 AH thats 4 hours bike light time at high brightness. I will probably buy a smaller LIon battery source eventually.
Key to my plan was getting all this electronics and a DPDT switch inside the exisiting housing. So I bought a sub-miniture 3 position switch with waterproof cover.
Step 4: Build the Electronics
It is also necessary to modify the circuit of the LED plate. I unsoldered the diode and resistor. I replaced the diode with a piece of tined wire. I didn't remove the capacitors as I thought they may give some switch spike suppression.
After a lot of trial fitting and very short lead soldering I was able to slide the electronics into the capsule, poking the switch lever and battery lead through the terminal holes, replace the electronics cap, poking two fine wires through the existing cap hole, ready to solder to the plate on reassembly.
Step 5: Reassembly
Apart from drilling out the terminal holes a bit bigger and chamfering the base, the only other alteration I made to the exterior was to place some silver tape around half of the lens surround. This stops light leaking out of the side of the lens (through the heat sink blades) distracting the rider. I left the bottom half of the lens able to leak light on the front wheel - as this can only help with visibility.
If you want to make the hole unit waterproof now is the time to add a fine silicon film on the bottom of the heat sink (where it mates with the electronics capsule) and to the bottom edge of the lens (where it mates with the heat sink). There is a slight cutout in the lens edge which seems to be a "breathing" point. This also need taping or sealing. The switch I bought came with a waterpoofing cap, and the battery cable hole was sized to take a tightly fitting strain relief that with a little silicon, would be completely waterproof.
The only tricky part of reassembly is the soldering of the LED plate terminals to the flying leads from the electronics. I did this before tightening the screws into the heatsink to ensure the iron heat didn't get soaked into the whole unit. If you feel you may have touched the LED surfaces, you will need to carefully polish them up with a lint free cloth.
To connect to the lead coming from the battery I used standard inline (laptop) DC power connectors.
Step 6: Bike Bracket
First impressions on use are - it is very bright. My main concern is blinding drivers, I will keep it pointing down steeply and will only use High Power mode when there is no one coming in the other direction.
Let me know if you want more info and I'll try to get some decent pics and upload my scribbled diagrams.