"He Wears Black and Has a Beard"

CrapBike II: The Upgrades

Toby Roworth

May 12, 2014


After a few hundred miles my bike was starting to show signs of fatigue. It also still didn't have a lights, which made riding at night even more dangerous than riding in the day. With Rascal Engine Repair Week 2 approaching, I had to get these fixed, so I could get between my Dad's place and Rachel's abode.
Whilst messing about on Brunel's meadow I noticed a slight crack at the front end of the top tube. By slight, I mean 2mm wide and 20mm long:

I always knew fatigue could be a risk, but my design spec prohibited adding significant amounts of new metal to the build, so I ran the risk - it mostly paid off, given that the bike got me to work without trouble last year. But the stress beneath that weld was quite significant, and thus the crack chose there to form.
The legend that is Guy Fitch, metal fabrication technician at Brunel, welded up the crack for me and added an extra bit of sheet over the top to add some extra strength. His welding is far better than mine (as you'd hope), this the beautiful beads. The downside to this repair was the loss of my livery, which will now be mostly in Guy's lungs, or maybe the extraction system.
I planned to put a photo here, but apparently don't have one.
Whilst Guy was welding, I made a new front bracket for the seat - nothing special, just a piece of bent ally to screw to the sides of the seat - but it works far better than the old one. I also added a tube for my backpack to rest on, as it was hitting the wheel if heavily loaded.
Afterwards, I got a couple of bits for my lights. The constant current driver circuit, which I've started using frequently, is based on a guy called dan's circuit. This can also be used with PWM control, so it's quite useful. Here's my interpretation:

The circuit basically uses the MOSFET as a variable resistor, and the NPN to sense the current flowing in R2, which then controls the MOSFET. The link explains it further, but it basically increases the resistance when the current's too high, and lowers it again when it's too low, fast enough so the LED sees a constant resistance, and thus a constant current.
The MOSFET I used was a P4NK80Z, as it's the biggest one I could find at uni - it's rated for 3A and 800V, so will easily handle a couple of LEDs, and should handle a lot more. The NPN is just a BC237. However, the choice of transistors is quite open, so if recreating these lights I'd suggest using whatever's closest to hand and seeing if that works.
The LED shown is actually two LEDs in series - a white Luxeon for the front LED. and a red one for the back. The two jumpers allow the current to be set between three brightnesses, by changing which resistors are in the circuit - when both jumpers are on, the resistors are in parallel, giving the third setting. These resistors should really have been bigger than the 0.25 watt (or maybe 0.5W) ones I used, but these were nearest to me, and haven't burned out on me yet...
I've found the lowest setting is adequate for all but the darkest paths, where the brighter setting was a bit better. The LEDs can, in theory, take 0.4 amps, which would require a 1.25 ohm resistor for R2, but 0.2 A seemed enough for these lights, and helps the battery last longer.

I built the circuit on stripboard (note the voltage regulator I accidentally soldered on - this is unnecessary) and fitted it into an Altoids tin, as is traditional. I then glued the red LED into the pannier tube I added earlier and the white LED into a little jam jar I had lying in my toolbox, which I then cable tied to the handlebars.

I should point out how, technically riding my bike like this is still illegal. The lights don't necessarily conform to BS 6102/3 (although they might be pretty close to most of the requirements), and (more worryingly) I still haven't got round to fitting a rear reflector. However, I've seen very few lights that do claim conformance to the standard, and even fewer policemen on the roads and paths I cycle on, so I'm not too concerned. For an in-depth look at cycle lighting law, there's a fascinating article here.
To give everything a proper test, my flatmate and I cycled down to Little Britain Lake, through some muddy paths, and then visited the M25: