Diode light on wheels

Since bicycle became my main means of local transport, i have been seeking ways to improve its lighting system - especially the brightness of the headlight. A possible improvement would be to replace the incandescence bulb, in the original headlight fixture, by a light emitting diode (LED).
The latter type of lamp features better efficiency (in converting electric power to light), longer lifetime and very robust construction - to name just a few advantages. However, unlike conventional filament lamps, high-power LEDs are far less forgiving in electical supply. If over-powered - be it for a split second - they tend to heat and burn. Then, the main issue is to regulate the (direct) current through the LED, by use of current-limiting electronics. Two popular simple versions are shown here.
In the first circuit, LM317 - an adjustable voltage regulator - powers the LED set in line with a resistor which creates a voltage drop. By construction, the LM317 chip includes a compensating circuit that keeps the voltage difference across its “GND” and “OUT” pins at a pre-set 1.25 Volt. Therefore, choosing the value of the resistor (say R1, in Ohm) sets the current (I1 in Ampere) at a constant value I1=1.25/R1.
The second circuit works in a similar way: the LED is driven by a power transistor Q1 which acts as a variable resistor. Its state (between pass and cut-off) is set by a second transistor Q2 which monitors the voltage drop on a resistor connected in line with the LED. In this version, Q2 saturates (thus driving Q1 into the cut-off region) around I1*R1=0.7 Volt. Again, choosing value of R1 sets the value of I1.
To start with, bike electical systems typically include a dynamo, producing alternating current (AC). On the other hand, LEDs are polarised devices, operating on direct current (DC) - the one i used is a 1 Watt star rated at 350 mA. Both circuits above use a simple AC-DC converter, consisting of a full-wave rectifier bridge followed by a smoothing capacitor C1. The value of C1 relates to the frequency of the AC current produced by the dynamo (which, in turn, is proportional to the bike’s speed). I found out my bike’s hub dynamo (a 28-pole Shimano DH 3N70, rated at 6 Volt / 3 Watt) alternates at approximately 1,7 Hertz per km/h. For a typical speed of 10 km/h, the period is 60 ms, which - combined with an internal resistance of 8 Ohm leads to a rough-cut estimate of an ideal capacitor in the order of several milli-Farad. This is far beyond the space allowance of the fixture, thereby this simple learning: use the largest capacitor that mechanically fits. Point (and pray).
I decided to keep the original headlamp casing and mirror. I removed the original halogen lamp and squeezed the electronics (in this case, the LM317 circuit above) inside the body of the headlamp. For the actual LED element (a 1 Watt star, powered at 3V-330mA), to go through, i had to widen the hole of the plastic cup supporting the reflector. The largest capacitor fitting the fixture was an electrolytic 2200uF/25V.
As expected, the light flickers at low speeds - however, once on the go, the white LED produces a cool bright beam.

Estival bracelets: res parvae

As always, some merit lies in the small things. In the case of 2011 summer bracelets, i found out that the grommets on the ready-made barrel clasps tend to deform and let one or more strands go loose. For a more solid solution, one can make full-loop grommets using brass (or similar alloy) wire.
The end of the wire that goes inside the barrel can be secured by tying it in a knot or, simply, bending a small number of folds. This way, the grommet can turn freely inside the barrel clasp (which facilitates fastening the bracelet on the wrist).
The other end of the wire is shaped into a (single or double) loop, some five or six mm in diameter. At the closure of the loop, the wire is wound once or twice around the "stem" which exits the barrel.
By the way, i am only glad to report that the bracelets have been a success - demand keeps rising!

Estival bracelets

Summer is a great season for small projects. I use to come up with a craft theme every year. This time round, it's simple, colourful jewels - ladies' bracelets, to be exact.
I start with a length of waxed polyester thread. It comes in many colours, is relatively inexpensive, very strong, resistant to wear and easy to work with. Polyester melts under heat, so the end of the thread can be conveniently shaped (e.g. flat to slide in a needle's eye or in a thin rod to string a bead). I have been using waxed polyester for leather stitching (handsewing) for years and it never let me down.
Craft shops hold threaded barrel clasps for bracelets, necklaces etc. When properly screwed-in, they are quite reliable. Made from brass alloy, they usually come in silver and gold colour. Typically, on their "outside" extremities they present wire loops (or "grommets") to attach a thread or wire.
The rest is just beads - spaced at will and according to taste. In this case, i used coloured oyster shell fragements, short pieces of 1/16" brass tubing (for the two bottom examples in the photo) and glass and silver beads (for the green bracelet on top). A simple overhand knot on each side of a bead fastens it in place.
Each bracelet has seven or nine thread runs (each approx. 16 cm long, for a typical female wrist), i.e. three- or four- and-a-half loops of thread around the clasps' grommets. An odd number of strands means that threading finishes on opposite ends of the bracelet. The rest of the thread is then used to tie the loops together (e.g. with a couple of half-hitches around the bunch).