Cyclists’ pulsing LED lights are eye-catching and much more conspicuous than a steady light, so it was decided to make provision for the other half who don’t have one. The aim was to use cheap, recycled components only. At £3.99 ($6.50) the most expensive bit was the 24-LED work light with a magnet and a retractable hook. The DB3 diac could be less easy to find; one was liberated from an 18-watt Philips CFL. Don’t buy the lower wattage types as you’ll find that the Philips 8 W and 11 W versions normally don’t have a DB3 diac.
The prototype was originally built with a 2SD1266 transistor which was replaced by the more common BD433, a TO126 device which should be cooled adequately. The part most likely to cause sourcing difficulties is the ferrite toroid core. The one used has an inner and outer diameter of 9.16 mm and 17.76 mm respectively and a thickness of 6.63 mm. It was liberated from a scrap PC motherboard — ask the local computer shop for an old dead motherboard to salvage components.
Electrically the circuit has been made as simple as it can possibly be — a bog standard blocking oscillator. The downside is you have to wind 60 turns of very thin wire on a toroid! The winding wire was liberated from a 6-volt power adapter. Put the two 12 turn windings on first (0.5 mm/AWG24). The 60 turn winding is easier if you wind half one way then half the other. Cut about one meter (just over three feet) of the thin wire (0.1 mm/AWG38) and feed it through the toroid, then hold the two ends and let the weight of the core find the middle. Hold the first start ( middle ) and wind 30 turns (tape the loose end to nAA battery and let it hang to one side so it doesn’t get caught up in the end you’re winding); when you’ve wound 30 turns, free up the other end and feed that through 30 times to make a total of 60 turns.
For the collector & base windings, the easiest way to avoid phasing the windings is to wind on the first 12 turns, then pull out a loop and tightly twist it back to the core before putting on the second lot of 12 turns. The center tap is the +1.5 V power connection and as the two windings are identical either end can be collector or base — that only leaves phasing the secondary. If the circuit only flashes a few times a minute, reverse the leads and it will flash from about normal for a cycle light or you can turn the 470 Ω pot until it flashes dizzyingly fast. The 180 Ω resistor is to protect the transistor from the excessive base current.
On the secondary side, the pulses are rectified by a UF4007 diode to charge an electrolytic capacitor. Every time the voltage on the capacitor reaches about 32 V the diac triggers and dumps the charge into the 24 parallel connected LEDs. The DB3 diac carries pulses of 2 A which is plenty to flash the LEDs. The 47 µF electrolytic was selected empirically for a good pulse brightness, bumping this up to 100 µF would make the pulses really intense — but for how long!
As an afterthought, while the pulsing light is very conspicuous to other road users on well-lit roads, it’s not so good for actually seeing where you’re going on unlit cycleways away from the main roads and street lamps. The obvious solution is a second flash rate pot and a changeover switch. The unit as is can be adjusted to a flash rate not that far short of the persistence of vision. A switch to change over to the maximum flash rate would make it easy to navigate an unlit cycleway in total darkness.
Along with the usual reminder about type approval and road legality in some countries, readers should be advised that the most rapid flash rate can be irritating (even confusing!) to drivers and should only be used in unlit areas away from the road. It is also worth pointing out that the maximum flash rate puts a greater strain on the components — especially the battery.