You might be an electronic engineer if… (III)

… you grow so tired of shoddy, plastic LED-based bike lights which break (or fall) off, get pinched if you forget to take them into the supermarket with you and fail on contact with rainwater that you over-engineer a lighting system for a bike which is itself not mechanically sound.

Principles:

• Bright enough to see by, not just be seen
• No chassis return
• Weatherproof
• Permanently secured
• Hard-wired connections
• Rechargeable in-situ
• Scope for expansion, like dynamo top-up, but otherwise kept simple

Front Lamp

This ‘retro’ type parabolic lamp was bought as a 6V 2.4W part (for connection to a conventional dynamo) but fitted with a 12V 5W bulb and confirmed to operate at a sensible temperature.  It’s clamped to the (pre-existing) lamp holder using a bolt through a plastic sleeve and nylon washers to preserve isolation from the frame.

Rear Lamp

This pannier-mounted 200mW LED lamp was originally powered by 2 AA batteries.  PCB modification allows operation from 5V which is provided from the 12V supply through a 1 Watt monolithic DC/DC converter.  In total this is 63% efficient (electrically speaking) — not great but about as efficient as the original 3V design and far better than keeping the original LED configuration and using a larger shunt resistor (which would be 17.5% efficient).  The efficiency is academic, however, compared to the incandescent front lamp.

Battery Box (which seems wonky in this photo for some reason)

An IP54-rated die-cast aluminium box is U-bolted to the frame and houses the batteries, fuse, connections, switch and charger socket.  Initially I’d planned to use a “4.8 AH” (2.4 AH in tests) 12V lithium ion battery but ditched it when found ailing and imminently doomed components inside the atrociously designed charger.

Instead I’m using ten “3 AH” NiMH AA cells which in practice only provide one hour of lighting at 500 mA.  I suspect the deficit is a combination of false advertising (the cells were from eBay) and high load de-rating.  I’m currently considering alternatives.

Three core, 3 A mains cable may seem excessive compared to the flimsy wires typical of bike lights but it’s robust, weatherproof, has negligible voltage drop at 500 mA and the extra core provides future expandability.  Strain relief grommets are used throughout for fatigue reduction and weatherproofing at cable entry points.

The whole project was an exercise in simple, robust engineering.  A more electronically-complicated project is in the pipeline.  Stay tuned for more boredom.

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