Issue 143 June 2002
Invisible Components

Start • Directed Lighting • Battery Basics • Down The Drain • Invisible Resistors • Sources & PDF

Directed Lighting

I picked a General Electric ESX/CG halogen MR16 spotlight to get a tight beam pattern that illuminates the road without lighting up the trees. The reflector has a cover glass that protects the bulb, which is vital because an exposed halogen bulb can shatter when a drop of water hits it. Although Mary doesn’t plan to ride in the rain, such things can happen. The cover glass also keeps dust and grime off the reflector.

The lamp isn’t specified for outdoor use or a bicycle’s vibration, so I won’t be surprised if it doesn’t reach its 4000-hour rated life. On the other hand, it’s cheap and readily available, which makes up for a lot. Smaller reflector bulbs, in the 6-V MR11 series, should work equally well.

As a rule of thumb, if your design depends on exotic parts, you should expect procurement troubles at some point during your product’s life. While stranding your wife in the darkness may not be worse than stranding your customers, I strongly recommend avoiding either outcome.

The bulb draws a continuous 20 W from a 12-V supply, which can be either AC or DC. Although many people think that bike headlights can be powered from a pedal-turned generator, it turns out that riding requires 0.1 hp, about 75 W, on a continuous basis. Adding 20 W to that just for lighting isn’t practical—just ask my wife!