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This article is from the Misc Bicycles FAQs, by various authors.

8.10 Low-Power Hi-Vis Never-fail Tailights

From: doconnor@srg.UUCP (<uunet!srg!titania!doconnor>)

When my wife's tailight burned out and we couldn't find a correct-wattage
bulb to replace it, I put together a replacemet using LEDs that has now
been tested enough that I'm posting it to the net. This set-up is very
bright ( brighter than the original 1/2watt incandescant ) but draws
only a fraction of the power ( 1/8 of a watt ). It's designed for a
generator (i.e. 6V AC) not a battery. A battery set-up would need less
than half the components.

The LEDs are from Radio Shack, part number forgotten, but they are called
"Hi-Brightness" LEDs, not the normal kind, and are rated at 5000mcd
(milli-candellas) of brightness. The cost I think $1.95 each. Most RS
seem to only keep two or three in stock, it seems, so I had to visit
two stores to get enough. They draw 20ma at 1.8-2.8 volts.

Parts list :

5 - Hi-brightness LEDs
2 - 6.3V 1 Watt Zener diodes
1 - 10 Ohm resistor

Diagram:

                  Resistor
  Wire from gen ---[====]---+-------+--------+
                            |       |        |
                            |   LED v    LED -
                            -       -        ^
                     Zener  ^       |   LED  |
                            |       +--->|---+
                     Zener  v       |        |
                            -       -        v
                            |   LED ^    LED -
                            |       |        |
  Wire to frame ------------+-------+--------+

I simply soldered all the componets together using the component leads,
using shrink-tubing and electrical tape for insulation, and stuffed
the hole thing into the tail-light housing.

People with knowledge of electronics will already realize that what
I've built is a bridge rectifier of LEDs with a fifth LED wired
across the output. The resistor and back-to-back Zeber diodes provide
voltage limiting ( to about 7 volts for either polarity input, up to
a safe maximium of about 8.6Volts RMS input ) which is within the
maximum forward voltage spec of the diodes bridge. The power penalty
for doing this ( the 10 ohm resistor ) is I*I*R = 20ma * 20ma * 10 =
4mw. Current draw is 20ma, so power disipation is 120milliwatts
( 1/8 watt) at 6V input.

It is much brighter than the old 1/2watt incandescant. Very visible.
It should never burn out and is immune to shock and vibration, at least
as much as the tailight housing is, I think.

The downside is that at low speed ( 5mph ) it goes out.

From: doconnor@srg.UUCP (<uunet!srg!titania!doconnor>)

News travels fast : hours after posting the actual generator-version
of this, a request for a 12V battery version arrived in my Email !

Now, I haven't avtually built such a beast, but the following
ought to work :

First, for a 6V battery system : I assume a negative ground (
for corrosion-resistance of course ), here's a schematic :

Parts list :
3 Hi-Brightness (5000mcd) Radio Shack LEDs
1 10 Ohm resistor

Schemtic:

                        resistor   LED  LED  LED
   wire_from_battery -----[===]----->|--->|--->|------- wire_to_frame

If it doen't work, try reversing the power connetcions. The
reverse breakdown voltage of the LEDs is high enough to prevent
any damage from accidental power supply polarity reversal. The
resistor is to limit the current, it actually could be deleted.
It wastes 4 milliwatts. Total power draw 20ma ( 120mw ). This setup
should tolerate voltages up to 8.4 volts. It will stop working
if the voltage goes much below 5.4 volts.

Second, for a 12V battery system : I again assume a negative ground.

Parts list :
6 Hi-Brightness (5000mcd) Radio Shack LEDs
1 10 Ohm resistor

Schemtic:

                     resistor   LED  LED  LED  LED  LED  LED
 wire_to_battery -----[===]----->|--->|--->|--->|--->|--->|--- wire_to_frame

This should draw 20ma, dissipate 240mw ( 1/4watt ). It will tolerate
voltages up to 16.8 volts. It will stop working if the voltage falls
much below 10.8 volts.

Last, a 12V system more tolerant of low voltage, but a bit wasteful :

Parts list :
5 Hi-Brightness (5000mcd) Radio Shack LEDs
1 47 Ohm resistor

Schemtic:

                     resistor   LED  LED  LED  LED  LED
 wire_to_battery -----[===]------>|--->|--->|--->|--->|--- wire_to_frame

This will draw 20ma, dissipate 240mw ( 1/4watt ). This is the same amount
of power drawn as for the 6-LED version, but only 5/6ths the light. It will
tolerate voltages up to 15 volts. It will stop working if the voltage falls
much below 9 volts.

Again, I haven't built these, but it's all pretty straitforward. I did
build the generator-driven version, where there are more unknowns.

 

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