Introduction:    I have been using a Supernova E-3 Pro hub dynamo driven bicycle head light for over a year on my commute to work and back.  I am happy with it and the light it puts out.   However, I was rather disappointed with Supernova's tail light offering as it does not have a reflector and and only has three small LEDs.  I had been using a 12V GU4 LED, mounted in a Cateye reflector, with the battery mounted in a water bottle.  After discovering the Avago Moonstone 1/2 W LED, I saw that it  had the optimal design and power consumption for a tail light made out of a reflector, like my 12 V version. 

Connecting the "Red Giant" up to the tail light leads of the Supernova E3 light is very easy.   The  Supernova takes the AC power from the hub dynamo and supplies a stable 6.0V DC supply to the tail light leads.    The red lead is "+" and the blue lead is "-".   Technically do you not need to build any diodes or capacitors into the lamp when connecting it to the Supernova E3.   The stand light feature of the Supernova will drive the "Red Giant" as a stand light too.

I made the battery powered design after a friend from our triathlon club had mentioned he thought all of the commercial tail LED lights were not bright enough.  He wanted a brighter tail light for more safety on the road when it was dark, even at the expense of shorter battery life.  The battery powered "Red Giant" is just a modified version of the hub dynamo type.   To keep things simple, there is no switch.   Just clip the "9V" clip on the standard battery pack to turn the light on.  To turn it off, disconnect the "9V" clip. 

Both types of tail lights are extremely bright and visible from a long way off and can be built for less than $5 excluding the hub dynamo or rechargeable batteries.    

On this page you will find out how you too can make the "Red Giant".  It is the perfect companion to your Supernova hub dynamo bike light!

The Reflector
I have found the Cateye brand reflectors work well for this application.   Cateye has three styles of reflector backs.   The "Post" style is my favorite type, then the "slide lock".   The "tapping screw" type should not be used as it would be difficult to bring out the power lead out the back since the "tapping" screw would be in the way. 

The following part numbers should work well:

Post mount:  RR-165-BPR (round) RR-180-BPR (square), RR-290-BPR
Slide lock  :  RR-165-BSLR (round) RR-180-BSLR (square), RR-290-BPR

I have only used the RR-165-BPR and the RR-165-BSLR so far.

The LED
The LED is a Avago "Moonstone" ASMT-MW62 1/2 W 3.7V, which is compact, flat and perfect for the tight space inside a reflector.   It is also has the appropriate power range so as not to take away too much power from the head light.  Being an LED, it is much more efficient than a normal light bulb and will most likely never burn out.  Moonstone LED Conrad 180869-62 1.80 €

Other Materials
You will need a 2 conductor 20-22 gauge cable to run to your Supernova tail light leads, two part epoxy to glue the reflector back together and some 2 part epoxy putty to hold the parts in place inside the lamp and a 12-18 ohm resistor rated at 1/2 watt or a little more.  For certain designs, where AC power comes directly from the hub dynamo, a small full bridge rectifier and a small resistor  9-13 ohm will be needed

Applications
There are three possible power sources for the Red Giant:

1.  Used in conjunction with a Supernova headlight:  For this design you do not need to to rectify the AC power from the hub dynamo.   The Supernova lamp provides leads for a tail light and it is a stable 6V DC current.  However, you will need to add a current limiting resistor in the circuit to keep from pushing too much current through the LED and burning it out.   . 

2.  Use with 4 AA or AAA rechargeable batteries:   Rechargeable batteries generally deliver 1.2 V so with 4 batteries you would have 4.8 V instead of 6V.     Here are a few current measurements using fresh charged AAA NiMh batteries.  For a bright light, but shorter battery life you can use the small resistors.   For less light and longer battery life, you can use a larger resistor.   A diode can substitute for a resistor here too. 

Diode                 150 ma
6 ohm resistor  220 ma
9 ohm resistor  150 ma
13 ohm resistor 100 ma

3.  Use with a hub dynamo: AC power output:    In this case you need to rectify the AC power source with a full bridge rectifier.     The  design is detailed below and is very easy to assemble. 

R1 =  12 - 18 ohm (use with Super Nova E3) >1/2W

R2 =  9 - 13 ohm (4 NiMh battery pack) >1/2W
D1 = diode 50V 1A  1N4001

R3 = 9 - 13 ohm (hub dynamo)  >1/2W
BR1 = SMD bridge rectifier 50V 1A B40S

Moonstone LED Conrad 180869-62 1.80 €

Resistor sizing calculation:
R = (V_S - V_L- V_D) / I 

I = 75 - 150 mA  (V_L = 3.35 - 3.70 V)