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· Registered
9,680 Posts
Discussion Starter · #1 ·
Thought this might be of some interest, why not design and make your own display for blinkers, tail lights, or running lights. You could convert your stock or aftermarket lights to LED's. If you're handy with a soldering iron and can follow a wiring diagram you can do it. The hard part is figuring out the proper dropping resistors. Thanks to the handy LED series/parallel array wizard it takes the guess work out of play.

has a wide variety of 3mm or 5mm LED's in various colors. You would need a small blank circuit board (usually green in color) and a thin strip of aluminum to mount the LED's. The circuit board can be purchased at Radio Shack or you can be creative.

The cost is minimal and the worst thing would be a blown fuse if you screw up.

· Old, Bold rider
2002 Honda 919, 1976 Yamaha XT500 cafe, TC-JAG TZ250
2,496 Posts
First, some basics:

Current is expressed in Amps, so for small values such as 20 milliamps (mA), divide by 1000, or just slide the decimal place 4 places to the left, meaning 20mA = .02 amps.

In a series circuit the voltage varies depending on where you are measuring it, but the current is the same anywhere it's measured in the circuit. An example: six LEDs connected in series would need 12V to light, but only draws 20mA, the same amount as any one of the LEDs.

In a parallel circuit the voltage is the same wherever it's measured, but the current varies depending on where it's measured. An example: six LEDs connected in parallel would need only 2V to light, but would need 120mA (6 * .02) to light. Both of these principles will be important when making a series / parallel array.

When designing an LED array be sure to use a system voltage of 13.8 volts -- it's an industry standard value for an automotive electrical system while the engine is running, also valid for motorcycles. If you want to check your voltage do so with the engine turning at least 3,000 RPM and design accordingly.

And Ohm's law:

Voltage = Current times Resistance.

Current = Voltage divided by Resistance.

Resistance = Voltage divided by Current.

Wattage = Voltage times Current, or Resistance times Current squared, or Voltage squared divided by Resistance. This is useful to determine the wattage of a resistor. Always round up to the higher wattage value.


The Wizard is a nice find, but it does have it's limitations as far as resistor values are concerned: it rounds up to the nearest common value of 10% resistor, usually resulting in less brightness than the LED is capable of.
An example from the calculator: one LED (2volt at 20mA) driven by a system voltage of 13.8 volts returns a resistor value of 680 ohms. This arrangement will give an LED current of 17.6mA, and as LED brightness drops off significantly with a decrease in current the brightness may not be what you had in mind.

To calculate the actual resistance needed first you need to subtract the LED voltage (2V) from the system voltage (13.8) and divide that by the LED current in amps. 11.8 / .02 = 590 ohms. If you are connecting two or more LEDs in series multiply the individual forward voltages by the number of LEDs, subtract that number from the system voltage, then proceed with the resistance calculation. You will undoubtedly come up with a resistance value that isn't available but can be made with two resistors in series (just add the values together), or parallel (a bit more complicated: it's 1 / (1 / one resistor + 1 / the other resistor). In this case you could connect a 200 ohm and 390 ohm resistor in series to hit it right on the head. Whenever possible use 5% tolerance resistors, there are more values to choose from than 10% tolerance.

Things get a bit more complicated when you want a dual brightness light such as a brake / taillight or driving / turn signal light. In these cases you must design two separate circuits and distribute the lower brightness array LEDs amongst the higher brightness array LEDs to insure even distribution of light in both modes. This can make the circuit board design a real bear, and requires some experimentation to get the LEDs in the right position for even light.

One last thing: However you decide to interconnect the LEDs, make sure all the connections are secure enough to survive vibration and shock loads common when riding -- a single failed connection could stop one series array from lighting, or blank out the whole light. Obviously, you will have to have the proper soldering iron (30 watt or thereabouts is about right), use only rosin core electronic grade solder, and know exactly what you are doing when soldering not only to make a secure connection, but also to prevent overheating the solid state LEDs which will either kill them immediately or significantly shorten their life.


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