# Edwin Holman

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• Edwin Holman commented on CarlosOliveira's instructable Arduino Uno - Basic Program9 months ago

Enlightening entry into the world of Arduino!To prevent the led from burning-up it is highly recommended you add a resistor in line with either one of its legs. The value of the resistor depends on the color of the light your selected led is designed for to emit, as color = the wavelength of the light-particles // photons technically corresponds to a voltage. For example: the green led is emitting photons at around 560 nanometer, which corresponds to an energy per photon of about 2.2 electron-volt (eV) (for converting wavelength in eV see for example http://halas.rice.edu/conversions). It is at this "pressure" that you want to drive your led. The I/O port of the Arduino will provide a "pressure" of 5.0 volt, sufficient for generating photons of 248 nm (ultraviolet)...see more »Enlightening entry into the world of Arduino!To prevent the led from burning-up it is highly recommended you add a resistor in line with either one of its legs. The value of the resistor depends on the color of the light your selected led is designed for to emit, as color = the wavelength of the light-particles // photons technically corresponds to a voltage. For example: the green led is emitting photons at around 560 nanometer, which corresponds to an energy per photon of about 2.2 electron-volt (eV) (for converting wavelength in eV see for example http://halas.rice.edu/conversions). It is at this "pressure" that you want to drive your led. The I/O port of the Arduino will provide a "pressure" of 5.0 volt, sufficient for generating photons of 248 nm (ultraviolet). The "over-pressure" or difference between the 2.2 and 5.0 volt = 2.8 volt needs a valve that will destroy // consume this. A resistor does the job. Question is which value // that is how many ohms? The answer depends on the current your led will safely consume at its driving voltage. For example: If this current is 20 mA = 0.020 Ampere, this current must correspond to 2.8 volt of "pressure" being destroyed in the resistor when being forced through it. Omh's law gives us the value of the resistor: Volt = current x resistance --> V = I x R or R = V/I, filling in the numbers, in our case 2.8 / 0.020 = 140 ohm, your resistor of choice. A red led at 640 nm = 1.94 eV has an overpressure of 5.00 - 1.94 = 3.06 V and at 20 mA needs 3.06/0.020=153 ohm of resistance in line with a 5 volt source. Now sniffing through your box with resistors you pick the one that has the value closest to the calculated value or slightly above (not below...).Regards,Edwin Holmanwww.dessa.com.br