{"id":7155,"date":"2013-05-01T10:31:37","date_gmt":"2013-05-01T14:31:37","guid":{"rendered":"http:\/\/embedded-lab.com\/blog\/?p=7155"},"modified":"2014-01-31T12:30:36","modified_gmt":"2014-01-31T17:30:36","slug":"netduino-day-7-pulse-width-modulation-and-color-formation-using-rgb-led","status":"publish","type":"post","link":"https:\/\/embedded-lab.com\/blog\/netduino-day-7-pulse-width-modulation-and-color-formation-using-rgb-led\/","title":{"rendered":"Netduino Day 7 – RGB LED color formation using Pulse Width Modulation"},"content":{"rendered":"

Pulse Width Modulation (PWM) is a digital method of delivering a varying amount of power to a load, and hence can be used to control the brightness of an LED or speed of a DC motor. Controlling the power, we will make this tutorial colorful using a Red Green Blue (RGB) LED. Each segment (lead) of an RGB-LED will get a PWM signal via RgbLed<\/em> class allowing us to illuminate the RGB-LED with any color defined by RGB. This class can also generate random colors. Using the RgbLed class, some standard colors can also be sent to a RGB-LED.<\/p>\n

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RGB-LED illuminating a purple color<\/p><\/div>\n

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Circuit Setup and Theory<\/h2>\n

To understand PWM, let’s understand the terms that make a modulated pulse width signals. The very important one is Duty Cycle. In lay man terms this duty cycle is a value in percent of ON status compared to OFF status. From the figure below we can see the formula to calculate the duty cycle.<\/p>\n

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PWM wave and its terminology<\/p><\/div>\n

Now, let’s co-relate the above to our project. Let’s say we like to produce a “grey” color. We know the RGB value for grey is 128, 128, and 128. This means, if we mix 50% of red (50% of 256 is 128), 50% of green and 50% of blue, we will get a grey color. The value of 50% came from 128\/256*100 %. With respect to above figure, if Ton<\/sub> = 50% for Red leg, Ton<\/sub> = 50% of Green leg and Ton<\/sub> = 50% of Blue leg of a RGB LED then we will have a gray color out from a RGB-LED. The calculated value is nothing but the duty cycle of a PWM. In the RgbLed class, we basically adjust the duty cycle to illuminate the RGB-LED with a desired color.<\/p>\n

Here is a DutyCycle<\/strong><\/span><\/a> Excel spreadsheet to understand it little further and it also somewhat followed in the RgbLed class.<\/p>\n