Nuts and Volts - May 2014

devices so that they produce results functionally similar to a variable voltage.

Figure 11 illustrates this concept with four pulse trains (pulses that repeat over a set interval know as the period) where the vertical represents voltage (zero to + 5) and the horizontal represents time. In the first pulse train, the pulse is turned on to + 5 volts 10% of the time and off to zero volts 90% of the time. In that case, the average voltage over time is 10% of five volts or 0.5 volts.

The second pulse train is 30% on and 70% off, giving an average of 1.5 volts. The third pulse is 50% on and

50% off, and averages to 50% of five volts or 2. 5 volts.

The last one is 90% on and 10% off for a 4. 5 volts average. [Please note this is a simplification of what happens in real electronic systems and is meant to help understand what is going on at this point in the discussion.] These digital signals are known as PWM (Pulse Width Modulation). To output PWM signals with an Arduino, you use the analogWrite(write_value); function. This function takes the write_value variable which can be from zero to 255, and uses that number to set the width of the pulses as shown in Figure 12. There are three terms you need to describe what is happening: frequency, period, and pulse width.

The frequency is the number of times a repeating event occurs per unit time. The period is the duration of one cycle of that repeating event. We use the term Hz (Hertz) to indicate how many times the repeating event occurs in one second. The Arduino analog Write function outputs a pulse train with a frequency of about 490 Hz that equates to a period of about 2 ms.

We see in Figure 12 that each of these pulse trains creates a constant DC (Direct Current) voltage that does not change over time. Since we can set the pulse to any of 256 widths, that means we can have 256 discrete DC voltages. If we use analog Write(0);, we get a voltage of zero; if we use analog Write(255), we get the maximum voltage that (in our case) is five volts. If we use analog Write(127) — half way between the two — then we get half of five volts, or 2. 5 volts.

You can calculate the write_value to use in analog Write(write_value) to give a percent of time the pulse is high by multiplying the percent times 255. Thus, for a 90% on time, you multiply 0.9x255 = 229.5. Since we are using integers, you round it up to 230. This is shown in Figure 12 as analog Write(230).