Nuts and Volts - August 2010

the average voltage value of the pulse DC across the waveform period. For example, if we have the pulse high for 50% of the period consistently and the pulses are + 3.3V, then we are supplying (on the average) 50% of 3.3V to the load or 1.65V. In order to change the pulse width while the system is running, we simply write into OCXRS and it automatically loads into OCXR on the next cycle.

We’ll look at how to apply this idea in the following applications (all hardware hook-ups and source code is provided at www.nutsvolts.com).

■ FIGURE 3. LED intensity (RGB) experiment hook-up.

Demo 1. RGB LED color control using PWM • Using changing PWM to control DC level to an LED, thereby controlling its brightness and (in the case of RGB) its color.

Demo 2. Running four R/C servos simultaneously • This simply provides a 1.5 msec to 2 msec pulse width over a 50 Hz period to control servo position.

Demo 3. Independent dual motor controls • This allows for two DC motors to be independently controlled in both speed and direction.

■ FIGURE 4. Servo experiment hook-up.

RGB PWM Experiment

In the original introductory demo code for the Experimenter, we had an RGB demo. This is a similar exercise with a little more focus on the software and hardware elements. This experiment uses three of the five OCX modules — OC1, OC2, and OC3 — to independently drive each LED (red, green, and blue) that is put together in a single RGB LED package. With PWM on each of the LEDs, we can control the individual light intensity for each one to realize up to 16M colors. The code is contained in RGBDEMO, and the hook-up is shown is Figure 3. There are several key C functions:

ColorGreen_off(),ColorBlue_on (), ColorBlue_off()

— individual functions per LED type for on or off.

• Set_ColorRed (setting), Set_ColorGreen (setting), Set_ColorBlue (setting) — Sets the PWM duty cycle of the designated color red, green, blue, thereby changing combined color. Designated color (or LED) is selected via pushbutton by user. Current LED is stored as a number 1 to 3 in variable sel_color.

We use the LCD and pushbutton C libraries for the display and selection of the LED. We use the ADC library to digitalize the pot value connected to pin 10 to get the brightness value (see earlier 16-bit Experimenter articles for explanation on these C libraries and their appropriate functions).