STM32 Mbed CodeIDE (6/8): PWM brightness and RGB control

Many devices need levels between off and on: dimmer lights, color indicators, motors, buzzers, and other outputs that should feel adjustable. In this lesson, PWM lets you control brightness and color intensity, then connect that visible output to a live analog control.

Mbed uses PwmOut for PWM pins:

PwmOut red(PA_8);
red.period_ms(2);
red.write(0.25f);

PwmOut red(PA_8) creates a Mbed PWM output object. period_ms(2) sets how often the pulse pattern repeats, and write(0.25f) sets how much of each repeat is on. For an RGB LED, using the same period on red, green, and blue makes the channels behave as one coordinated color output.

write() takes a duty cycle from 0.0 to 1.0:

- 0.0f: always off
- 0.25f: on for 25 percent of each cycle
- 1.0f: always on

This lesson uses the RGB PWM pins PA_8, PA_9, and PA_10.

It also uses PC_0 as the analog control pin in the second half of the lesson. In the lab UI, PC_0 is the Pot. pot1 slider. This is different from lesson 5, which used PC_1 / Pot. pot2.

#include "mbed.h"

PwmOut red(PA_8);
PwmOut green(PA_9);
PwmOut blue(PA_10);

void set_rgb(float r, float g, float b) {
    red.write(r);
    green.write(g);
    blue.write(b);
}

int main() {
    red.period_ms(2);
    green.period_ms(2);
    blue.period_ms(2);

    printf("PWM RGB fixed levels start\n");

    while (true) {
        set_rgb(0.20f, 0.00f, 0.00f);
        printf("state=low_red r=20 g=0 b=0\n");
        ThisThread::sleep_for(2000ms);

        set_rgb(0.20f, 0.50f, 0.10f);
        printf("state=mixed r=20 g=50 b=10\n");
        ThisThread::sleep_for(2000ms);

        set_rgb(0.00f, 0.00f, 0.80f);
        printf("state=high_blue r=0 g=0 b=80\n");
        ThisThread::sleep_for(2000ms);
    }
}

Fill three rows, one per printed RGB state. Printed percentages means the three duty-cycle percentages shown after r=, g=, and b=; Observed output is the camera color/brightness. The table may display extra rows: leave unused rows completely blank. If a color is subtle, choose partly or no instead of guessing.

Now combine analog input and PWM output. The potentiometer controls the red channel duty cycle. The serial percentage is your strongest evidence if camera exposure makes brightness subtle.

#include "mbed.h"

AnalogIn pot(PC_0);
PwmOut red(PA_8);
PwmOut green(PA_9);
PwmOut blue(PA_10);

int main() {
    red.period_ms(2);
    green.period_ms(2);
    blue.period_ms(2);

    green.write(0.0f);
    blue.write(0.0f);

    printf("Potentiometer controlled PWM start\n");

    while (true) {
        float reading = pot.read();
        // TODO 1: replace 0.0f with the analog-to-duty mapping.
        float duty = 0.0f;
        int percent = (int)(duty * 100.0f);

        // TODO 2: replace 0.0f with the mapped duty variable.
        red.write(0.0f);
        printf("red_percent=%d\n", percent);

        ThisThread::sleep_for(500ms);
    }
}

Fill exactly the three required cases: low, middle, and high potentiometer positions. Serial percent is the integer after red_percent= and has unit %; Observed brightness is a broad camera judgment, not a numeric measurement; Visibility records whether the camera evidence was clear. Leave any extra unused rows completely blank.