STM32 Mbed CodeIDE (5/8): analog voltage warning

This lesson uses PC_1, one of the analog input pins in the STM32 Mbed CodeIDE guide. In the remote lab UI, this is the Pot. pot2 slider. Move that slider slowly and watch the console value change.

Pin checklist for this lesson:

- Analog input: Pot. pot2 -> PC_1
- Green LED 1: PB_13
- Green LED 2: PB_14
- Green LED 3: PB_15

Mbed code:

AnalogIn pot(PC_1);
float reading = pot.read();
int millivolts = (int)(reading * 3300.0f);

AnalogIn pot(PC_1) creates a Mbed input object for a measured voltage, similar to how DigitalOut created output objects for LEDs. pot.read() takes one sample and returns a normalized value from about 0.0 to 1.0. Multiplying by 3300.0f converts that fraction into approximate millivolts for a 3.3 V input range. The program prints integer millivolts, not floating-point text.

Approximate thresholds:

- below 1000 mV: low
- 1000-1999 mV: medium
- 2000-2999 mV: high
- 3000 mV and above: simulated overvoltage/full-scale warning, shown by blinking all three green LEDs

The low state still turns one green LED on. In this lesson, that means "LOW warning level", not "no output."

Expected warning behavior:

- < 1000 mV: printed level LOW; one green LED on.
- 1000-1999 mV: printed level MEDIUM; two green LEDs on.
- 2000-2999 mV: printed level HIGH; three green LEDs on.
- >= 3000 mV: printed level OVERVOLTAGE; all three green LEDs blink on and off together.

The helper function set_led() writes 0 for on and 1 for off because these green LED outputs are active-low in the lab examples.

#include "mbed.h"

AnalogIn pot(PC_1);

DigitalOut led1(PB_13, 1);
DigitalOut led2(PB_14, 1);
DigitalOut led3(PB_15, 1);

void set_led(DigitalOut &led, bool on) {
    led = on ? 0 : 1;
}

void set_leds(int count) {
    set_led(led1, count >= 1);
    set_led(led2, count >= 2);
    set_led(led3, count >= 3);
}

int main() {
    printf("Analog voltage warning start\n");
    bool overvoltage_flash = false;

    while (true) {
        float reading = pot.read();
        int millivolts = (int)(reading * 3300.0f);

        if (millivolts < 1000) {
            overvoltage_flash = false;
            set_leds(1);
            printf("millivolts=%d level=LOW\n", millivolts);
        } else if (millivolts < 2000) {
            overvoltage_flash = false;
            set_leds(2);
            printf("millivolts=%d level=MEDIUM\n", millivolts);
        } else if (millivolts < 3000) {
            overvoltage_flash = false;
            set_leds(3);
            printf("millivolts=%d level=HIGH\n", millivolts);
        } else {
            overvoltage_flash = !overvoltage_flash;
            set_leds(overvoltage_flash ? 3 : 0);
            printf("millivolts=%d level=OVERVOLTAGE flash=%d\n",
                   millivolts,
                   overvoltage_flash);
        }

        ThisThread::sleep_for(1000ms);
    }
}

The program above is one continuous main.cpp file. If your PDF view splits the code block across two pages, continue copying with the next line on the following page.

Fill at least four table rows while you move the potentiometer. Use different positions when possible: low, medium, high, and the highest value you can reach. In the Reading column, write a short label such as low position, middle position, high position, or highest value. For each row, copy the printed millivolt value and level from the console, then write the LED state you saw in the camera.

Example serial lines:

millivolts=742 level=LOW
millivolts=1840 level=MEDIUM
millivolts=2510 level=HIGH
millivolts=3120 level=OVERVOLTAGE flash=1

For level=OVERVOLTAGE, observe for at least two seconds before filling the row. flash=1 means all three green LEDs are on for that sample; flash=0 means all three are off for that sample. Record the overall behavior as "all three blinking", not as a single still image.

Example table row: middle position | 1840 | MEDIUM | two green LEDs on | yes.

Change one threshold by 200 mV. For example, change the medium/high boundary from 2000 mV to 2200 mV by editing this line:

} else if (millivolts < 2000) {

to:

} else if (millivolts < 2200) {

This moves the MEDIUM/HIGH boundary upward. Values below 2200 mV should now print MEDIUM; values at 2200 mV or above should print HIGH until the overvoltage threshold.

Compile and upload again. Test at least two potentiometer positions near the changed boundary: one just below the new boundary and one just above it. Copy both serial lines as evidence. If you cannot get very close to the boundary, record the closest value below and the closest value above, then explain that limitation.