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Paul Rodolf P. Castor

Published June 8, 2026

RT-Spark ADC Voltage Monitor

A simple RT-spark-based voltage monitoring system was developed to measure battery and USB voltages using the ADC.

IntermediateFull instructions provided2 hours53

Things used in this project

Hardware components

Breadboard (generic)

Resistor 1M ohm

Jumper wires (generic)

Panasonic Coin Cell CR1220

optional brand

Voltmeter, DC Voltage

you can also use digital multimeter

RT-Thread RT-Spark Development Board

Software apps and online services

RT-Thread STM32CubeIDE

version 1.19.0

Story

Introduction

This activity focused on using the Analog-to-Digital Converter (ADC) of the RT-Spark development board to measure the voltage of a coin cell battery and the USB power supply. A voltage divider circuit was implemented to safely scale the input voltage before it was applied to the ADC input.

Project Overview

The STM32 ADC can only measure voltages within its reference voltage range. Since the USB supply voltage is approximately 5V, directly connecting it to the ADC would exceed the maximum input voltage. To address this limitation, a voltage divider consisting of two high-value resistors was used to scale the voltage down to a safe level.

In this project, the ADC built into the STM32F407ZGT6 microcontroller on the RT-Spark board was used to measure input voltages. The ADC converts the analog voltage into a digital value ranging from 0 to 4095 (12-bit resolution), which can then be translated into an actual voltage through software calculations.

ADC Setup

First, create a new STM32 project using STM32CubeIDE.

1 / 3

The ADC was then configured by enabling ADC1 and assigning pin PA3 as the analog input channel used for voltage measurements.

Open Analog settings from the left sidebar and set PA3 to ADC1 Channel 3, keep the default GPIO settings, use a Clock Prescaler of 8, set the Sampling Time to 480 Cycles, and disable Continuous Conversion Mode. Then generate code for the project.

Circuit Setup

The voltage divider was connected between the voltage source and ADC input pin PA3.

1 / 2 • Since the ADC can only measure voltages within its reference range (approximately 3.3V), directly connecting the 5V USB supply would damage the ADC input or produce invalid readings. To solve this problem, two 1 MΩ resistors were connected in series to divide the voltage by approximately one-half before it reached the ADC input.

Experimental Results

Battery Voltage Measurements

The battery voltage was first measured using a digital multimeter.

VBattery = 3.059 V
VADCIn = 1.513 V
VRefH = 3.277 V

After running the provided firmware, the ADC calculated the battery voltage as:

ADC Computed Voltage (vbat) = 3.026 V

The ADC measurement closely matched the multimeter reading, demonstrating good accuracy.

Resistor Measurements

The actual resistor values were measured using a multimeter:

R1 = 986, 312 Ω
R2 = 977, 291 Ω

Based on these measured values, the calculated scale factor was:

Scale Factor = 2.009230618

Accuracy Improvement

The firmware was updated using the measured resistor values. After calibration:

Measured Battery Voltage = 3.059 V
Calculated Battery Voltage = 3.008 V

The resulting error was:

Error = 0.051 V (51 mV)

This small error indicates that the system achieved a high level of measurement accuracy.

Measurements Without Averaging

Five consecutive battery voltage measurements were recorded:

vbat(1) = 2.98V
vbat(2) = 2.87V
vbat(3) = 2.95V
vbat(4) = 3.019V
vbat(5) = 3.03V

The measured voltages were not constant and shows noticeable variations between readings.

Measurements With 10-Sample Averaging

The firmware was modified to average ten ADC samples before calculating the voltage.

Five measurements were recorded:

vbat(1) = 3.015V
vbat(2) = 3.013V
vbat(3) = 3.016V
vbat(4) = 3.012V
vbat(5) = 3.011V

Small variations were still present, but the readings became significantly more consistent. The averaging process reduced noise and produced more stable measurements.

USB Voltage Measurements

The USB supply voltage was measured using a multimeter.

VUSB = 5.05 V
VBatDiv = 2.407 V

The voltage divider successfully reduced the USB voltage to a level that could be safely measured by the STM32 ADC.

Results and Discussion

The experiment showed how an ADC and a voltage divider can be used to measure battery and USB voltages using an STM32F407ZGT6 microcontroller. It was observed that the actual resistor values were slightly different from their rated values, which affected the voltage calculations. After updating the code with the measured resistor values, the voltage readings became more accurate.

The results showed that using the actual resistor values and applying software averaging improved the accuracy and stability of the measurements. Overall, the activity provided hands-on experience with ADC configuration, voltage divider circuits, voltage measurement, and basic signal processing in embedded systems.

Schematics

Code

 /* USER CODE BEGIN 2 */
  unsigned int value = 0;
  float halfVolts = 0;
  float sourceVolt = 3.3;
  float volt = 0;

  float average = 0;
  float summation = 0;
  /* USER CODE END 2 */

  /* Infinite loop */
  /* USER CODE BEGIN WHILE */
  while (1)
  {
    /* USER CODE END WHILE */

    /* USER CODE BEGIN 3 */
	  summation = 0;   // Reset before taking 10 samples

	      for(int i = 0; i < 10; i++)
	      {
	          HAL_ADC_Start(&hadc1);
	          HAL_ADC_PollForConversion(&hadc1, 10);

	          value = HAL_ADC_GetValue(&hadc1);

	          halfVolts = (value * sourceVolt) / 4095.0;
	          volt = halfVolts * 2.0092;   // Fixed capitalization

	          summation += volt;
	      }

	      average = summation / 10.0;

	      HAL_Delay(100);
  }
  /* USER CODE END 3 */
}

Credits

MA REGINE BERMUDO

7 projects • 0 followers

Paul Rodolf P. Castor

57 projects • 9 followers

RT-Spark ADC Voltage Monitor