8. Chapter Potentiometer & LED

Earlier we learned how to use ADC and PWM. In this chapter, we learn to control the brightness of an LED by using a potentiometer.

8.1. Project Soft Light

In this project, we will make a soft light. We will use an ADC Module to read ADC values of a potentiometer and map it to duty cycle ratio of the PWM used to control the brightness of an LED. Then you can change the brightness of an LED by adjusting the potentiometer.

8.1.1. Component List

  1. Raspberry Pi (with 40 GPIO) x1

  2. GPIO Extension Board & Ribbon Cable x1

  3. Breadboard x1

Rotary potentiometer x1

Rotary-potentiometer

Resistor 10kΩ x2

Resistor-10kΩ

ADC module x1 (Only one)

ADC-module-1 or ADC-module-2

LED x1

red-led

Jumper Wire M/M x17

jumper-wire

Resistor 220Ω x1

res-220R

8.1.2. Circuit with ADS7830

Schematic diagram

ADS7830-Schematic-2

Hardware connection. If you need any support,please feel free to contact us via:

support@freenove.com

ADS7830-fritizing-2

8.1.3. Circuit with PCF8591

Schematic diagram

PCF8591-Schematic-2

Hardware connection.

PCF8591-fritizing-2

8.1.4. Code

8.1.4.1. C Code Softlight

If you did not configure I2C, please refer to Chapter 7. If you did, please move on.

First, observe the project result, and then learn about the code in detail.

Hint

If you have any concerns, please contact us via: support@freenove.com

  1. Use cd command to enter 08.1.1_Softlight directory of C code.

$ cd ~/Freenove_Kit/Code/C_Code/08.1.1_Softlight

  1. Use following command to compile “Softlight.cpp” and generate executable file Softlight.

$ g++ Softlight.cpp -o Softlight -lwiringPi -lADCDevice

  1. Then run the generated file Softlight.

$ sudo ./Softlight

After the program is executed, adjusting the potentiometer will display the voltage values of the potentiometer in the Terminal window and the converted digital quantity. As a consequence, the brightness of LED will be changed.

The following is the code:

 1/**********************************************************************
 2* Filename    : Softlight.cpp
 3* Description : Use potentiometer to control LED
 4* Author      : www.freenove.com
 5* modification: 2020/03/07
 6**********************************************************************/
 7#include <wiringPi.h>
 8#include <stdio.h>
 9#include <softPwm.h>
10#include <ADCDevice.hpp>
11
12#define ledPin 0
13
14ADCDevice *adc;  // Define an ADC Device class object
15
16int main(void){
17    adc = new ADCDevice();
18    printf("Program is starting ... \n");
19
20    if(adc->detectI2C(0x48)){    // Detect the pcf8591.
21        delete adc;                // Free previously pointed memory
22        adc = new PCF8591();    // If detected, create an instance of PCF8591.
23    }
24    else if(adc->detectI2C(0x4b)){// Detect the ads7830
25        delete adc;               // Free previously pointed memory
26        adc = new ADS7830();      // If detected, create an instance of ADS7830.
27    }
28    else{
29        printf("No correct I2C address found, \n"
30        "Please use command 'i2cdetect -y 1' to check the I2C address! \n"
31        "Program Exit. \n");
32        return -1;
33    }
34    wiringPiSetup();
35    softPwmCreate(ledPin,0,100);
36    while(1){
37        int adcValue = adc->analogRead(0);    //read analog value of A0 pin
38        softPwmWrite(ledPin,adcValue*100/255);    // Mapping to PWM duty cycle
39        float voltage = (float)adcValue / 255.0 * 3.3;  // Calculate voltage
40        printf("ADC value : %d  ,\tVoltage : %.2fV\n",adcValue,voltage);
41        delay(30);
42    }
43    return 0;
44}

In the code, read the ADC value of potentiometer and map it to the duty cycle of PWM to control LED brightness.

1int adcValue = adc->analogRead(0);    //read analog value of A0 pin
2softPwmWrite(ledPin,adcValue*100/255);    // Mapping to PWM duty cycle