Firstly, thanks for all the help previously it’s massively enabled me to understand and make progress with my projects.
The code below (for me as a novice) started out as an alternative idea instead of using 8 round GC9A01 displays by using the example in TFT_eSPI/examples/320x240/TFT_Meters as a template. The basic display works giving me confidence to progress to the next step by using sensing devices (looking at using Bluetooth) to provide real world measurements.
But each time I make a change (for example add a comment) when I build the code I get the following warnings and I’d like to know is there an alternative coding method as the part of the code exists in the the original meter code.
src/main.cpp: In function 'void setup()':
src/main.cpp:133:27: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
plotLinear("Bar", 0, 80);
^
src/main.cpp:134:31: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
plotLinear("Bar", 1 * d, 80);
^
src/main.cpp:135:31: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
plotLinear("Bar", 2 * d, 80);
^
src/main.cpp:136:31: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
plotLinear("Bar", 3 * d, 80);
^
src/main.cpp:137:31: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
plotLinear("`C", 4 * d, 80);
^
src/main.cpp:138:31: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
plotLinear("Bar", 5 * d, 80);
^
src/main.cpp:139:31: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
plotLinear("Volt", 6 * d, 80);
^
src/main.cpp:140:31: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]
plotLinear("ANY", 7 * d, 80);
^
Linking .pio\build\esp32doit-devkit-v1\firmware.elf
Retrieving maximum program size .pio\build\esp32doit-devkit-v1\firmware.elf
Checking size .pio\build\esp32doit-devkit-v1\firmware.elf
Advanced Memory Usage is available via "PlatformIO Home > Project Inspect"
RAM: [= ] 7.0% (used 22840 bytes from 327680 bytes)
Flash: [== ] 23.2% (used 303497 bytes from 1310720 bytes)
Building .pio\build\esp32doit-devkit-v1\firmware.bin
esptool.py v4.9.0
Creating esp32 image...
Merged 2 ELF sections
Successfully created esp32 image.
Here is the code example (modified) I’m using. Please feel free to use.
/*
Example animated analogue meters using a ILI9341 TFT LCD screen
Needs Font 2 (also Font 4 if using large scale label)
Make sure all the display driver and pin connections are correct by
editing the User_Setup.h file in the TFT_eSPI library folder.
#########################################################################
###### DON'T FORGET TO UPDATE THE User_Setup.h FILE IN THE LIBRARY ######
#########################################################################
Modified: 25/06/2026
Verion: Basic Display Configuration
*/
#include <Arduino.h>
#include <TFT_eSPI.h> // Hardware-specific library
TFT_eSPI tft = TFT_eSPI(); // Invoke custom library
#define TFT_GREY 0x5AEB
////#define LOOP_PERIOD 35 // Display updates every 35 ms
#define LOOP_PERIOD 20 // Display updates every 20 ms
uint32_t updateTime = 0; // Time for next update in millis() function
int value[8] = {0, 0, 0, 0, 0, 0,0 ,0}; //Array to hold
int old_value[8] = { -1, -1, -1, -1, -1, -1,-1, -1};
int d = 0;
// #########################################################################
// Draw a linear meter on the screen
// #########################################################################
void plotLinear(char *label, int x, int y)
{
int w = 36;
tft.drawRect(x, y, w, 155, TFT_GREY);
tft.fillRect(x + 2, y + 19, w - 3, 155 - 38, TFT_WHITE);
tft.setTextColor(TFT_CYAN, TFT_BLACK);
tft.drawCentreString(label, x + w / 2, y + 2, 2);
for (int i = 0; i < 110; i += 10)
{
tft.drawFastHLine(x + 20, y + 27 + i, 6, TFT_BLACK);
}
for (int i = 0; i < 110; i += 50)
{
tft.drawFastHLine(x + 20, y + 27 + i, 9, TFT_BLACK);
}
tft.fillTriangle(x + 3, y + 127, x + 3 + 16, y + 127, x + 3, y + 127 - 5, TFT_RED);
tft.fillTriangle(x + 3, y + 127, x + 3 + 16, y + 127, x + 3, y + 127 + 5, TFT_RED);
tft.drawCentreString("---", x + w / 2, y + 155 - 18, 2);
}
// #########################################################################
// Adjust 8 (was 6) linear meter pointer positions
// #########################################################################
void plotPointer(void)
{
byte pw = 16;
////int dy = 187;
int dy = 106; //Change this to align text
int xTxt = 165; //Change this to align text
int yTxt = 10; //Change this to align text
tft.setTextColor(TFT_CYAN, TFT_BLACK);
tft.setRotation(0);
tft.drawString("Left Front", xTxt, yTxt,2);
tft.drawString("Left Rear", xTxt,yTxt+40,2);
tft.drawString("Right Front", xTxt,yTxt+80,2);
tft.drawString("Right Rear", xTxt,yTxt+120,2);
tft.drawString("Coolant", xTxt,yTxt+160,2);
tft.drawString("Oil ", xTxt,yTxt+200,2);
tft.drawString("Battery", xTxt,yTxt+240,2);
tft.drawString("OtherMeas't", xTxt,yTxt+280,2);
tft.setRotation(1);
tft.setTextColor(TFT_GREEN, TFT_BLACK);
// Move the 8 pointers one pixel towards new value
for (int i = 0; i < 8; i++)
{ // Turn Floats (numbers) into Strings (characters)
// dtostrf(float_value, min_width, num_digits_after_decimal, where_to_store_string)
char buf[8]; dtostrf(value[i], 4, 0, buf);
//drawRightString(const char *string, int32_t dX, int32_t poY, uint8_t font)
/*
const char* string: String to be displayed as array of char, null terminated.
ìnt32_t dX: end x coord.
ìnt32_t poY: y coord.
uìnt32_t font: Font to be used.
*/
tft.drawRightString(buf, i * 40 + 36 - 5, 106 - 27 + 155 - 18, 2);
int dx = 3 + 40 * i;
if (value[i] < 0) value[i] = 0; // Limit value to emulate needle lower end stop.
if (value[i] > 100) value[i] = 100; // Limit value to emulate needle upper end stop.
while (!(value[i] == old_value[i])) {
////dy = 187 + 100 - old_value[i]; Original Example Code
dy = 106 + 100 - old_value[i];
if (old_value[i] > value[i])
{
tft.drawLine(dx, dy - 5, dx + pw, dy, TFT_WHITE);
old_value[i]--;
tft.drawLine(dx, dy + 6, dx + pw, dy + 1, TFT_RED);
}
else
{
tft.drawLine(dx, dy + 5, dx + pw, dy, TFT_WHITE);
old_value[i]++;
tft.drawLine(dx, dy - 6, dx + pw, dy - 1, TFT_RED);
}
}
}
}
void setup(void) {
////Serial.begin(115200); // For debug
tft.init();
tft.setRotation(1);
tft.fillScreen(TFT_BLACK);
// Draw 8 linear meters, original exaple code draws 6.
// Declare a typedef uint8_t value of 40.
byte d = 40;
//Position each meters quantity or dimension.
plotLinear("Bar", 0, 80);
plotLinear("Bar", 1 * d, 80);
plotLinear("Bar", 2 * d, 80);
plotLinear("Bar", 3 * d, 80);
plotLinear("`C", 4 * d, 80);
plotLinear("Bar", 5 * d, 80);
plotLinear("Volt", 6 * d, 80);
plotLinear("ANY", 7 * d, 80);
updateTime = millis(); // Next update time
}
void loop() {
int angle =0;
int period = 60;
if (updateTime <= millis()) {
updateTime = millis() + LOOP_PERIOD;
// Create a Sine wave for testing
d += 2; if (d >= 360) d = 0;
value[0] = 50 + 50 * sin((d + angle) * 0.0174532925);
angle = angle+period;
value[1] = 50 + 50 * sin((d + angle) * 0.0174532925);
angle = angle+period;
value[2] = 50 + 50 * sin((d + angle) * 0.0174532925);
angle = angle+period;
value[3] = 50 + 50 * sin((d + angle) * 0.0174532925);
angle = angle+period;
value[4] = 50 + 50 * sin((d + angle) * 0.0174532925);
angle = angle+period;
value[5] = 50 + 50 * sin((d + angle) * 0.0174532925);
angle = angle+period;
value[6] = 50 + 50 * sin((d + angle) * 0.0174532925);
angle = angle+period;
value[7] = 50 + 50 * sin((d + angle) * 0.0174532925);
if (angle >= 420) angle = 0; // 8 * 60
plotPointer();
}
}
Hope this makes sense, thanks in advance.
Degs
