ESP32 TX Voltage Monitoring RC Car Preparation

/* Program by: RANDY N. CAÑADA, DEC. 2025
ESP32 wiring (VSPI example)
MOSI -> GPIO23
MISO -> GPIO19
SCK  -> GPIO18
CSN  -> GPIO5
CE   -> GPIO17

ESP32 (Transmitter + Feedback Receiver)

OLED 1306 CONNECTIONS:
-can be operated in 3.3vdc / 5 vdc
sda=21,scl=22
*/

#include <SPI.h>
#include <nRF24L01.h>
#include <RF24.h>
#include <Wire.h>
#include <Adafruit_GFX.h>
#include <Adafruit_SSD1306.h>

// --- RF24 setup ---
RF24 radio(17, 5); // CE, CSN
const byte address[][6] = {"00001", "00002"}; // ESP32->Arduino, Arduino->ESP32

int buttonPin = 13;

// --- OLED setup ---
#define SCREEN_WIDTH 128
#define SCREEN_HEIGHT 64
Adafruit_SSD1306 display(SCREEN_WIDTH, SCREEN_HEIGHT, &Wire, -1);

// --- Battery thresholds ---
const float VBAT_MIN = 7.0;   // cutoff for 2S Li-ion
const float VBAT_MAX = 8.4;   // full charge for 2S Li-ion

void setup() {
  Serial.begin(9600);
  pinMode(buttonPin, INPUT_PULLUP);

  // --- Radio init ---
  radio.begin();
  radio.setPALevel(RF24_PA_MIN);
  radio.openWritingPipe(address[0]);   // Send commands to Arduino
  radio.openReadingPipe(1, address[1]); // Listen for feedback

  // --- OLED init ---
  if (!display.begin(SSD1306_SWITCHCAPVCC, 0x3C)) {
    Serial.println("SSD1306 allocation failed");
    while (1);
  }
  display.clearDisplay();
  display.setTextSize(3);
  display.setTextColor(SSD1306_WHITE);
  display.setCursor(20,0);
  display.println("Remote");
  display.setCursor(20,35);
  display.println("Ready!");
  display.display();
  delay(1500);
}

void loop() {
  // --- Send command ---
  radio.stopListening();
  if (digitalRead(buttonPin) == LOW) {
    const char text[] = "LED ON";
    radio.write(&text, sizeof(text));
    Serial.println("Sent: LED ON");
  } else {
    const char text[] = "LED OFF";
    radio.write(&text, sizeof(text));
    Serial.println("Sent: LED OFF");
  }

  // --- Switch to RX mode and wait for feedback ---
  radio.startListening();
  unsigned long start_time = millis();
  while (!radio.available()) {
    if (millis() - start_time > 500) {
      Serial.println("No feedback received.");
      display.clearDisplay();
      display.setTextSize(2);
      display.setCursor(0,0);
      display.println("OFFLINE!");
      display.display();
      return;
    }
  }

  char feedback[32] = "";
  radio.read(&feedback, sizeof(feedback));
  Serial.print("Battery feedback: ");
  Serial.println(feedback);

  // --- Parse feedback (format: "V=1.45V I=1.4mA") ---
  float busVoltage = 0.0, current_mA = 0.0;
  sscanf(feedback, "V=%fV I=%fmA", &busVoltage, &current_mA);

  // Calculate power and battery percentage
  float power_W = busVoltage * (current_mA / 1000.0);
  int pct = batteryPercent(busVoltage);

  // --- Serial Monitor output ---
  Serial.print("V: "); Serial.print(busVoltage, 2); Serial.println("V");
  Serial.print("I: "); Serial.print(current_mA, 1); Serial.println(" mA");
  Serial.print("P: "); Serial.print(power_W, 1); Serial.println(" W");
  Serial.print("Batt: "); Serial.print(pct); Serial.println("%");
  Serial.println("-----------------------------");

  // --- OLED output ---
  display.clearDisplay();
  display.setCursor(0,0);
  display.setTextSize(2);
  display.print("V: "); display.print(busVoltage, 2); display.println("V");
  display.print("I: "); display.print(current_mA, 1); display.println(" mA");
  display.print("P: "); display.print(power_W, 1); display.println(" W");

  // --- Graphical Battery Bar ---
  int barX = 0;
  int barY = 48;
  int barWidth = 100;
  int barHeight = 12;

  // Outline
  display.drawRect(barX, barY, barWidth, barHeight, SSD1306_WHITE);

  // Fill based on pct
  int fillWidth = map(pct, 0, 100, 0, barWidth - 2);
  display.fillRect(barX + 1, barY + 1, fillWidth, barHeight - 2, SSD1306_WHITE);

  // Percentage text
  display.setCursor(104, 48);
  display.setTextSize(1);
  display.print(pct);
  display.print("%");

  // --- Battery warnings ---
  if (pct >= 15 && pct <= 20) {
    display.clearDisplay();
    display.setTextSize(2);
    display.setCursor(28,0);
    display.print("Charge");
    display.setCursor(42,20);
    display.print("Your");
    display.setCursor(28,40);
    display.print("Battery");
    Serial.println("⚠️ Battery Low!");
  }
  else if (pct <= 5) {
    display.clearDisplay();
    display.setTextSize(2);
    display.setCursor(52,1);
    display.print("LOW");
    display.setCursor(28,32);
    display.print("BATTERY!");
    Serial.println("❌ Battery Depleted!");
  }

  display.display();
  delay(1000);
}

// ------------------------------------------------------
// REALISTIC NON-LINEAR BATTERY PERCENTAGE FUNCTION
// For 2S Li-ion (7.0V empty, 8.4V full)
// ------------------------------------------------------
int batteryPercent(float v) {
  if (v >= 8.40) return 100;
  if (v >= 8.30) return 90;
  if (v >= 8.20) return 80;
  if (v >= 8.10) return 70;
  if (v >= 8.00) return 60;
  if (v >= 7.90) return 50;
  if (v >= 7.80) return 40;
  if (v >= 7.60) return 30;
  if (v >= 7.40) return 20;
  if (v >= 7.20) return 10;
  if (v >= 7.00) return 5;
  return 0;
}