ESP-NOW Photoresistor Integration

Read a photoresistor on the sender board and blink an LED on the receiver board when the light level drops below 600.

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How the Data Flows

SENDER                          RECEIVER
──────                          ────────
analogRead(A0) → lightVal
lightVal < 600 → myData.d=true  
esp_now_send() ────────────────→ OnDataRecv()
                                  myData.d==true → blink LED

The bool d field is the flag — sender sets it, receiver acts on it. Field b carries the raw sensor value for logging.

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SENDER — SenderESP.ino

Integrate the photoresistor by reading the sensor each loop and packing the value into the struct before sending.

#include <esp_now.h>
#include <WiFi.h>

uint8_t broadcastAddress[] = {0xe4, 0xb3, 0x23, 0xc5, 0x3d, 0xa4};

typedef struct struct_message {
  char a[32];
  int b;       // used for raw lightVal
  float c;
  bool d;      // true = dark (below threshold)
} struct_message;

struct_message myData;
esp_now_peer_info_t peerInfo;

// Photoresistor globals
int lightPin = A0;
int lightVal = 0;
int threshold = 600;

void OnDataSent(const wifi_tx_info_t *tx_info, esp_now_send_status_t status) {
  Serial.print("\r\nLast Packet Send Status:\t");
  Serial.println(status == ESP_NOW_SEND_SUCCESS ? "Sent OK" : "Send Failed");
}

void setup() {
  Serial.begin(115200);

  // Photoresistor setup
  analogSetAttenuation(ADC_11db);
  pinMode(lightPin, INPUT);

  WiFi.mode(WIFI_STA);

  if (esp_now_init() != ESP_OK) {
    Serial.println("Error initializing ESP-NOW");
    return;
  }
  esp_now_register_send_cb(OnDataSent);
  memcpy(peerInfo.peer_addr, broadcastAddress, 6);
  peerInfo.channel = 0;
  peerInfo.encrypt = false;
  if (esp_now_add_peer(&peerInfo) != ESP_OK) {
    Serial.println("Failed to add peer");
    return;
  }
}

void loop() {
  // Read sensor
  lightVal = analogRead(lightPin);
  Serial.print("Light value: ");
  Serial.println(lightVal);

  // Pack into struct
  strcpy(myData.a, "LIGHT SENSOR");
  myData.b = lightVal;                  // raw reading
  myData.c = 1.2;
  myData.d = (lightVal < threshold);    // true = dark

  esp_err_t result = esp_now_send(broadcastAddress, (uint8_t *) &myData, sizeof(myData));
  if (result == ESP_OK) Serial.println("Sent with success");
  else Serial.println("Error sending the data");

  delay(2000);
}

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RECEIVER — RecieverESP.ino

Check the bool d flag in the callback and blink the LED if dark, turn it off if bright.

#include <esp_now.h>
#include <WiFi.h>

typedef struct struct_message {
  char a[32];
  int b;
  float c;
  bool d;      // true = below threshold (dark)
} struct_message;

struct_message myData;

int ledPin = 4;   // change to match your LED pin

void OnDataRecv(const uint8_t * mac, const uint8_t *incomingData, int len) {
  memcpy(&myData, incomingData, sizeof(myData));
  Serial.print("Light value received: ");
  Serial.println(myData.b);
  Serial.print("Dark? ");
  Serial.println(myData.d ? "YES - blinking" : "NO - LED off");

  if (myData.d) {
    // Blink 3 times when dark
    for (int i = 0; i < 3; i++) {
      digitalWrite(ledPin, HIGH);
      delay(200);
      digitalWrite(ledPin, LOW);
      delay(200);
    }
  } else {
    digitalWrite(ledPin, LOW);
  }
}

void setup() {
  Serial.begin(115200);
  pinMode(ledPin, OUTPUT);

  WiFi.mode(WIFI_STA);
  if (esp_now_init() != ESP_OK) {
    Serial.println("Error initializing ESP-NOW");
    return;
  }
  esp_now_register_recv_cb(esp_now_recv_cb_t(OnDataRecv));
}

void loop() {
  // nothing needed here
}

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Key Changes Summary

Location	What was added
Sender globals	lightPin, lightVal, threshold variables
Sender setup()	analogSetAttenuation() and pinMode() for sensor
Sender loop()	analogRead(), pack value into myData.b and myData.d
Receiver globals	ledPin variable
Receiver setup()	pinMode(ledPin, OUTPUT)
Receiver OnDataRecv()	Check myData.d, blink or turn off LED